embodiments of the present invention will be described with reference to the accompanying drawings . firstly , an electrophotographic apparatus of one side printing type will be explained with reference to fig2 . in the figure , a reference numeral 1 depicts a photosensitive drum which forms a toner image thereon through the electrophotographic process and is supported by a supporting axis so that the photosensitive drum 1 rotates to a direction indicated by an arrow &# 34 ; a &# 34 ; at a constant speed . a charger 2 is disposed to oppose to the surface of the photosensitive drum 1 thereby to uniformly charge the surface of the photosensitive drum 1 which passes the charger 2 in opposite relation . a laser beam 3 , for exposing the surface of the photosensitive drum 1 which is charged uniformly , is modulated by a print information signal supplied from an information processing apparatus thereby to form an electrostatic latent image on the surface of the photosensitive drum 1 . a development apparatus 4 is disposed to oppose to the surface of the photosensitive drum 1 on which the electrostatic latent image is formed . the development apparatus 4 has a developing function that powder toner are made adhered to the surface of the photosensitive drum 1 by the electrostatic power of the electrostatic latent image thereby to form a toner image . a sheet hopper 7 receives , in a folded state , a continuous recording material ( printing sheet ) 6 of belt - like configuration to which the toner image is transferred and fixed thereby to perform image printing thereon . transfer rollers 8a , 8b forming a part of a print sheet transfer means take in the print sheet 6 from the hopper 7 and send to the photosensitive drum 1 . the print sheet 6 sent out from the transfer rollers 8a , 8b is made in contact with the surface of the photosensitive drum 1 so that the toner image is transferred to the surface of the print sheet from the photosensitive drum 1 . a transfer unit 5 applies electric charge , whose polarity is in opposite to that of the toner image , to the rear surface of the print sheet 6 being in contact with the photosensitive drum 1 , thereby to generate on the rear surface of the print sheet 6 electrostatic force for moving ( transferring ) the toner image formed on the surface of the photosensitive drum 1 to the surface of the print sheet 6 . transfer rollers 9a , 9b forming the other part of the print sheet transfer means send the print sheet 6 , on which the toner image has been transferred , to a preheater 10 . the preheater 10 is disposed at the upstream side of fixing rollers 11 and made in contact with the rear surface of the print sheet 6 holding the toner image thereon , thereby to preheat the print sheet 6 . the preheater 10 is arranged for example in a manner that an electric heater is mounted to the rear surface of a heat transfer member with which the rear surface of the print sheet 6 contacts . the preheater 10 heats the print sheet 6 to such a degree that the toner image adhered to the print sheet 6 is softened while the print sheet 6 moves in a state of being in contact with the surface of the preheater 10 . the temperature of the preheater 10 can be adjusted by controlling the current supplied to the electric heater by a temperature control device 16 . the pair of fixing rollers 11 formed by a heat roller 11a and a pressure roller 11b urged to each other heat and pressurize the print sheet 6 which was made in contact with the preheater 10 and preheated , thereby fixing the toner image on the surface of the print sheet 6 . the print sheet 6 sent from the fixing rollers 11 is pulled in by puller rollers 12a , 12b and then folded and received within a stacker 13 or ejected out of the electrophotographic apparatus . fig1 shows an arrangement of an electrophotographic printing system of both sides printing type formed by using two electrophotographic apparatuses a and b of one side printing type each arranged in the aforesaid manner . the electrophotographic apparatus a of the former stage and the electrophotographic apparatus b of the rear stage are arranged side by side so as to be connected in a subordinate relation . a print sheet reversing apparatus c is disposed between the electrophotographic apparatus a of the former stage and the electrophotographic apparatus b of the rear stage . the print sheet reversing apparatus reverses the print sheet 6 , which has been printed on its major surface and ejected from the electrophotographic apparatus a of the former stage , and transfers it to the electrophotographic apparatus b of the rear stage . then , the electrophotographic apparatus b of the rear stage prints an image on the rear surface of the print sheet 6 . in such an electrophotographic printing system , the electrophotographic apparatus a of the former stage takes in the print sheet 6 from the hopper 7 and prints an image thereon , and ejects the print sheet 6 thus printed to the outside of the apparatus . then , the electrophotographic apparatus b of the rear stage prints an image on the print sheet 6 taken in from the print sheet reversing apparatus c thereby to have the print sheet thus printed received within the stacker 13 . a print controller 23 forming a part of a higher - rank control apparatus in a control system of the electrophotographic printing system controls the two electrophotographic apparatuses a and b in accordance with print information supplied from a host computer 24 which is a higher - rank unit of the print controller . in the present invention , in order to form the electrophotographic printing system of both sides printing type by using the aforesaid two electrophotographic apparatuses a and b of one side printing type , the fixing apparatus of at least the electrophotographic apparatus b of the rear stage is arranged in a manner shown in fig3 and 4 . fig3 is a side view showing the state of the fixing apparatus at the time of printing operation , and fig4 is a side view showing the state of the fixing apparatus at the time of stopping the printing operation . the fixing apparatus is formed by a suction apparatus 14 for attracting the rear surface of the print sheet 6 holding a toner image ( not shown ) thereon , the preheater 10 for preheating the print sheet 6 while adhering the print sheet 6 thereto by the suction power of the suction apparatus 14 , the pair of fixing rollers 11 formed by the heat roller 11a and the pressure roller 11b urged to each other , and a recording material lifting apparatus 15 . the recording material