Abstract:
An image forming apparatus including a plurality of photoconductive elements arranged on an inclined plane with respect to the ground, a plurality of developing devices, each of the plurality of developing devices corresponding to one of the plurality of photoconductive elements and configured to develop an image on a corresponding one of the plurality of photoconductive elements and a plurality of mirrors and a polygon mirror disposed inside a single housing and configured to reflect light beams to the plurality of photoconductive elements, the housing being arranged parallel to the inclining plane and below the plurality of photoconductive elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation Application of, and claims the benefit of priority under 35 U.S.C. § 120 from, U.S. application Ser. No. 11/334,427, filed Jan. 19, 2006, which is a Continuation of U.S. Pat. No. 7,027,762, issued Apr. 11, 2006, which is a Continuation of U.S. Pat. No. 6,898,408, issued May 24, 2005, and claims the benefit of priority under 35 U.S.C. § 119 from Japanese Patents Applications No. 2001-374541, filed Dec. 7, 2001 and 2002-322502, filed Nov. 6, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus operable in a duplex print mode for printing images on both sides of a sheet or recording medium. 
   2. Description of the Related Art 
   It is a common practice with an image forming apparatus operable in a duplex print mode to transfer a toner image from an image carrier to one surface of a sheet, fix the toner image, turn the sheet via, e.g., a turn path, and again feed the sheet to form another toner image on the other side of the sheet. The problem with this type of apparatus is that the sheet cannot be reliably conveyed due to the switching of the sheet conveying direction and the curl of the sheet ascribable to the fixation of the toner image on one side of the sheet. 
   In light of the above, Japanese Patent Laid-Open Publication No. 1-209470 discloses an image forming apparatus including a first and a second image carrier for transferring toner images to both sides of a sheet and then fixing them at the same time. More specifically, in the apparatus taught in this document, a first image formed on a photoconductive element is transferred to an image transfer belt by first image transferring means. Subsequently, a second toner image formed on the photoconductive element is transferred to one side of a sheet. Thereafter, the first image is transferred from the belt to the other side of the sheet by second image transferring means. The sheet carrying the toner images on both sides thereof is conveyed to a fixing unit. 
   However, the procedure taught in the above document is not practicable without causing the image transfer belt to make two turns. More specifically, the second image begins to be formed only after the image transfer belt has made one full turn, resulting in lower productivity in the duplex print mode. This is particularly true when full-color images are formed on both sides of a sheet. 
   Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-160703. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image forming apparatus capable of executing a full-color duplex print mode without lowering productivity. 
   An image forming apparatus capable of forming images on both sides of a recording medium of the present invention includes a first image carrier on which a toner image to be formed, and a second image carrier to which the toner image is transferred from the first image carrier. The toner image transferred from the image carrier to the second image carrier is transferred to one side of the recording medium while a toner image is transferred from the first image carrier to the other side of the recording medium. After the toner image has been transferred from the first image carrier to the second image carrier, the running condition of the second image carrier is varied. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a section showing an image forming apparatus embodying the present invention; 
       FIG. 2  is a section showing another specific configuration of an image forming section included in the illustrative embodiment; 
       FIG. 3  is a section showing still another specific configuration of the image forming section; 
       FIG. 4  is a section showing a modification of the illustrative embodiment; 
       FIGS. 5A through 5F  demonstrate a specific operation of the illustrative embodiment; 
       FIGS. 6A through 6F  demonstrate another specific operation of the illustrative embodiment; 
       FIGS. 7A through 7F  demonstrate still another specific operation of the illustrative embodiment; 
       FIGS. 8A and 8B  are graphs comparing the illustrative embodiment and a conventional image forming apparatus as to printing time; 
       FIGS. 9A through 9F  demonstrate a specific operation representative of an alternative embodiment of the present invention; 
       FIGS. 10A through 10F  demonstrate another specific operation available with the alternative embodiment; 
       FIG. 11  is a perspective view showing a specific configuration of a mechanism for selectively moving an intermediate image transfer belt into or out of contact with a photoconductive drum; 
       FIG. 12  is a perspective view showing a specific configuration of a mechanism for obviating the offset of the belt; 
       FIGS. 13A through 13C  are side elevations showing the operation of the mechanism of  FIG. 12 ; 
       FIG. 14  is a view showing a specific configuration of an image forming apparatus including a first image carrier implemented as a belt; 
       FIG. 15  is a section showing one of image forming units included in the apparatus of  FIG. 14 ; 
       FIGS. 16A and 16B  are sections showing a specific configuration for selectively moving a second image carrier included in the apparatus of  FIG. 14  into or out of contact with the first image carrier; 
       FIGS. 17A and 17B  are fragmentary sections showing another specific configuration for moving the second image carrier; 
