hereinafter , exemplary embodiments of the present invention will be described in detail with reference to the appended drawings . fig1 is a diagram illustrating an overall configuration of an image forming system 500 to which the present exemplary embodiment is applied . the image forming system 500 includes an image forming apparatus 1 that forms an image on a paper p , and a post - processing apparatus 2 that performs post - processing of the paper p on which an image is formed by the image forming apparatus 1 . first , the image forming apparatus 1 includes an image forming unit 10 that forms an image on the basis of image data , an image reading unit 11 that reads an image from an original document to generate read image data , a paper supply unit 12 that supplies the paper p , one example of a sheet , to the image forming unit 10 , a user interface unit 13 that receives operations by a user and presents information to the user , and a main controller 14 that controls operation of the entirety of the image forming system 500 . the image forming apparatus 1 further includes a paper relaying unit 15 that relays the paper p output from the image forming unit 10 after image formation to the post - processing apparatus 2 . the post - processing apparatus 2 , one example of a sheet processing apparatus , includes a transport unit 3 that receives the paper p after image formation from the image forming apparatus 1 and transports the paper p , a folding unit 4 that performs , if necessary , folding of the paper p brought in from the transport unit 3 , and a post - processing unit 5 that performs another type of post - processing of the paper p brought in from the folding unit 4 . the post - processing apparatus 2 includes a laminated paper supply unit ( interposer ) 6 that supplies laminated paper used as a booklet cover and the like to the transport unit 3 . the post - processing apparatus 2 includes a paper processing controller 7 that controls operation of each functional unit of the post - processing apparatus 2 . the transport unit 3 includes a decurler 3 a , one example of an output unit , an adjusting unit , and a reversing unit , that rectifies curl ( curvature ) of the paper p brought in from the image forming apparatus 1 . the folding unit 4 includes a folding functional unit 4 a that performs folding such as inner tri - folding ( so - called “ c - folding ”) and outer tri - folding ( so - called “ z - folding ”) of the paper p brought in from the transport unit 3 . the post - processing unit 5 includes a hole piercing functional unit 5 a that performs , if necessary , hole piercing ( punching ) for piercing two holes , four holes , and the like on the paper p brought in from the folding unit 4 . the post - processing unit 5 includes an end stitching functional unit 5 b that stacks the paper p passing through the hole piercing functional unit 5 a in required quantities to form a paper stack and performs stitching ( end stitching ) with a staple on an end portion of the formed paper stack . the post - processing unit 5 includes a center stitching functional unit 5 c that stacks the paper p passing through the hole piercing functional unit 5 a in required quantities to form a paper stack , performs stitching ( center stitching ) with a staple on a central portion of the formed paper stack , and performs folding of the central portion ( center stitched portion ) of the paper stack to form a booklet ( performs a simple bookbinding work ). while the paper processing controller 7 in this example is illustratively configured to be provided in the post - processing unit 5 in the post - processing apparatus 2 , the paper processing controller 7 may be provided in other units ( the transport unit 3 , the folding unit 4 , and the laminated paper supply unit 6 ) constituting the post - processing apparatus 2 . while the paper processing controller 7 in this example is illustratively configured to be provided in the post - processing apparatus 2 , the paper processing controller 7 may be provided in the image forming apparatus 1 . the main controller 14 and the paper processing controller 7 may not be separately provided . the main controller 14 may double as the paper processing controller 7 . fig2 is a diagram illustrating a configuration of the image forming unit 10 provided in the image forming apparatus 1 . the image forming unit 10 , one example of an image forming unit and a supply unit , includes a toner image forming unit 10 a that performs an electrophotographic process to form a toner image , an intermediate transfer belt 10 b that is rotatably provided to face the toner image forming unit 10 a , and a first transfer unit 10 c that first transfers the toner image formed by the toner image forming unit 10 a to the intermediate transfer belt 10 b . the image forming unit 10 includes a second transfer unit 10 d that second transfers the toner image first transferred to the intermediate transfer belt 10 b to the paper p ( refer to fig1 ), and a fixing unit 10 e , one example of a heating unit , that fixes the second transferred toner image to the paper p . the image forming unit 10 includes a supply path ra along which the paper p transported from the paper supply unit 12 ( refer to fig1 ) is supplied to the second transfer unit 10 d , an output path rb along which the paper p transported from the second transfer unit 10 d is output to the paper relaying unit 15 ( refer to fig1 ) through the fixing unit 10 e , and a reversal path rc that is connected to the downstream side of the fixing unit 10 e on the output path rb and to the supply path ra and along which the paper p passing through the fixing unit 10 e is reversed inside out by reversing a transport direction thereof and supplied to the second transfer unit 10 d again . the image forming apparatus 1 that may be used alone is used in the present exemplary embodiment by being incorporated in the image forming system 500 . when the image forming apparatus 1 is used alone , a paper output unit is provided at a position where the paper relaying unit 15 is attached , and the paper p after image formation is output face down to above the image forming apparatus 1 . thus , the output path rb in the image forming unit 10 is provided to extend upward in fig2 up to the fixing unit 10 e and bend ( curve ) rightward in fig2 after passing through the fixing unit 10 e . the angular difference between the transport direction of the paper p on an outlet side of the fixing unit 10 e , that is , the part corresponding to the most upstream side of the output path rb , and the transport direction of the paper p in the part corresponding to the downstream side of the output path rb in the transport direction at a distance of 200 mm to 250 mm from the outlet side of the fixing unit 10 e is set to be greater than or equal to 45 degrees on the output path rb , one example of a curved transport path . a transport roller 10 f that transports the paper p transported from the paper supply unit 12 ( refer to fig1 ) is provided on the upstream side , in the transport direction , of the supply path ra from the part of the supply path ra where the supply path ra joins the reversal path rc . a registration roll 10 g that aligns the position of the paper p supplied to the second transfer unit 10 d is provided on the downstream side , in the transport direction , of the supply path ra from the part of the supply path ra where the supply path ra joins the reversal path rc . an output roll 10 h that forwardly rotates to guide the paper p passing through the fixing unit 10 e to the paper relaying unit 15 ( refer to fig1 ) or reversely rotates to guide the paper p to the reversal path rc is provided in a forwardly and reversely rotatable manner near the part of the output path rb where the output path rb and the reversal path rc branch off from each other . multiple reversal transporting rolls 10 j that guide the paper p brought in from the output path rb to the supply path ra are provided on the reversal path rc . fig3 is a diagram illustrating types of folding performed by the folding functional unit 4 a provided in the folding unit 4 . the folding functional unit 4 a of the present exemplary embodiment performs envelope folding in which the paper p is tri - folded into a nearly trisected ( 1 : 1 : 1 ) state in order to accommodate the paper p in an envelope , for example , and z - folding in which the paper p is tri - folded into a nearly 2 : 1 : 1 state by bi - folding one panel of the bi - folded paper p , thereby forming the folded shape of the paper p into the z shape . the folding functional unit 4 a of the present exemplary embodiment performs , as the envelope folding , envelope z - folding in which the outer side of the paper p is tri - folded to form the folded shape of the paper p into the z shape , and envelope c - folding in which the inner side of the paper p is tri - folded to form the folded shape of the paper p into the c shape . when the envelope folding ( the envelope z - folding and the envelope c - folding ) and the z - folding are performed in the present exemplary embodiment , a first fold f 1 is formed by performing first folding of the paper p , and a second fold f 2 is formed by performing second folding of the paper p in which the first fold f 1 is formed . therefore , folding is required to be performed twice in the envelope folding ( the envelope z - folding and the envelope c - folding ) and the z - folding . fig4 a and 4b are diagrams illustrating types of the envelope c - folding . fig4 a illustrates outer image c - folding , and fig4 b illustrates inner image c - folding . the outer image c - folding illustrated in fig4 a is one of the types of the envelope c - folding illustrated in fig3 in which an image im ( toner image ) formed one side of the paper p is positioned on the outer side of the paper p after folding . the inner image c - folding illustrated in fig4 b is one of the types of the envelope c - folding illustrated in fig3 in which the image im ( toner image ) formed one side of the paper p is positioned on the inner side of the paper p after folding . the image im ( toner image ) formed on one side of the paper p is positioned on the outer side and the inner side of the paper p after folding in the envelope z - folding and the z - folding other than the envelope c - folding . the outer tri - folding includes the envelope z - folding and the z - folding , and the inner tri - folding includes the envelope c - folding ( the outer image c - folding and the inner image c - folding ) in the present exemplary embodiment . fig5 is a diagram illustrating a configuration of the folding functional unit 4 a in the folding unit 4 . the folding functional unit 4 a of the present exemplary embodiment includes a straight transport path r 1 that linearly straightly connects a paper inlet ( in ) into which the paper p is brought from the transport unit 3 ( refer to fig1 ) and a paper outlet ( out ) from which the paper p is transported to the post - processing unit 5 ( refer to fig1 ), and a detour transport path r 2 that is provided to branch off from midway of the straight transport path r 1 and detour below the straight transport path r 1 . the folding functional unit 4 a includes a first folding mechanism 41 that performs the first folding of the paper p , and a second folding mechanism 42 that performs the second folding of the paper p . the first folding mechanism 41 and the second folding mechanism 42 are provided on the detour transport path r 2 . an intake roller ( transport roller ) 43 is provided in the inlet part of the straight transport path r 1 , and a dispensing roller ( transport roller ) 44 is provided midway of the straight transport path r 1 in the folding functional unit 4 a . a first switching gate 45 for switching the transport path of the paper p is provided in the part of the folding functional unit 4 a where the straight transport path r 1 and the detour transport path r 2 branch off from each other . the detour transport path r 2 includes an inlet detour transport path r 21 that extends downward from the part where the straight transport path r 1 and the detour transport path r 2 branch off from each other , an intermediate detour transport path r 22 that branches off in the c shape from midway of the inlet detour transport path r 21 , and a return detour transport path r 23 that branches off from midway of the intermediate detour transport path r 22 and returns to the straight transport path r 1 . the first folding mechanism 41 is configured of a skew correction roller ( doubles as a transport roller ) 41 a that is provided midway of the inlet detour transport path r 21 and is provided immediately before a first folding position in a manner capable of nipping and releasing the paper p , a first end guide 41 b that is provided on the terminal side of the inlet detour transport path r 21 in a manner capable of moving upward and downward , and a first folding roll 41 c that is provided on the inlet detour transport path r 21 side of the intermediate detour transport path r 22 . the second folding mechanism 42 includes a push roller ( doubles as a transport roller ) 42 a that is provided midway of the intermediate detour transport path r 22 and is provided immediate before a second folding position , a second end guide 42 b that is provided on the terminal side of the intermediate detour transport path r 22 in a manner capable of moving upward and downward , and a second folding roll 42 c that is provided on the intermediate detour transport path r 22 side of the return detour transport path r 23 . multiple transport rollers 47 are provided on the detour transport path r 2 in addition to the skew correction roller 41 a and the push roller 42 a . a second switching gate 46 that switches between permitting passage to the return detour transport path r 23 and not permitting passage thereto is provided immediately after the second folding roll 42 c . a paper accommodating device 48 that accommodates the paper p subjected to the envelope folding ( the envelope z - folding or the envelope c - folding ) is provided below the second switching gate 46 . the first folding roll 41 c functions as one example of a first folding unit , and the second folding roll 42 c functions as one example of a second folding unit in the present exemplary embodiment . the first folding roll 41 c and the second folding roll 42 c respectively function as one example of an inner tri - folding unit and one example of an outer tri - folding unit . fig6 is a diagram illustrating a layout of the folding functional unit 4 a . paper which is the target of the envelope folding in the present exemplary embodiment has a maximum size corresponding to jis a4 and a minimum size equal to 8 . 5 × 11 [ inches ]. paper which is the target of the z - folding in the present exemplary embodiment has a maximum size equal to 11 × 17 [ inches ] and a minimum size corresponding to jis b4 . the reference sign l in fig6 is the length of passage of paper from the intake roller 43 to the first end guide 41 b and is set to be greater than or equal to the maximum size of the paper which is the target of the z - folding ( 11 × 17 [ inches ] in the present example ). a reference sign l 1 is the length of passage of paper from the intake roller 43 to the skew correction roller 41 a and is set to be less than or equal to the minimum size of the paper which is the target of the envelope folding ( 8 . 5 × 11 [ inches ] in the present example ). a reference sign l 2 is the length of passage of paper from the skew correction roller 41 a to a first nip position a of the first folding roll 41 c and is set to be less than or equal to ⅓ of the minimum size of the paper which is the target of the envelope folding ( 8 . 5 × 11 [ inches ] in the present example ). a reference sign l 3 is the length of passage of paper from the first nip position a of the first folding roll 41 c to the first end guide 41 b and is set to be equal to ⅓ of the size of the paper which is the target of the envelope z - folding , ⅔ of the size of the paper which is the target of the envelope c - folding , or ¼ of the size of the paper which is the target of the z - folding . a reference sign l 4 is the length of passage of paper from the first nip position a of the first folding roll 41 c to the push roller 42 a and is set to be less than or equal to ½ of the minimum size of the paper which is the target of the z - folding ( size corresponding to jis b4 in the present example ). a reference sign l 5 is the length of passage of paper from the push roller 42 a to a second nip position b of the second folding roll 42 c and is set to be less than or equal to ⅓ of the minimum size of the paper which is the target of the envelope folding ( 8 . 5 × 11 [ inches ] in the present example ). a reference sign l 6 is the length of passage of paper from the second nip position b of the second folding roll 42 c to the second end guide 42 b and is set to be equal to ⅔ of the size of the paper which is the target of the envelope folding or ¼ of the size of the paper which is the target of the z - folding . fig7 is a block diagram related to control of the image forming system 500 of the present exemplary embodiment . an instruction signal corresponding to an instruction received from the user is input from the user interface unit 13 into the main controller 14 provided in the image forming apparatus 1 . the main controller 14 outputs control signals to the image forming unit 10 , the image reading unit 11 , the paper supply unit 12 , and the paper relaying unit 15 provided in the image forming apparatus 1 . the main controller 14 outputs a control signal to the paper processing controller 7 provided in the post - processing apparatus 2 . a control signal is input from the main controller 14 provided in the image forming apparatus 1 into the paper processing controller 7 provided in the post - processing apparatus 2 . the paper processing controller 7 outputs control signals to the transport unit 3 , the folding unit 4 , the post - processing unit 5 , and the laminated paper supply unit 6 provided in the post - processing apparatus 2 . the paper processing controller 7 outputs a control signal to the main controller 14 provided in the image forming apparatus 1 . next , a folding operation performed by the folding unit 4 ( folding functional unit 4 a ) will be described . the folding unit 4 of the present exemplary embodiment , as described above , is capable of selectively performing , as folding , three processes ( the envelope z - folding , the envelope c - folding , and the z - folding ). hereinafter , the processes will be described in order . fig8 a to 8c are diagrams illustrating progress in the envelope z - folding . it is assumed that a job in which the paper p of a small size put into an envelope ( for example , jis a4 short edge feed ( sef )), after the image im is formed thereon by the image forming apparatus 1 , is subjected to the envelope z - folding by the folding unit 4 and is then output to the paper accommodating device 48 is performed . first , a toner image ( the image im ) is transferred and fixed to the paper p in the image forming apparatus 1 , and