an example of the invention will be explained in detail as follows , referring to fig1 - fig6 ( d ). incidentally , although the present example is an image forming apparatus of an electrophotographic type , the invention is not limited to this but is related to an image forming apparatus having the structure for conveying a recording sheet to the recording position such as an ink jet system . fig1 is a functional block diagram showing an example of an electric structure of primary portions of an image forming apparatus in an example of the invention . fig2 is a block diagram showing a structure of the whole of an image forming apparatus in an embodiment of the invention . fig3 is a sectional side view showing the mechanical structure of an image forming apparatus in an example of the invention . the whole of an image forming apparatus will be explained as follows , referring to fig2 and fig3 . in fig2 the numeral 10 is an automatic document conveyance means ( hereinafter referred to as adf ) which is a means to convey a document to read it . the numeral 20 is an image reading section where a document is read through optical scanning and image data are generated . the numeral 50 is an image forming section where the image data are recorded on recording medium ( hereinafter referred to as a transfer sheet ) p through an electrophotographic system . the numeral 100 is cpu serving as a control means which controls each section , and cpu controls each section in accordance with an unillustrated processing program . the numeral 110 is an operation section where there are conducted various operations such as selection and designation for the number of copies , enlargement ratio and reduction ratio , and transfer sheet sizes ( a4 , a4r , b5 , b5r ), and start of copying , and results of the operation are transmitted to cpu 100 . the numeral 120 is an adf control section which controls document conveyance conducted by adf 10 , and it controls the state of document conveyance in accordance with a directive of cpu 100 . the numeral 130 is an image memory acting as a memory means which stores compressed image data , and it is composed of a memory capable of storing ( housing ) and reading ( outputting ). the numeral 140 is a read image processing section which conducts processing of image data obtained by image reading section 20 through its reading . the numeral 150 is a compression / expansion circuit which conducts compression processing for writing image data processed by the read image processing section into image memory 130 and expansion processing for expanding the compressed image data which have been read out of the image memory 130 . the numeral 160 is a recorded image processing section which processes , for image forming , the image data which have been expanded and have been returned to original state . the numeral 170 is a drive control means which controls the state of driving of a driving source which will be described later , and conducts conveyance control for a transfer sheet , while , 180 represents first and second driving sources ( sma , smb ) which drive conveyance rollers for conveying a transfer sheet based on the control conducted by the drive control means 170 . fig3 is a structure diagram showing the sectional structure of an image forming apparatus used in an embodiment of the invention . in fig3 on document placing section 11 of adf 10 capable of conveying both sides of a document , there are placed plural documents “ d ” each being placed with its surface representing the first page facing upward . the first document fed out through roller 12 a and roller 12 b is conveyed through roller 13 . in this case , a document surface of document “ d ” is illuminated by light source 23 , and its reflected light forms an image on a light - receiving surface of ccd 28 which serves as a photoelectric transfer means through mirrors 24 , 25 and 26 and through image forming optical system 27 . image reading section 20 is composed of an optical system having therein light source 23 , mirrors 24 , 25 and 26 , image forming optical system 27 and ccd 28 and of an unillustrated optical system driving means . in fig3 when document “ d ” which is placed on platen glass 21 with its surface to be read facing downward is read , the optical system is moved along the platen glass 21 for reading . when reading document “ d ” while conveying it , the reading is conducted under the condition that light source 23 and mirror 24 are fixed under second platen glass 22 . image data of document “ d ” obtained through the reading are sent from ccd 28 to unillustrated read image processing section 140 . when both sides of document “ d ” are conveyed by adf 10 , the first page of the document “ d ” is read , and then , the document is reversed and conveyed again to roller 13 through reversing roller 14 so that an image on the reverse side of the document is read by image reading section 20 , and image data obtained through reading are sent to read image processing section 140 . the document “ d ” from which an image on its surface and that on its reverse side have been read in the aforesaid manner is reversed again by reversing roller 14 , and is ejected to be stacked on sheet ejection tray 16 , with the surface of the document facing downward . image data obtained by image reading section 20 through its reading in the aforesaid manner are subjected to prescribed image processing at read image processing section 140 , then are subjected to compression conducted by compression / expansion circuit 150 , and are stored in image memory 130 . on the other hand , transfer sheet “ p ” is fed out of sheet - feeding cassette 30 wherein transfer sheets are stacked by conveyance roller 181 , and is conveyed to image forming section 50 . further , transfer sheet “ p ” is fed out of bypass feeding tray 31 by conveyance roller 183 , and is conveyed to image forming section 50 . transfer sheet “ p ” conveyed to image forming section 50 is fed to the surface of photoreceptor drum 51 representing an image carrier , after being synchronized in terms of timing by registration roller 185 located in the vicinity of an entrance of the image forming section . namely , there are provided a first conveyance path extending from sheet feeding cassette 30 to registration roller 185 and a second conveyance path extending from bypass feeding tray 31 to registration roller 185 . image data are inputted in image writing section 40 from recorded image processing section 160 , and a laser beam corresponding to the image data is applied to photoreceptor drum 51 from a laser diode in the image writing section 40 to form an electrostatic latent image . when this electrostatic latent image is developed by developing section 53 , a toner image is formed on the photoreceptor drum 51 . this toner image is transferred onto transfer sheet “ p ” by transfer section 54 representing an image recording means located under the photoreceptor drum 51 . then , transfer sheet “ p ” which is in contact with the photoreceptor drum 51 is separated therefrom by separation section 55 . transfer sheet “ p ” separated from the photoreceptor drum 51 is conveyed by conveyance mechanism 58 and enters fixing section 59 where the toner image is fixed by heat and pressure . thus , an image is formed on transfer sheet “ p ” . incidentally , when sheet - reversing and sheet - refeeding is needed in the case of double - sided image forming , the transfer sheet “ p ” on which a toner image has been fixed is conveyed downward through guide 61 , and enters reversing section 63 . then , the transfer sheet “ p ” which has entered reversing section 63 is fed out again by reversing conveyance roller 65 , and is sent to image forming section 50 again through reversing conveyance path 64 . in the image forming section 50 where image forming for one side of the document “ d ” has been completed , toner remaining on and sticking to photoreceptor 51 is removed by cleaning section 56 to be ready for the succeeding image forming . under this state , the other surface ( surface on which an image has not been formed yet ) of the transfer sheet “ p ” is conveyed in image forming section 50 and an image is formed . the transfer sheet “ p ” separated from photoreceptor drum 51 at separation section 55 enters fixing section 59 again through conveyance mechanism 58 to be fixed . transfer sheet “ p ” on which image forming on the reverse side and on the surface has been finished , or transfer sheet “ p ” on which image forming on one side has been finished is ejected out of the apparatus . details of the circumference of drive control means 170 will be explained now , referring to fig1 . as shown in fig1 drive control means 170 is equipped with oscillator 171 which generates basic clock having prescribed frequency , frequency divider 172 which divides basic clock according to setting from cpu 100 , frequency divider 173 which divides basic clock according to setting of dividing ratio from cpu 100 , driver 174 which generates driving pulses for driving stepping motor sma from frequency division signals divided by the frequency divider 172 , and driver 175 which generates driving pulses for driving stepping motor smb from frequency division signals divided by the frequency divider 173 . incidentally , in the present embodiment , improvement of synchronization and cost reduction are realized by only one oscillator 171 , which is preferable . however , it is also possible that each of driver 174 and driver 175 is provided with each oscillator . further , loop forming roller 182 and registration roller 185 constitute conveyance rollers for conveying by changing speed under the condition of interposing one transfer sheet . in this case , the registration roller 185 is driven by the first driving source ( stepping motor sma ), while the loop forming roller 182 is driven by the second driving source ( stepping motor smb ). for the purpose of realizing an independent driving source as in the foregoing , frequency divider 172 and frequency divider 173 of drive control means 170 are structured so that frequency dividing ratio of each of them is established from cpu 100 independently . incidentally , transmission section 190 is a means which can distribute ( switch ) turning force of stepping motor smb to loop forming roller 182 closer to the first conveyance path and to loop forming roller 184 closer to the second conveyance roller . it is therefore a driving mechanism for supplying turning force to either one of conveyance rollers depending on a direction of rotation ( regular direction / reverse direction ) of stepping motor smb . as this driving mechanism , it is effective , for example , that each of conveyance rollers is provided with a one - way clutch which is opposite in terms of direction to that provided on the other conveyance roller . fig4 ( a )- 4 ( d ) represent characteristic diagrams showing the speed of rotation of stepping motor smb ( fig4 ( a )), the conveyance speed for a transfer sheet by loop forming roller 182 ( peripheral speed of loop forming roller 182 : fig4 ( b )), the conveyance speed for a transfer sheet by loop forming roller 184 ( peripheral speed of loop forming roller 184 fig4 ( c )) and the speed of rotation of stepping motor smb ( fig4 ( d )). incidentally , the speed of rotation of stepping motor smb ( fig4 ( d )) corresponds to the conveyance speed for a transfer sheet by registration roller 185 . in this case , during a period of a in fig4 ( a )- 4 ( d ), stepping motor smb rotates in the cw ( clockwise ) direction , and a rotation in this cw direction is transmitted to loop forming roller 182 through transmission section 190 . due to this , the conveyance speed of the first conveyance speed ( 2s ) is obtained by loop forming roller 182 . in this period of a , transfer sheet “ p ” is made to hit registration roller 185 on the part of the first conveyance path , and the transfer sheet “ p ” is conveyed until it forms a loop . during a period of b in fig4 ( a )- 4 ( d ), stepping motor smb rotates in the cw ( clockwise ) direction , and a rotation in this cw direction is transmitted to loop forming roller 182 through transmission section 190 . due to this , conveyance speed ( s ) is obtained by loop forming roller 182 . in this period of b , stepping motor sma is also rotating , and the second conveyance speed ( s ) is obtained by registration roller 185 . namely , in this period of b , transfer sheet “ p ” is conveyed at constant conveyance speed ( s ) while it is interposed between loop forming roller 182 and registration roller 185 . in the meantime , the first driving source which drives registration roller 185 and the second driving source which drives loop forming roller 182 are independent of each other , and when conveying transfer sheet “ p ” to photoreceptor 51 , there is conducted control so that the registration roller 185 is driven by the first driving source and the loop forming roller 182 is driven by the second driving source independently of each other . namely , since switching of the conveyance speed by the loop forming roller 182 is realized by control of the second driving source , it is not necessary to use a conventional magnetic clutch , and neither dispersion nor delay of the response time is caused . further , since the loop forming roller 182 is driven by the stepping motor smb , switching between conveyance speeds ( 2s and s ) is easy , and it is possible to control accurately . further , since the first driving source and the second driving source are controlled separately in drive control means 170 , it is possible to control to switch the conveyance speed for loop forming roller 182 independently of registration roller 185 . since the control to made the first conveyance speed to be different from the second conveyance speed is conducted on the part of loop forming roller 182 , it is possible to control to switch the conveyance speed for loop forming roller 182 independently of registration roller 185 . since the first conveyance speed ( 2s ) at which the transfer sheet “ p ” hits registration roller 185 to form a loop is controlled to be higher than the second conveyance speed ( s ) for conveying from registration roller 185 to photoreceptor 51 , quick conveyance of a transfer sheet can be realized , and image forming at high speed is possible . when a stepping motor is used as a driving source not only on the part of the loop forming roller 182 but also on the part of the registration roller 185 , it is easy to maintain and control the conveyance speed , and it is possible to maintain the state of synchronization with the loop forming roller 182 when conveying transfer sheet “ p ” to photoreceptor 51 . during a period of c in fig4 ( a )- 4 ( d ), stepping motor smb rotates in the ccw ( counterclockwise ) direction , and a rotation in this ccw direction is transmitted to loop forming roller 184 through transmission section 190 . due to this , the conveyance speed of the first conveyance speed ( 2s ) is obtained by loop forming roller 184 . in this period of c , transfer sheet “ p ” is made to hit registration roller 185 on the part of the second conveyance path , and the transfer sheet “ p ” is conveyed until it forms a loop . during a period of d in fig4 ( a )- 4 ( d ), stepping motor smb rotates in the ccw ( counterclockwise ) direction , and a rotation in this ccw direction is transmitted to loop forming roller 184 through transmission section 190 . due to this , second conveyance speed ( s ) is obtained by loop forming roller 184 . in this period of d , stepping motor sma is also rotating , and the second conveyance speed ( s ) is obtained by registration roller 185 . namely , in this period of d , transfer sheet “ p ” is conveyed at constant conveyance speed ( s ) while it is interposed between loop forming roller 184 