lifting apparatus 15 is formed by a base member 15b supported , for example , by a fixed member ( not shown ) of the fixing apparatus and a projection member 15a of pin - like configuration fixed on the upper surface of the base member 15b . an opening portion 16 such as a through hole or a slit having a size capable of being engaged with the projection member 15a is provided at a portion of the preheater 10 opposing to the tip portion of the projection member 15a . the opening portion is arranged in a manner that the projection member 15a protrudes from the surface of the preheater 10 through the opening portion 16 when the preheater 10 moves down to the position shown in fig4 . the preheater 10 is formed by a high - temperature preheater 10a disposed at a position closer to the heat roller 11a along the transferring direction of the print sheet 6 and a low - temperature preheater 10b disposed at the upstream side with respect to the high - temperature preheater 10a along the transferring direction of the print sheet . the projection member 15a of the recording material lifting apparatus 15 is provided so as to be movable relative to the surface of the high - temperature preheater 10a . in addition to the suction apparatus 14 , a suction apparatus 17 is provided at the lower portion of the low - temperature preheater 10b so as to adhere the print sheet 6 thereto . in the above arrangement , a toner image transferred on the surface of the print sheet 6 is preheated by the low - temperature preheater 10b and the high - temperature preheater 10a , then heated and pressurized by the heat roller 11a and the pressure roller 11b thereby to be fixed to the print sheet . the suction apparatus 14 provided just before the preheater 10 and the suction apparatus 17 provided at the lower portion of the low - temperature preheater 10b serve to improve the adhesion property between the print sheet 6 and the preheater 10 . such a technical concept can also be applied to the fixing apparatus of the electrophotographic apparatus a of the former stage . in this case , in the fixing apparatus of the electrophotographic apparatus a of the former stage , since there is no toner image on the surface side of the print sheet 6 to which the preheater 10 contacts , it is preferable to provide the suction apparatus at the lower portion of the high - temperature preheater 10a as well as at the lower portion of the low - temperature preheater 10b . at the time of stopping the printing operation ( that is , at the time of non - fixing operation ), as a general operation , the pressure roller 11b is separated from the heat roller 11a thereby to release the urged state between these rollers , and further the preheater 10 and the suction apparatuses 14 , 17 move away from the print sheet transfer path simultaneously or almost simultaneously . at this time , the print sheet 6 placed on the high - temperature preheater 10a is pushed up by the projection member 15a of the recording material lifting apparatus 15 provided at the high - temperature preheater 10a . in this case , the print sheet 6 placed on the low - temperature preheater 10b , whose temperature is set to such a value that the toner image on the print sheet is not adhered to the preheater 10 , is kept to be adhered to the low - temperature preheater 10b by the suction apparatuses 14 and 17 . in this manner , in the stopping state of the printing operation , the print sheet 6 is kept to be adhered to the low - temperature preheater 10b . accordingly , at the time of restarting the printing operation , it is possible to restore the print sheet 6 , which was once separated from the preheater at the time of stopping the printing operation , accurately to the original position at the time of stopping the printing operation . although the setting temperature of the low - temperature preheater 10b depends on the characteristics of the toner used in the electrophotographic apparatus , it is possible to increase the setting temperature almost to 80 ° c . in the present embodiment . further , the setting temperature of the high - temperature preheater 10a can be increased almost to 110 ° c . the setting temperature of the high - temperature preheater 10a corresponds to the setting temperature of an electrophotographic apparatus used in the case of performing the one - side printing . accordingly , according to the present invention , the fixing intensity of the electrophotographic apparatus of the rear stage can be made substantially same as that of electrophotographic apparatus of the former stage . when the setting temperatures of the preheaters are set in the aforesaid manner , it becomes possible unlike the prior art to prevent the toner from being adhered to the surfaces of the a preheaters without providing on the surfaces of the preheaters layers formed by material such as ptfe or pfa compound to which toner is hardly adhered . although , in the aforesaid embodiment , the preheater 10 is formed by the high - temperature preheater 10a and the low - temperature preheater 10b , the present invention is not limited thereto . that is , the preheater 10 may one arranged to have a single setting tempearature . fig5 shows a stopping state of the printing operation of another embodiment using a preheater 10 in which only one setting temperature can be set . in this embodiment of such an arrangement , the deviation of the print sheet 6 due to the lift - up from the preheater 10 is prevented only by a suction apparatus 14 provided just before the preheater 10 . in this case , the preheater 10 is a high - temperature preheater . as described above , according to the present invention , the fixing intensity of the electrophotographic apparatus of the rear stage can be made substantially same as that of the electrophotographic apparatus of the former stage . further , according to the present invention , at the time of restarting the printing operation , it is possible to restore the print material , which was once separated from the preheater at the time of stopping the printing operation , accurately to the original position at the time of stopping the printing operation , so that it is possible to provide a printed material without any unevenness of fixed image . the foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed , and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention . the embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated . it is intended that the scope of the invention be defined by the claims appended hereto , and their equivalents .