the paper p is relayed to the post - processing apparatus 2 . the paper p after image formation that is output from the image forming apparatus 1 is relayed to the folding unit 4 via the transport unit 3 after , if necessary , curl thereof is rectified by the decurler 3 a . the first switching gate 45 is provided in the folding unit 4 ( folding functional unit 4 a ) in a position permitting movement of the paper p to the detour transport path r 2 as illustrated in fig5 in accordance with a job specification “ envelope z - folding mode ” received through the user interface unit 13 . the first end guide 41 b and the second end guide 42 b are moved to target positions thereof , and the second switching gate 46 is provided in a position preventing movement of the paper p to the return detour transport path r 23 . particularly , the positions of the first end guide 41 b and the second end guide 42 b are adjusted in such a manner that both l 3 and l 6 illustrated in fig6 are equal to ⅓ of the size of the target paper ( jis a4 sef in the present example ). the paper p , in this state , is transported from the straight transport path r 1 to the inlet detour transport path r 21 and is guided to the first folding mechanism 41 via the skew correction roller 41 a , and the leading edge of the paper p abuts the first end guide 41 b and stops as illustrated in fig8 a . at this point , when the paper p is transported to the first folding roll 41 c in a curved attitude in a case where the paper p is put in a skewed manner in the image forming apparatus 1 or skewed midway of the transport path of the transport unit 3 , the accuracy of the position of a fold is decreased . therefore , skew correction is performed of the paper p by using the skew correction roller 41 a in the present exemplary embodiment . this skew correction method includes causing the leading edge of the paper p to abut the first end guide 41 b while transporting the paper p in a nipped manner with the skew correction roller 41 a , transporting the paper p a few mm ( for example , approximately 5 mm ) further therefrom to form a loop on the leading edge side of the paper p , and releasing nipping of the skew correction roller 41 a . in this case , when the nipping by the skew correction roller 41 a is released , the loop formed in the paper p is straightened . thus , the leading edge of the paper p becomes horizontal along the first end guide 41 b , and the trailing edge of the paper p becomes horizontal following the leading edge of the paper p . accordingly , skew of the paper p is corrected . next , the paper p after the end of skew correction is again nipped by the skew correction rollers 41 a and is transported at a slightly higher speed than the first folding roll 41 c to buckle the paper p in a space in front of the first folding roll 41 c and to form a loop . the paper p is transported to the first folding roll 41 c and is subjected to the first folding ( forming the first fold f 1 of the envelope z - folding illustrated in fig3 ) in the first nip position a ( refer to fig6 ). next , as illustrated in fig8 b , the paper p after the end of the first folding is guided to the second folding mechanism 42 . the paper p is transported through the push roller 42 a , and the leading edge thereof , the first fold f 1 made by the first folding , is then caused to abut the second end guide 42 b to form a loop on the trailing edge side of the paper p . then , the paper p is transported to the second folding roll 42 c and is subjected to the second folding ( forming the second fold f 2 of the envelope z - folding illustrated in fig3 ) in the second nip position b ( refer to fig6 ). at this point , skew correction is not required since skew correction is previously performed in front of the first folding roll 41 c . then , the paper p subjected to the envelope z - folding after the end of the second folding is guided by the second switching gate 46 and is output to the paper accommodating device 48 ( refer to fig5 ) as illustrated in fig8 c . an envelope z - folding operation for one sheet of the paper p is completed as described heretofore . fig9 a to 9c are diagrams illustrating progress in the envelope c - folding . it is assumed that a job in which the paper p of a small size put into an envelope ( for example , jis a4 sef ), after the image im is formed thereon by the image forming apparatus 1 , is subjected to the envelope c - folding by the folding unit 4 and is then output to the paper accommodating device 48 is performed . first , in the same manner as described in the case of the envelope z - folding above , a toner image ( the image im ) is transferred and fixed to the paper p in the image forming apparatus 1 , and the paper p after image formation is relayed to the post - processing apparatus 2 . the paper p after image formation that is output from the image forming apparatus 1 is relayed to the folding unit 4 via the transport unit 3 after , if necessary , curl thereof is rectified by the decurler 3 a . the first switching gate 45 is provided in the folding unit 4 ( folding functional unit 4 a ) in a position permitting movement of the paper p to the detour transport path r 2 as illustrated in fig5 in accordance with a job specification “ envelope c - folding mode ” received through the user interface unit 13 . the first end guide 41 b and the second end guide 42 b are moved to target positions thereof , and the second switching gate 46 is provided in a position preventing movement of the paper p to the return detour transport path r 23 . particularly , the position of the first end guide 41 b is adjusted differently from the envelope z - folding mode in such a manner that l 3 illustrated in fig6 are equal to ⅔ of the size of the target paper ( jis a4 sef in the present example ). at this point , the position of the second end guide 42 b is the same as that in the envelope z - folding mode . the paper p , in this state , is transported from the straight transport path r 1 to the inlet detour transport path r 21 and is guided to the first folding mechanism 41 via the skew correction roller 41 a , and the leading edge of the paper p abuts the first end guide 41 b and stops as illustrated in fig9 a . next , the paper p is subjected to skew correction by using the skew correction roller 41 a and is then subjected to the first folding ( forming the first fold f 1 of the envelope c - folding illustrated in fig3 ) by the first folding roll 41 c in the first nip position a ( refer to fig6 ). next , as illustrated in fig9 b , the paper p after the end of the first folding is guided to the second folding mechanism 42 . the paper p is transported through the push roller 42 a , and the leading edge thereof , the first fold f 1 made by the first folding , is then caused to abut the second end guide 42 b to form a loop on the trailing edge side of the paper p . then , the paper p is transported to the second folding roll 42 c and is subjected to the second folding ( forming the second fold f 2 of the envelope c - folding illustrated in fig3 ) in the second nip position b ( refer to fig6 ). then , the paper p subjected to the envelope c - folding after the end of the second folding is guided by the second switching gate 46 and is output to the paper accommodating device 48 ( refer to fig5 ) as illustrated in fig9 c . an envelope c - folding operation for one sheet of the paper p is completed as described heretofore . fig1 a to 10c are diagrams illustrating progress in the z - folding . it is assumed that a job in which the paper p of a size larger than that in the case of the envelope z - folding and the envelope c - folding described above ( for example , jis a3 sef ), after the image im is formed thereon by the image forming apparatus 1 , is subjected to the z - folding by the folding unit 4 and is then output to the post - processing unit 5 ( refer to fig1 ) is performed . first , in the same manner as described in the case of the envelope z - folding and the envelope c - folding above , a toner image ( the image im ) is transferred and fixed to the paper p in the image forming apparatus 1 , and the paper p after image formation is relayed to the post - processing apparatus 2 . the paper p after image formation that is output from the image forming apparatus 1 is relayed to the folding unit 4 via the transport unit 3 after , if necessary , curl thereof is rectified by the decurler 3 a . the first switching gate 45 is provided in the folding unit 4 ( folding functional unit 4 a ) in a position permitting movement of the paper p to the detour transport path r 2 as illustrated in fig5 in accordance with a job specification “ z - folding mode ” received through the user interface unit 13 . the first end guide 41 b and the second end guide 42 b are moved to target positions thereof , and the second switching gate 46 is provided in a position , different from that in the envelope folding ( the envelope z - folding and the envelope c - folding ), permitting movement of the paper p to the return detour transport path r 23 . particularly , the positions of the first end guide 41 b and the second end guide 42 b are adjusted differently from the envelope folding in such a manner that l 3 and l 6 illustrated in fig6 are equal to ¼ of the size of the target paper ( jis a3 sef in the present example ). the paper p , in this state , is transported from the straight transport path r 1 to the detour transport path r 2 and is guided to the first folding mechanism 41 via the skew correction roller 41 a , and the leading edge of the paper p abuts the first end guide 41 b and stops as