and registration roller 185 . in this case , effects obtained by independent driving between registration roller 185 and loop forming roller 184 in the period c and the period d are the same as those in the aforesaid period a and period b . the following effects are further obtained in comparison between period a — period b and period c — period d . in this case , a common driving source is used by plural loop forming rollers provided respectively on plural conveyance paths , and switching of the direction of rotation of this common driving source makes it possible to drive plural loop forming rollers independently of each other . due to this , even in the case of plural conveyance paths , it is not necessary to use a magnetic clutch , and neither dispersion nor delay of the response time is caused , when switching each loop forming roller to be driven . since there is conducted control to switch each loop forming roller to plural conveyance speeds , it is possible to control to switch the conveyance speed of each loop forming roller independently of a registration roller . it is also possible to make one of plural conveyance paths to be a conveyance path for conveying a transfer sheet coming from a bypass feeding tray , which makes it possible to convey at high speed also from the bypass feeding tray . as explained in detail above , the first driving source for driving a registration roller and the second driving source for driving a loop forming roller are independent of each other in the invention , and when conveying a transfer sheet to a photoreceptor , there is conducted to control to convey by driving the registration roller with the first driving source and by driving the loop forming roller with the second driving source . namely , since switching of the conveyance speed by the loop forming roller is realized by control of the second driving source , it is not necessary to use a magnetic clutch , and neither dispersion nor delay of the response time is caused . in the invention , a common driving source is used by plural loop forming rollers provided respectively on plural conveyance paths , and switching of the direction of rotation of this common driving source makes it possible to drive plural loop forming rollers independently of each other . due to this , even in the case of plural conveyance paths , it is not necessary to use a magnetic clutch , and neither dispersion nor delay of the response time is caused , when switching plural loop forming rollers to be driven and conveyance speeds . another example of the invention will be explained in detail as follows , referring to fig7 - fig9 ( b ). fig7 is a block diagram showing an example of electric structure of primary portions of an image forming apparatus in another example of the invention , and fig8 is a sectional side view showing the mechanical structure of an image forming apparatus in another example of the invention . the mechanical structure of another example will be explained as follows , referring to fig8 focusing on the points which are different from the aforesaid example shown in fig3 . transfer sheet “ p ” to be conveyed to image forming section 50 is made to hit registration roller 185 located in the vicinity of the image forming section , and a loop is formed on the transfer sheet by loop forming roller 186 , then , the transfer sheet is synchronized in terms of timing , and is conveyed to photoreceptor drum 51 representing an image carrier . conveyance roller 182 a and conveyance roller 183 a represent conveyance rollers which are driven respectively by different driving sources ( stepping motor sma and stepping motor smb ) to convey a transfer sheet by switching the speed under the condition that they interpose the transfer sheet . incidentally , roller 182 b and roller 183 b are driven rollers which are rotated respectively by the rotations of the conveyance rollers 182 a and 183 a . setting from cpu 100 for frequency dividing means setting for changing , based on the same pattern , the drive frequency in increasing and decreasing the speeds of stepping motors sma and smb and for making the stepping motors to drive . fig9 ( a ) and 9 ( b ) represent an illustration showing patterns of drive frequency in increasing and decreasing the speeds of stepping motors sma and smb . incidentally , conveyance roller 182 a driven by stepping motor sma and conveyance roller 183 a driven by stepping motor smb correspond respectively to conveyance roller 189 and reversing conveyance roller 191 both located at positions shown in fig8 . a period of a in fig9 ( a ) and 9 ( b ) shows the state wherein transfer sheet “ p ” ejected out of fixing section 59 enters reversing section 63 and is conveyed toward a reversing conveyance roller . in this period a , stepping motors sma and smb rotate in the cw ( clockwise ) direction , and the transfer sheet is conveyed by conveyance roller 189 and reversing conveyance roller 191 at the second conveyance speed ( s ) until the trailing edge of the transfer sheet is ejected out of fixing section 59 completely . when sensor 400 detects that the trailing edge of the transfer sheet has been ejected out of fixing section 59 completely , switching to the first conveyance speed ( 2 s ) is conducted in the period b in fig9 ( a ) and 