       FIGS. 18A through 18C  show specific timing marks formed on the second image carrier and means for sensing the timing marks; 
       FIG. 19  is a timing chart representative of a specific operation of the apparatus shown in  FIG. 14 ; 
       FIG. 20  demonstrates specific speed control over a stepping motor assigned to the second image carrier; 
       FIG. 21  is a section showing a unit, which includes the second image carrier of the apparatus shown in  FIG. 14 , in an open position; 
       FIG. 22  is a section showing another specific configuration of the image forming apparatus including another specific configuration of a fixing device; 
       FIG. 23  is a fragmentary section showing a unit, which includes the second image carrier of the apparatus shown in  FIG. 22 , in an open position; 
       FIG. 24  is a section showing another specific configuration of the image forming apparatus; 
       FIG. 25  is a perspective view showing a plurality of image forming apparatuses each having any one of the configurations of  FIGS. 14 ,  22  and  24  and interconnected by a network; 
       FIG. 26  is a view showing another specific configuration of the image forming apparatus in which a first image carrier is implemented as a plurality of image carriers; 
       FIG. 27  is a section showing a second image carrier included in the apparatus of  FIG. 26 ; and 
       FIG. 28  is a fragmentary view showing a specific configuration of a mechanism for moving the second image carrier of  FIG. 27  into and out of contact with the first image carrier. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1  of the drawings, an image forming apparatus embodying the present invention is shown and implemented as a printer by way of example. As shown, the printer, generally  100 , includes a photoconductive drum or first image carrier  1  positioned at substantially the center of the printer body. Arranged around the drum  1  are a drum cleaner  2 , a discharger  3 , a charger  4 , and a revolver type developing unit (revolver hereinafter)  5 R. An optical writing unit  7  is positioned above the drum  1  and scans the surface of the drum  1  with a laser beam L at a position between the charger  4  and the revolver  5 R. 
   A belt unit  20  is arranged below the drum  1  and includes an intermediate image transfer belt or second image carrier  10 . In the illustrative embodiment, the intermediate image transfer belt (simply belt hereinafter)  10  is angularly movable into or out of contact with the drum  1  in a direction indicated by a double-headed arrow K in  FIG. 1 . When image formation is not under way, the belt  10  is spaced from the drum  1  so as not to curl or otherwise deform or adversely effect the drum  1 . The belt  10  should preferably be releasable from the drum  1  in the event of jam processing as well. 
   The belt  10  is passed over rollers  11 ,  12  and  13 . A moving mechanism, which will be described later, causes the belt  10  to angularly move about the roller  11  into or out of contact with the drum  1  in the direction K. The belt  10  is heat-resistant, coated with PFA (perfluoroalcoxy), and provided with resistance of 10 5 Ω·cm to 10 12 Ω·cm that allows toner to be transferred to the belt  10 . In the illustrative embodiment, a mark, not shown, is provided on the belt  10  for controlling the system. In the event of power-up, the timing mark on the belt  10  is sensed to bring the belt  10  to a preselected reference or initial position. 
   Back rollers  14  and  15 , cooling means  16 , a fixing roller  18  and first image transferring means  21  are arranged inside of the loop of the belt  10 . The fixing roller  18  accommodates a heater or similar heat source and fixes a toner image carried on a sheet. The first image transferring means  21  faces the drum  1  with the intermediary of the belt  10  for transferring a toner image formed on the drum  1  to the belt  10  or a sheet. The belt  10  is driven by a stepping motor  53  (see  FIG. 11 ) via the drive roller  11 . The stepping motor  53  is independent of a motor that drives the drum  1  and other rotary members. Second image transferring means  22 , a fixing device  22  and a belt cleaner  25  are positioned outside of the loop of the belt  10 . The fixing device  30  includes a fixing roller  19  also accommodating a heater or similar heat source and fixes a toner image carried on a sheet. A mechanism, not shown, causes the fixing device  30  to angularly movable about a fulcrum  30   a  into or out of contact with the fixing roller  18  with the intermediary of the belt  10  (and sheet) in a direction indicated by a double-headed arrow G. 
   The belt cleaner  25  assigned to the belt  10  includes a cleaning roller  25   a , a blade  25   b  and toner conveying means 
     25   c  and removes toner left on the belt  10  after image transfer. The toner conveying means  25   c  conveys the toner collected in the belt cleaner  25  to a container not shown. The belt cleaner  25  is angularly movable about a fulcrum  25   d  in a direction indicated by a double-headed arrow H. A mechanism, not shown, causes the belt cleaner  25  to move into or out of contact with the belt  10  in the direction H. 
   The drum  1 , drum cleaner  2 , charger  4  and revolver  5 R may be constructed into a single process cartridge replaceable when its life ends. 
   A sheet cassette  26  is positioned in the lower portion of the printer body and can be pulled out to the front in the direction perpendicular to the sheet surface of  FIG. 1 . Sheets or recording media P are stacked on the cassette  26 . A pickup roller  27  is positioned above the right end, as viewed in  FIG. 1 , of the sheet cassette  26 . A manual sheet feed tray  35  is mounted on the right side, as viewed in  FIG. 1 , of the printer body. The manual sheet feed tray  35  includes a bottom plate  37  loaded with sheets P and constantly biased toward a pickup roller  36 . 