illustrated in fig1 a . next , the paper p is subjected to skew correction by using the skew correction roller 41 a and is then subjected to the first folding ( forming the first fold f 1 of the z - folding illustrated in fig3 ) by the first folding roll 41 c in the first nip position a ( refer to fig6 ). next , as illustrated in fig1 b , the paper p after the end of the first folding is guided to the second folding mechanism 42 . the paper p is transported through the push roller 42 a , and the leading edge thereof , the first fold f 1 made by the first folding , is then caused to abut the second end guide 42 b to form a loop on the central side of the paper p . then , the paper p is transported to the second folding roll 42 c and is subjected to the second folding ( forming the second fold f 2 of the z - folding illustrated in fig3 ) in the second nip position b ( refer to fig6 ). then , the paper p subjected to the z - folding after the end of the second folding is transported by the second switching gate 46 to the return detour transport path r 23 and is then transported to the post - processing unit 5 ( refer to fig1 ) as illustrated in fig1 c . a z - folding operation for one sheet of the paper p is completed as described heretofore . fig1 a and 11b are diagrams illustrating image output modes performed by the image forming apparatus 1 . fig1 a illustrates the paper p that is output from the image forming apparatus 1 in “ regular output mode ”, and fig1 b illustrates the paper p that is output from the image forming apparatus 1 in “ reversed output mode ”. description will be provided with reference to fig1 and fig2 as well . first , in the regular output mode , the image im is transferred by the second transfer unit 10 d to a front side pa of the paper p that is supplied from the paper supply unit 12 and is transported into the supply path ra . the paper p that is subjected to heating ( fixing ) by passing through the second transfer unit 10 d and the fixing unit 10 e and is relayed from the output path rb to the paper relaying unit 15 . the regular output mode forms the image im on the front side pa of the paper and does not form the image im on a rear side pb of the paper p as illustrated in fig1 a . in this case , an upper side ts of the paper p that is output from the paper relaying unit 15 in a transport direction x corresponds to the rear side pb ( where the image im is not formed ), and a lower side us of the paper p corresponds to the front side pa ( where the image im is formed ). downcurl that is illustrated as “ convex upward ” in fig1 a is generated in the regular output mode in the paper p output from the image forming unit 10 through the paper relaying unit 15 when the paper p passes through the fixing unit 10 e ( heated ) for the first time and then passes through a curved part of the output path rb . as a result , the rear side pb ( where the image im is not formed ) corresponding to the upper side ts of the paper p supplied from the image forming apparatus 1 to the transport unit 3 becomes convex , and the front side pa ( where the image im is formed ) corresponding to the lower side us of the paper p becomes concave . in the reversed output mode , the image im is transferred by the second transfer unit 10 d to the front side pa of the paper p that is supplied from the paper supply unit 12 and is transported into the supply path ra in the same manner as the regular output mode . the difference from the regular output mode is that in the reversed output mode , the paper p on which the image im is heated ( fixed ) by passing through the second transfer unit 10 d and the fixing unit 10 e is guided to the output path rb , is then guided from the output path rb to the reversal path rc by reversing the transport direction , and is guided from the reversal path rc to the supply path ra again . accordingly , the paper p that is brought again into the supply path ra is reversed inside out in addition to the transport direction . next , the second transfer unit 10 d does not transfer the image im to the rear side pb of the paper p transported again into the supply path ra and passes the paper p . the paper p that is subjected to heating again by passing through the second transfer unit 10 d and the fixing unit 10 e and is relayed from the output path rb to the paper relaying unit 15 . the reversed output mode , in the same manner as the regular output mode , forms the image im on the front side pa of the paper and does not form the image im on the rear side pb of the paper p . in this case , the upper side ts of the paper p that is output from the paper relaying unit 15 in the transport direction x corresponds to the front side pa ( where the image im is formed ), and the lower side us of the paper p corresponds to the rear side pb ( where the image im is not formed ). downcurl that is illustrated as “ convex upward ” is generated in the reversed output mode in the paper p output from the image forming unit 10 through the paper relaying unit 15 when the paper p passes through the fixing unit 10 e ( heated ) for the second time and then passes through a curved part of the output path rb . as a result , the front side pa ( where the image im is formed ) corresponding to the upper side ts of the paper p supplied from the image forming apparatus 1 to the transport unit 3 becomes convex , and the rear side pb ( where the image im is not formed ) corresponding to the lower side us of the paper p becomes concave . the lower side us of the paper p corresponds to one side of the sheet , and the upper side ts of the paper p corresponds to the other side of the sheet in the present exemplary embodiment . the lower side us of the paper p corresponds to a first facing side , and the upper side ts of the paper p corresponds to a second facing side in the present exemplary embodiment . in the image forming system 500 of the present exemplary embodiment , the image output mode in the image forming apparatus 1 and a setting for the decurler 3 a in the transport unit 3 are configured to be switched according to the folding mode set in the image forming system 500 on the basis of a job specification received through the user interface unit 13 . fig1 is a diagram illustrating a relationship among the folding mode , the image output mode , and the setting for the decurler 3 a in the present exemplary embodiment . first , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the decurler 3 a in the transport unit 3 is set to “ maintain a curl direction ( convex upward )” in a case where “ folding is not performed ” of the paper p by the folding unit 4 ( folding mode : no ). the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the decurler 3 a in the transport unit 3 is set to “ maintain a curl direction ( convex upward )” in a case where “ envelope z - folding ” is performed of the paper p by the folding unit 4 ( folding mode : envelope z - folding ). the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the decurler 3 a in the transport unit 3 is set to “ change a curl direction ( convex downward )” in a case where “ outer image c - folding ” is performed of the paper p by the folding unit 4 ( folding mode : outer image c - folding ( envelope c - folding )). the image output mode of the image forming apparatus 1 is set to “ reversed output mode ”, and the decurler 3 a in the transport unit 3 is set to “ change a curl direction ( convex downward )” in a case where “ inner image c - folding ” is performed of the paper p by the folding unit 4 ( folding mode : inner image c - folding ( envelope c - folding )). the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the decurler 3 a in the transport unit 3 is set to “ maintain a curl direction ( convex upward )” in a case where “ z - folding ” is performed of the paper p by the folding unit 4 ( folding mode : z - folding ). relationship between folding mode and curl of paper a relationship between the folding mode and curl of the paper p in the image forming system 500 of the present exemplary embodiment will be described . four folding modes ( the envelope z - folding mode , an outer image c - folding mode ( the envelope c - folding ), an inner image c - folding mode ( the envelope c - folding ), and the z - folding mode ) may be selectively performed in the image forming system 500 of the present exemplary embodiment . fig1 a to 13c are diagrams illustrating a relationship between the paper p subjected to folding and curl of the paper p in the envelope z - folding mode . fig1 a illustrates the paper p at the intake thereof in which the paper p is transported in the transport direction x and passes through the intake roller 43 provided on the straight transport path r 1 of the folding functional unit 4 a . fig1 b illustrates the paper p after first fold formation in which the first fold f 1 is formed by passing the paper p through the first folding mechanism 41 provided on the detour transport path r 2 of the folding functional unit 4 a . fig1 c illustrates the paper p after second fold formation in which the second fold f 2 is formed by passing the paper p through the second folding mechanism 42 provided on the detour transport path r 2 of the folding functional unit 4 a . the relationships illustrated in fig1 a to 13c also apply to fig1 a to 14c to fig1 a to 16c described below . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the decurler 3 a in the transport unit 3 is set to “ maintain a curl direction ( convex upward )” in the envelope z - folding mode . thus , the paper p at the intake thereof is such that the upper side ts of the paper p corresponds to the rear side pb ( where the image im is not formed ) and that the lower side us of the paper p corresponds to the front side pa ( where the image im is formed ) in accordance with “ regular output mode ” as illustrated in fig1 a ( refer to fig1 a as well ). the paper p at the intake thereof is “ convex upward ”, that is , the rear side pb ( where the image im is not formed ) corresponding to the upper side ts being convex and the front side pa ( where the image im is formed ) corresponding to the lower side us being concave , in accordance with the setting “ maintain a curl direction ” for the decurler 3 a as illustrated in fig1 a . next , the paper p enters the first nip position a in such a manner that the concave front side pa comes in contact with the first folding roll 41 c provided in the first folding mechanism 41 . as a result , the first fold f 1 that looks like a mountain fold when seen from the front side pa side of the paper p ( a valley fold when seen from the rear side pb side of the paper p ) is formed in a part of the paper p passing through the first nip position a on the leading edge side of the paper p from the center as illustrated in fig1 b . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the outer side of the folded paper p after the first fold f 1 is formed in the paper p . then , the paper p in which the first fold f 1 is formed enters the second nip position b in such a manner that the convex rear side pb comes in contact with the second folding roll 42 c provided in the second folding mechanism 42 . as a result , the second fold f 2 that looks like a mountain fold when seen from the rear side pb side of the paper p ( a valley fold when seen from the front side pa side of the paper p ) is formed in a part of the paper p passing through the second nip position b on the trailing edge side of the paper p from the center as illustrated in fig1 c . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the outer side ( around the first fold f 1 ) and the inner side ( around the second fold f 2 ) of the folded paper p after the second fold f 2 is formed in the paper p . fig1 a to 14c are diagrams illustrating a relationship between the paper p subjected to folding and curl of the paper p in the outer image c - folding mode . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the decurler 3 a in the transport unit 3 is set to “ change a curl direction ( convex downward )” in the outer image c - folding mode . thus , the paper p at the intake thereof is such that the upper side ts of the paper p corresponds to the rear side pb ( where the image im is not formed ) and that the lower side us of the paper p corresponds to the front side pa ( where the image im is formed ) in accordance with “ regular output mode ” as illustrated in fig1 a ( refer to fig1 a as well ). the paper p at the intake thereof is “ convex downward ”, that is , the rear side pb ( where the image im is not formed ) corresponding to the upper side ts being concave and the front side pa ( where the image im is formed ) corresponding to the lower side us being convex , in accordance with the setting “ change a curl direction ” for the decurler 3 a as illustrated in fig1 a . what is common to the outer image c - folding mode and the envelope z - folding mode ( refer to fig1 a to 13c ) is that the front side pa of the paper p at the intake thereof corresponds to the lower side us . the difference from the envelope z - folding mode is that curl of the paper p at the intake thereof is “ convex downward ” in the outer image c - folding mode , while curl of the paper p at the intake thereof is “ convex upward ” in the envelope z - folding mode . next , the paper p enters the first nip position a in such a manner that the convex front side pa comes in contact with the first folding roll 41 c provided in the first folding mechanism 41 . as a result , the first fold f 1 that looks like a mountain fold when seen from the front side pa side of the paper p ( a valley fold when seen from the rear side pb side of the paper p ) is formed in a part of the paper p passing through the first nip position a on the trailing edge side of the paper p from the center as illustrated in fig1 b . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the outer side of the folded paper p after the first fold f 1 is formed in the paper p . then , the paper p in which the first fold f 1 is formed enters the second nip position b in such a manner that the convex front side pa comes in contact with the second folding roll 42 c provided in the second folding mechanism 42 . as a result , the second fold f 2 that looks like a mountain fold when seen from the front side pa side of the paper p ( a valley fold when seen from the rear side pb side of the paper p ) is formed in a part of the paper p passing through the second nip position b on the leading edge side of the paper p from the center as illustrated in fig1 c . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the outer side ( mountain fold side ) of the paper p folded at two places after the second fold f 2 is formed in the paper p . fig1 a to 15c are diagrams illustrating a relationship between the paper p subjected to folding and curl of the paper p in the inner image c - folding mode . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ reversed output mode ”, and the decurler 3 a in the transport unit 3 is set to “ change a curl direction ( convex downward )” in the inner image c - folding mode . thus , the paper p at the intake thereof is such that the upper side ts of the paper p corresponds to the front side pa ( where the image im is formed ) and that the lower side us of the paper p corresponds to the rear side pb ( where the image im is not formed ) in accordance with “ reversed output mode ” as illustrated in fig1 a ( refer to fig1 b as well ). the paper p at the intake thereof is “ convex downward ”, that is , the front side pa ( where the image im is formed ) corresponding to the upper side ts being concave and the rear side pb ( where the image im is not formed ) corresponding to the lower side us being convex , in accordance with the setting “ change a curl direction ” for the decurler 3 a as illustrated in fig1 a . the difference from the envelope z - folding mode is that the front side pa of the paper p at the intake thereof corresponds to the upper side ts in the inner image c - folding mode , while the front side pa of the paper p at the intake thereof corresponds to the lower side us in the envelope z - folding mode ( refer to fig1 a to 13c ). the difference from the envelope z - folding mode is that curl of the paper p at the intake thereof is “ convex downward ” in the inner image c - folding mode , while curl of the paper p at the intake thereof is “ convex upward ” in the envelope z - folding mode . the difference from the outer image c - folding mode is that the front side pa of the paper p at the intake thereof corresponds to the upper side ts in the inner image c - folding mode , while the front side pa of the paper p at the intake thereof corresponds to the lower side us in the outer image c - folding mode ( refer to fig1 a to 14c ). what is common to the inner image c - folding mode and the outer image c - folding mode is that curl of the paper p at the intake thereof is “ convex downward ”. next , the paper p enters the first nip position a in such a manner that the convex rear side pb comes in contact with the first folding roll 41 c provided in the first folding mechanism 41 . as a result , the first fold f 1 that looks like a mountain fold when seen from the rear side pb side of the paper p ( a valley fold when seen from the front side pa side of the paper p ) is formed in a part of the paper p passing through the first nip position a on the trailing edge side of the paper p from the center as illustrated in fig1 b . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the inner side of the folded paper p after the first fold f 1 is formed in the paper p . then , the paper p in which the first fold f 1 is formed enters the second nip position b in such a manner that the convex rear side pb comes in contact with the second folding roll 42 c provided in the second folding mechanism 42 . as a result , the second fold f 2 that looks like a mountain fold when seen from the rear side pb side of the paper p ( a valley fold when seen from the front side pa side of the paper p ) is formed in a part of the paper p passing through the second nip position b on the leading edge side of the paper p from the center as illustrated in fig1 c . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the inner side ( valley fold side ) of the paper p folded at two places after the second fold f 2 is formed in the paper p . fig1 a to 16c are diagrams illustrating a relationship between the paper p subjected to folding and curl of the paper p in the z - folding mode . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the decurler 3 a in the transport unit 3 is set to “ maintain a curl direction ( convex upward )” in the z - folding mode . thus , the paper p at the intake thereof is such that the upper side ts of the paper p corresponds to the rear side pb ( where the image im is not formed ) and that the lower side us of the paper p corresponds to the front side pa ( where the image im is formed ) in accordance with “ regular output mode ” as illustrated in fig1 a ( refer to fig1 a as well ). the paper p at the intake thereof is “ convex upward ”, that is , the rear side pb ( where the image im is not formed ) corresponding to the upper side ts being convex and the front side pa ( where the image im is formed ) corresponding to the lower side us being concave , in accordance with the setting “ maintain a curl direction ” for the decurler 3 a as illustrated in fig1 a . what is common to the z - folding mode and the envelope z - folding mode ( refer to fig1 a to 13c ) is that the front side pa of the paper p at the intake thereof corresponds to the lower side us . what is common to the z - folding mode and the envelope z - folding mode is that curl of the paper p at the intake thereof is “ convex upward ”. what is common to the z - folding mode and the outer image c - folding mode ( refer to fig1 a to 14c ) is that the front side pa of the paper p at the intake thereof corresponds to the lower side us . the difference from the outer image c - folding mode is that curl of the paper p at the intake thereof is “ convex upward ” in the z - folding mode , while curl of the paper p at the intake thereof is “ convex downward ” in the outer image c - folding mode . the difference from the inner image c - folding mode is that the front side pa of the paper p at the intake thereof corresponds to the lower side us in the z - folding mode , while the front side pa of the paper p at the intake thereof corresponds to the upper side ts in the inner image c - folding mode ( refer to fig1 a to 15c ). the difference from the inner image c - folding mode is that curl of the paper p at the intake thereof is “ convex upward ” in the z - folding mode , while curl of the paper p at the intake thereof is “ convex downward ” in the inner image c - folding mode . next , the paper p enters the first nip position a in such a manner that the concave front side pa comes in contact with the first folding roll 41 c provided in the first folding mechanism 41 . as a result , the first fold f 1 that looks like a mountain fold when seen from the front side pa side of the paper p ( a valley fold when seen from the rear side pb side of the paper p ) is formed in a part of the paper p passing through the first nip position a on the leading edge side of the paper p from the center as illustrated in fig1 b . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the outer side of the folded paper p after the first fold f 1 is formed in the paper p . then , the paper p in which the first fold f 1 is formed enters the second nip position b in such a manner that the convex rear side pb comes in contact with the second folding roll 42 c provided in the second folding mechanism 42 . as a result , the second fold f 2 that looks like a mountain fold when seen from the rear side pb side of the paper p ( a valley fold when seen from the front side pa side of the paper p ) is formed in a central part of the paper p passing through the second nip position b as illustrated in fig1 c . at this point , since the image im is formed on the front side pa of the paper p , the image im is positioned on the outer side ( around the first fold f 1 ) of the paper p folded at two places and on the inner side ( around the second fold f 2 ) of the valley - folded paper p after the second fold f 2 is formed in the paper p . the reason why the curl direction of the paper p is changed from “ convex upward ” to “ convex downward ” before the envelope c - folding ( the outer image c - folding and the inner image c - folding ) is performed of the paper p in the present exemplary embodiment will be described . fig1 a and 17b are diagrams illustrating a relationship between curl of the paper p and the paper p after the envelope c - folding . fig1 a illustrates performing the envelope c - folding of the paper p that exhibits a shape of curl “ convex downward ” when brought into the folding unit 4 ( at the intake thereof to the intake roller 43 ), and fig1 b illustrates performing the envelope c - folding of the paper p that exhibits a shape of curl “ convex upward ” when brought into the folding unit 4 . first , as illustrated in fig1 a , when the folding functional unit 4 a performs the envelope c - folding of the paper p of which the lower side us is convex when brought in , a first fold f 1 side end portion ( trailing edge side ) of the paper p that is positioned on the inner side of the paper p from a second fold f 2 side end portion ( leading edge side ) of the paper p is curled in a direction approaching the second fold f 2 after the envelope c - folding . therefore , a folding defect called a dog ear is unlikely to arise in the first fold f 1 side end portion in a case where such a configuration is employed in forming the first fold f 1 and the second fold f 2 in the paper p using the folding functional unit 4 a . a jam of the paper p is also unlikely to arise by employing such a configuration in a case of forming the first fold f 1 and the second fold f 2 in the paper p using the folding functional unit 4 a . meanwhile , as illustrated in fig1 b , when the folding functional unit 4 a performs the envelope c - folding of the paper p of which the lower side us is concave when brought in , the first fold f 1 side end portion of the paper p that is positioned on the inner side of the paper p from the second fold f 2 side end portion of the paper p is curled in a direction receding from the second fold f 2 after the envelope c - folding . therefore , a folding defect called a dog ear is likely to arise in the first fold f 1 side end portion in a case where such a configuration is employed in forming the first fold f 1 and the second fold f 2 in the paper p using the folding functional unit 4 a . a jam of the paper p is also likely to arise by employing such a configuration in a case of forming the first fold f 1 and the second fold f 2 in the paper p using the folding functional unit 4 a . thus , the present exemplary embodiment employs the technique illustrated in fig1 a when the envelope c - folding is performed of the paper p . while the decurler 3 a is provided in the transport unit 3 in the present exemplary embodiment , the present invention is not limited thereto . the decurler 3 a may be incorporated in the folding unit 4 . while the paper p of which the curl is “ convex upward ” is subjected to rectification to render the curl “ convex downward ” using the decurler 3 a and is then subjected to the envelope c - folding in the present exemplary embodiment , the present invention is not limited thereto . for example , the paper p that is not curled may be subjected to rectification to have a curl “ convex downward ” by using the decurler 3 a and then may be subjected to the envelope c - folding . for example , the paper p of which the curl is previously “ convex downward ” may be subjected to rectification to render the curl further “ convex downward ” using the decurler 3 a and then may be subjected to the envelope c - folding . while the paper p of which the curl is “ convex upward ” is subjected to the envelope z - folding or the z - folding without being subjected to rectification of the curl by the decurler 3 a in the present exemplary embodiment , the present invention is not limited thereto . for example , the paper p of which the curl is “ convex upward ” may be subjected to rectification to render the curl “ convex downward ” using the decurler 3 a and then may be subjected to the envelope z - folding or the z - folding . the curl direction of the paper p is changed by using the decurler 3 a in the first exemplary embodiment . meanwhile , the present exemplary embodiment changes the curl direction of the paper p using an inverter 3 b ( refer to fig1 described later ) that reverses the paper p inside out . the same part as the first exemplary embodiment will be designated by the same reference sign and will not be described in detail in the present exemplary embodiment . fig1 is a diagram illustrating an overall configuration of the image forming system 500 to which the present exemplary embodiment is applied . the difference from the first exemplary embodiment is that the inverter 3 b instead of the decurler 3 a is provided in the transport unit 3 in the image forming system 500 . the inverter 3 b , one example of the output unit , the adjusting unit , and the reversing unit , reverses the transport direction of the paper p brought in from the image forming apparatus 1 to reverse the paper p inside out and transports the paper p to the folding unit 4 . in the image forming system 500 of the present exemplary embodiment , the image output mode in the image forming apparatus 1 and a setting for the inverter 3 b in the transport unit 3 are configured to be switched according to the folding mode set in the image forming system 500 on the basis of a job specification received through the user interface unit 13 . fig1 is a diagram illustrating a relationship among the folding mode , the image output mode , and the setting for the inverter 3 b in the present exemplary embodiment . first , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ not used ” in a case where “ folding is not performed ” of the paper p by the folding unit 4 ( folding mode : no ). the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ not used ” in a case where “ envelope z - folding ” is performed of the paper p by the folding unit 4 ( folding mode : envelope z - folding ). the image output mode of the image forming apparatus 1 is set to “ reversed output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ used ” in a case where “ outer image c - folding ” is performed of the paper p by the folding unit 4 ( folding mode : outer image c - folding ( envelope c - folding )). the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ used ” in a case where “ inner image c - folding ” is performed of the paper p by the folding unit 4 ( folding mode : inner image c - folding ( envelope c - folding )). the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ not used ” in a case where “ z - folding ” is performed of the paper p by the folding unit 4 ( folding mode : z - folding ). the present exemplary embodiment is different from the first exemplary embodiment in that the image output mode is set to “ reversed output ” in a case where the folding mode is “ outer image c - folding ” ( refer to fig1 ). the present exemplary embodiment is different from the first exemplary embodiment in that the image output mode is set to “ regular output ” in a case where the folding mode is “ inner image c - folding ” ( refer to fig1 ). a relationship between the folding mode and curl of the paper p in the image forming system 500 of the present exemplary embodiment will be described . four folding modes ( the envelope z - folding mode , an outer image c - folding mode ( the envelope c - folding ), an inner image c - folding mode ( the envelope c - folding ), and the z - folding mode ) may be selectively performed in the image forming system 500 of the present exemplary embodiment in the same manner as the first exemplary embodiment . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ not used ” in the envelope z - folding mode . thus , the paper p at the intake thereof to the folding unit 4 is such that the upper side ts of the paper p corresponds to the rear side pb ( where the image im is not formed ) and that the lower side us of the paper p corresponds to the front side pa ( where the image im is formed ) in accordance with “ regular output mode ” and the setting “ not used ” for the inverter 3 b ( refer to fig1 a as well ). the paper p at the intake thereof is “ convex upward ”, that is , the rear side pb ( where the image im is not formed ) corresponding to the upper side ts being convex and the front side pa ( where the image im is formed ) corresponding to the lower side us being concave , in accordance with the setting “ not used ” for the inverter 3 b . the state of the paper p at the intake thereof is the same as that of the first exemplary embodiment in the envelope z - folding mode ( refer to fig1 a ). therefore , in the envelope z - folding mode , the first fold f 1 illustrated in fig1 b is formed in the paper p illustrated in fig1 a , and then the second fold f 2 illustrated in fig1 c is formed in the paper p in the same manner as the first exemplary embodiment . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ reversed output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ used ” in the outer image c - folding mode . thus , the paper p at the intake thereof to the folding unit 4 is such that the upper side ts of the paper p corresponds to the rear side pb ( where the image im is not formed ) and that the lower side us of the paper p corresponds to the front side pa ( where the image im is formed ) in accordance with “ reversed output mode ” and the setting “ used ” for the inverter 3 b ( refer to fig1 a as well ). the paper p at the intake thereof is “ convex downward ”, that is , the rear side pb ( where the image im is not formed ) corresponding to the upper side ts being concave and the front side pa ( where the image im is formed ) corresponding to the lower side us being convex , in accordance with the setting “ used ” for the inverter 3 b . the state of the paper p at the intake thereof is the same as that of the first exemplary embodiment in the outer image c - folding mode as well ( refer to fig1 a ). therefore , in the outer image c - folding mode , the first fold f 1 illustrated in fig1 b is formed in the paper p illustrated in fig1 a , and then the second fold f 2 illustrated in fig1 c is formed in the paper p in the same manner as the first exemplary embodiment . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ used ” in the inner image c - folding mode . thus , the paper p at the intake thereof to the folding unit 4 is such that the upper side ts of the paper p corresponds to the front side pa ( where the image im is formed ) and that the lower side us of the paper p corresponds to the rear side pb ( where the image im is not formed ) in accordance with “ regular output mode ” and the setting “ used ” for the inverter 3 b ( refer to fig1 b as well ). the paper p at the intake thereof is “ convex downward ”, that is , the front side pa ( where the image im is formed ) corresponding to the upper side ts being concave and the rear side pb ( where the image im is not formed ) corresponding to the lower side us being convex , in accordance with the setting “ used ” for the inverter 3 b . the state of the paper p at the intake thereof is the same as that of the first exemplary embodiment in the inner image c - folding mode as well ( refer to fig1 a ). therefore , in the inner image c - folding mode , the first fold f 1 illustrated in fig1 b is formed in the paper p illustrated in fig1 a , and then the second fold f 2 illustrated in fig1 c is formed in the paper p in the same manner as the first exemplary embodiment . as illustrated in fig1 , the image output mode of the image forming apparatus 1 is set to “ regular output mode ”, and the inverter 3 b in the transport unit 3 is set to be “ not used ” in the z - folding mode . thus , the paper p at the intake thereof to the folding unit 4 is such that the upper side ts of the paper p corresponds to the rear side pb ( where the image im is not formed ) and that the lower side us of the paper p corresponds to the front side pa ( where the image im is formed ) in accordance with “ regular output mode ” and the setting “ not used ” for the inverter 3 b ( refer to fig1 a as well ). the paper p at the intake thereof is “ convex upward ”, that is , the rear side pb ( where the image im is not formed ) corresponding to the upper side ts being convex and the front side pa ( where the image im is formed ) corresponding to the lower side us being concave , in accordance with the setting “ not used ” for the inverter 3 b . the state of the paper p at the intake thereof is the same as that of the first exemplary embodiment in the z - folding mode as well ( refer to fig1 a ). therefore , in the z - folding mode , the first fold f 1 illustrated in fig1 b is formed in the paper p illustrated in fig1 a , and then the second fold f 2 illustrated in fig1 c is formed in the paper p in the same manner as the first exemplary embodiment . while the paper p is reversed inside out by reversing the transport direction of the paper p in the present exemplary embodiment , the present invention is not limited thereto . for example , a technique that reverses the paper p inside out , without reversing the transport direction of the paper p , by rotating the paper p around the transport direction of the paper p as an axis may be used . while the paper p of which the curl is “ convex upward ” is subjected to the envelope z - folding or the z - folding without being subjected to changing the curl by the inverter 3 b ( without being reversed inside out ) in the present exemplary embodiment , the present invention is not limited thereto . for example , the paper p of which the curl is “ convex upward ” may be subjected to changing to render the curl “ convex downward ” using the inverter 3 b and then may be subjected to the envelope z - folding or the z - folding . in this case , the image output mode at this point in time is required to be the reversed output mode , not the regular output mode . [ relationship between folding mode and state of paper in first and second exemplary embodiments ] a relationship between the folding mode and the state of the paper p in the first and second exemplary embodiments will be described . fig2 is a diagram illustrating a relationship between the folding mode and the state of the paper p in the first exemplary embodiment . fig2 is a diagram illustrating a relationship between the folding mode and the state of the paper p in the second exemplary embodiment . more specifically , fig2 and fig2 illustrate a relationship between the folding mode set in the image forming system 500 and the state of the paper p in each unit constituting the image forming system 500 in each folding mode . fig2 and fig2 do not describe a case where the folding mode is not set ( in a case of “ no ”). description will be provided of , in association with each folding mode , the side of the paper p on which the image im is formed by the image forming apparatus 1 ( referred to as “ image formed side ”), the state of the paper p when brought from the image forming apparatus 1 into the transport unit 3 ( referred to as “ when brought into the transport unit ”), the state of the paper p when brought from the transport unit 3 into the folding unit 4 ( referred to as “ when brought into the folding unit ”), the state of the paper p after the first folding by the first folding mechanism 41 ( referred to as “ first folding ”), and the state of the paper p after the second folding by the second folding mechanism 42 ( referred to as “ second folding ”). the state of the paper p “ when brought into the transport unit ” includes “ lower side ” and “ convex side ” of the paper p . the state of the paper p “ when brought into the folding unit ” includes “ lower side ” and “ convex side ” of the paper p . the state of the paper p in “ first folding ” includes the side on which the first fold f 1 is formed to be a mountain fold ( referred to as “ first fold formed side ( mountain fold )”) and the position in which the first fold f 1 is formed with respect to the second fold f 2 ( referred to as “ first fold formed position ”) when the transport direction of the paper p when brought into the folding unit 4 is used as a reference . the state of the paper p in “ second folding ” includes the side on which the second fold f 2 is formed to be a mountain fold ( referred to as “ second fold formed side ( mountain fold )”) and the position in which the second fold f 2 is formed with respect to the first fold f 1 ( referred to as “ second fold formed position ”) when the transport direction of the paper p when brought into the folding unit 4 is used as a reference . a “ side ” of the paper p is represented by “ front side pa ” or “ rear side pb ”, and “ fold formed position ” in the paper p is represented by “ leading edge side ” or “ trailing edge side ”. the “ leading edge side ” of the paper p illustrated in fig2 and fig2 refers to the leading edge side , in the transport direction , of the paper p that is brought from the transport unit 3 into the folding unit 4 ( the paper p that passes through the intake roller 43 provided on the straight transport path r 1 ). the “ trailing edge side ” of the paper p illustrated in fig2 and fig2 refers to the trailing edge side , in the transport direction , of the paper p that is brought from the transport unit 3 into the folding unit 4 ( the paper p that passes through the intake roller 43 ). therefore , the leading edge side of the paper p may have the transport direction reversed ( may be changed to the trailing edge side ) on the detour transport path r 2 . first , a relationship between the folding mode and the state of the paper p in the first exemplary embodiment will be described with reference to fig2 . the image formed side is “ front side pa ” in the envelope z - folding mode . the lower side of the paper p when brought into the transport unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the lower side of the paper p when brought into the folding unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the first fold formed side in the first folding is “ front side pa ”, and the first fold formed position is “ leading edge side ”. the second fold formed side in the second folding is “ rear side pb ”, and the second fold formed position is “ trailing edge side ”. the image formed side is “ front side pa ” in the outer image c - folding mode . the lower side of the paper p when brought into the transport unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the lower side of the paper p when brought into the folding unit is “ front side pa ”, and the convex side thereof is “ front side pa ”. the first fold formed side in the first folding is “ front side pa ”, and the first fold formed position is “ trailing edge side ”. the second fold formed side in the second folding is “ front side pa ”, and the second fold formed position is “ leading edge side ”. the image formed side is “ front side pa ” in the inner image c - folding mode . the lower side of the paper p when brought into the transport unit is “ rear side pb ”, and the convex side thereof is “ front side pa ”. the lower side of the paper p when brought into the folding unit is “ rear side pb ”, and the convex side thereof is “ rear side pb ”. the first fold formed side in the first folding is “ rear side pb ”, and the first fold formed position is “ trailing edge side ”. the second fold formed side in the second folding is “ rear side pb ”, and the second fold formed position is “ leading edge side ”. the image formed side is “ front side pa ” in the z - folding mode . the lower side of the paper p when brought into the transport unit is “ front side pa ”, and the convex side thereof is “ front side pa ”. the lower side of the paper p when brought into the folding unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the first fold formed side in the first folding is “ front side . pa ”, and the first fold formed position is “ leading edge side ”. the second fold formed side in the second folding is “ rear side pb ”, and the second fold formed position is “ trailing edge side ”. next , a relationship between the folding mode and the state of the paper in the second exemplary embodiment will be described with reference to fig2 . the image formed side is “ front side pa ” in the envelope z - folding mode . the lower side of the paper p when brought into the transport unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the lower side of the paper p when brought into the folding unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the first fold formed side in the first folding is “ front side pa ”, and the first fold formed position is “ leading edge side ”. the second fold formed side in the second folding is “ rear side pb ”, and the second fold formed position is “ trailing edge side ”. the image formed side is “ front side pa ” in the outer image c - folding mode . the lower side of the paper p when brought into the transport unit is “ rear side pb ”, and the convex side thereof is “ front side pa ”. the lower side of the paper p when brought into the folding unit is “ front side pa ”, and the convex side thereof is “ front side pa ”. the first fold formed side in the first folding is “ front side pa ”, and the first fold formed position is “ trailing edge side ”. the second fold formed side in the second folding is “ front side pa ”, and the second fold formed position is “ leading edge side ”. the image formed side is “ front side pa ” in the inner image c - folding mode . the lower side of the paper p when brought into the transport unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the lower side of the paper p when brought into the folding unit is “ rear side pb ”, and the convex side thereof is “ rear side pb ”. the first fold formed side in the first folding is “ rear side pb ”, and the first fold formed position is “ trailing edge side ”. the second fold formed side in the second folding is “ rear side pb ”, and the second fold formed position is “ leading edge side ”. the image formed side is “ front side pa ” in the z - folding mode . the lower side of the paper p when brought into the transport unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the lower side of the paper p when brought into the folding unit is “ front side pa ”, and the convex side thereof is “ rear side pb ”. the first fold formed side in the first folding is “ front side pa ”, and the first fold formed position is “ leading edge side ”. the second fold formed side in the second folding is “ rear side pb ”, and the second fold formed position is “ trailing edge side ”. next , the first exemplary embodiment and the second exemplary embodiment will be compared with each other with reference to fig2 and fig2 . first , the state of the paper p in each unit in the envelope z - folding mode is common to the first exemplary embodiment and the second exemplary embodiment . the state of the paper p in each unit in the z - folding mode is also common to the first exemplary embodiment and the second exemplary embodiment . the state of the paper p in each unit in the outer image c - folding mode is common to the first exemplary embodiment and the second exemplary embodiment except for when brought into the transport unit . the state of the paper p in each unit in the inner image c - folding mode is common to the first exemplary embodiment and the second exemplary embodiment except for when brought into the transport unit . the reason why the state of the paper p in the outer image c - folding mode and in the inner image c - folding mode is different between the first exemplary embodiment and the second exemplary embodiment is described as follows . first , in the outer image c - folding mode and in the inner image c - folding mode of the first exemplary embodiment , the decurler 3 a provided in the transport unit 3 is used to change the curl direction of the paper p without reversing the paper p inside out . meanwhile , in the outer image c - folding mode and in the inner image c - folding mode of the second exemplary embodiment , the inverter 3 b provided in the transport unit 3 is used to change the curl direction of the paper p by reversing the paper p inside out . thus , the image forming apparatus 1 of the first exemplary embodiment , for an outer image tri - folding mode that requires changing the curl direction , supplies the paper p to the transport unit 3 in the regular output mode and , for an inner image tri - folding mode that also requires changing the curl direction , supplies the paper p to the transport unit 3 in the reversed output mode . meanwhile , the image forming apparatus 1 of the second exemplary embodiment , for the outer image tri - folding mode that requires changing the curl direction , supplies the paper p to the transport unit 3 in the reversed output mode which is opposite to that of the first exemplary embodiment and , for the inner image tri - folding mode that also requires changing the curl direction , supplies the paper p to the transport unit 3 in the regular output mode which is opposite to that of the first exemplary embodiment . the foregoing description of the embodiments of the present invention has been provided for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise forms disclosed . obviously , many modifications and variations will be apparent to practitioners skilled in the art . the embodiments were chosen and described in order to best explain the principles of the invention and its practical applications , thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated . it is intended that the scope of the invention defined by the following claims and their equivalents .