9 ( b ). in this period b , conveyance roller 189 and reversing conveyance roller 191 convey large - sized transfer sheet “ p ” by switching the conveyance speed under the condition that they interpose the transfer sheet . the conveyance of the transfer sheet is continued until the trailing edge of the transfer sheet “ p ” reaches the position immediately before the reversing roller at the first conveyance speed . after that , in a period of c in fig9 ( a ) and 9 ( b ), stepping motor smb rotates reversely in the ccw ( counterclockwise ) direction , and when it is rotated reversely by reversing conveyance roller 191 at the negative first conveyance speed ( s ), the transfer sheet “ p ” is reversed and conveyed . in this case , stepping motor sma does not need to be driven , and it is stopped . however , the stepping motor sma may also be driven together with stepping motor smb . then , the stepping motor smb is stopped after the transfer sheet “ p ” is reversed and conveyed , to be ready for conveyance of the succeeding transfer sheet . in this case , the first driving source for driving conveyance roller 189 and the second driving source for driving reversing conveyance roller 191 are independent of each other . the first driving source drives conveyance rollers 187 and 188 in addition to the conveyance roller 189 , thereby , conveyance rollers 187 , 188 and 189 which are located at the downstream side of the fixing section and at the upstream side of the reversing conveyance roller 191 are driven by the same first driving source . this makes transfer sheet “ p ” to be interposed by these conveyance rollers 187 , 188 and 189 in the course of switching the conveyance speed , which is preferable because troubles in conveying transfer sheet “ p ” in the course of switching the conveyance speed can be prevented . further , when reversing the transfer sheet “ p ” ejected out of the fixing section to eject onto a sheet ejection tray 300 , it is preferable that the conveyance rollers 187 - 189 stated above are caused to act also as conveyance rollers when the first driving source is rotated reversely . in the case of reversing , when a transfer sheet is large in size , it is reversed and conveyed under the condition that it is interposed also by the conveyance roller 191 . it is therefore possible to prevent timing delay of the transfer sheet by making the drive frequency to be the same pattern as in the example of the present embodiment . as stated above , in the present embodiment , each of plural conveyance rollers is driven by an independent stepping motor , and driving frequency for plural stepping motors for increasing and decreasing the speeds of the stepping motors is changed based on the same pattern so that the stepping motors may drive rollers . therefore , two conveyance rollers which convey a transfer sheet while interposing it are in the state of synchronization with each other from the start to the stop . though an example of applying the invention to the drive control of reversing section 63 has bee described in the present example , it is naturally possible to apply to those other than the reversing section 63 . for example , the invention can be applied to drive control for a registration roller and a loop forming roller . it is therefore possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary , and overload for the driving source , damage of the transfer sheet and occurrence of abnormal noise are not caused . although each of plural conveyance rollers is driven by an independent stepping motor , the state of perfect synchronization of the conveyance rollers can be created because an oscillator is common . due to this , two conveyance rollers which convey a transfer sheet by interposing it are in the state of perfect synchronization from the start to the stop . although each of plural conveyance rollers is driven by an independent stepping motor in the present embodiment , it is possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary , and overload for the driving source , damage of the transfer sheet and occurrence of abnormal noise are not caused in the case of conveyance for sheet reversing and refeeding in double - sided image forming , even in the case that one of the plural conveyance rollers is made to be a reversing conveyance roller . as explained in detail above , each of plural conveyance rollers is driven by an independent stepping motor in the invention , and driving frequency for plural stepping motors for increasing and decreasing the speeds of the stepping motors is changed based on the same pattern so that the stepping motors may drive rollers . therefore , two conveyance rollers which convey a transfer sheet while interposing it are in the state of synchronization with each other from the start to the stop . due to this , it is possible to realize conveyance control wherein a difference between gripping forces of two conveyance rollers and a slack of a transfer sheet are not necessary , and overload for the driving source , damage of the transfer sheet and occurrence of abnormal noise are not caused , in the case of conveying a transfer sheet with two conveyance rollers by switching to the different conveyance speed while the transfer sheet is interposed by these two conveyance rollers .