   A registration roller pair  28  is located at the right-hand side, as viewed in  FIG. 1 , of the drum  1 . A guide  29  guides the sheet P fed from either one of the sheet cassette  26  and manual sheet feeder  35  toward the registration roller pair  28 . An electric unit E 1  and a control unit E 2  are positioned above the sheet cassette  26 . 
   A path selector  42  is positioned at the left-hand side, as viewed in  FIG. 1 , of the fixing device  30 . The path selector  42  is pivotable about a fulcrum  43  to steer the sheet P coming out of the belt unit  20  to either one of a stack portion  40  positioned on the top of the printer body and a print tray  44  mounted on the side of the printer body. More specifically, a solenoid or similar actuator, not shown, moves the path selector  42  to a position shown in  FIG. 1  for steering the sheet P toward the stack portion  40  or moves it in a direction indicated by an arrow J for steering the sheet P toward the print tray  44 . 
   A roller pair  33  is positioned above the path selector  42  for conveying the sheet P while a roller pair  34  is positioned above the roller pair  33  for driving the sheet P to the stack portion  40 . Guides  31   a  and  31   b  cooperate to guide the sheet P from the roller pair  33  to the roller pair  34 . A roller pair  32  is positioned at the left-hand side, as viewed in  FIG. 1 , of the path selector  42  for driving the sheet P out of the printer body to the print tray  44 . 
   The revolver  5 R includes four developing sections  5   a  through  5   d  and is rotatable counterclockwise, as viewed in  FIG. 1 , to locate one of the developing sections  5   a  through  5   d  at a developing position. The developing sections  5   a  through  5   d  each store toner of a particular color so as to implement full-color development. For example, the developing sections  5   a  through  5   d  store yellow toner, magenta toner, cyan toner and black toner, respectively. In a monochromatic. print mode, the developing section  5   d  is located at the developing position. 
   The operation of the illustrative embodiment will be described hereinafter. First, a duplex print mode for forming images on both sides of the sheet P will be described. As for a duplex print mode, a toner image formed first and a toner image formed next will be referred to as a first and a second toner image, respectively. Also, a first and a second side of the sheet to which the first and second toner images are transferred will be referred to as a first and a second side, respectively. 
   On the power-up of the printer  100 , the belt or second image carrier  10  is brought to its reference position on the basis of the mark mentioned earlier. The printer  100  receives image data from a host machine, e.g., a computer. The writing unit  7  emits the laser beam L toward a polygonal mirror  7   a , which is rotated by a motor, in accordance with the image data. The laser beam L is steered by the polygonal mirror  7   a  and incident to the surface of the drum  1 , which has been uniformly charged by the charger  4 , via a mirror  7   b , an f- 6  lens  7   c  and so forth. As a result, a latent image corresponding to the image data, is electrostatically formed on the drum  1 . 
   In a monochromatic print mode, the developing section  5   d  develops the latent image with the black toner for thereby producing a black toner image on the drum  1 . On the other hand, in a full-color print mode, the writing unit  7  first scans the charged surface of the drum  1  with the laser beam L in accordance with yellow image data, thereby forming a latent image. At this instant, the belt  10  is spaced from the drum  1 . The developing section  5   a  located at the developing position develops the above latent image with yellow toner to thereby produce a yellow toner image. Subsequently, a magenta toner image is formed on the drum  1  over the yellow toner image. Likewise, a cyan toner image and a black toner image are sequentially formed on the drum  1  in this order over the composite toner image existing on the drum, completing a full-color toner image. The drum  1  makes four rotations for forming the full-color toner image. It is to be noted that the order of colors mentioned above is only illustrative. 
   The first image transferring means  21  transfers the toner image, which is monochromatic or full-color, from the drum  1  to the surface of the belt  10 , which is running in synchronism with the rotation of the drum  1 . After the image transfer, the drum cleaner  2  removes the toner left on the drum  1 . Subsequently, the discharger  3  discharges the surface of the drum  1  for thereby preparing it for the next image forming cycle. 
   The belt  10 , carrying the toner image or first toner image thereon, turns counterclockwise as viewed in  FIG. 1 . At this instant, the second image transferring means  22 , fixing device  30  and belt cleaner  25  are maintained inoperative so as not to disturb the toner image carried on the belt  10 . For this purpose, such process units  22 ,  30  and  25  may be released from the belt  10  or electric inputs thereto may be shut off. 
   After the entire first toner image has been transferred from the drum  1  to the belt  10 , the belt  10  is released from the drum  10  and then turned in the reverse direction, i.e., clockwise in  FIG. 1  to the reference position. The distance of movement of the belt  1  is controlled on the basis of the number of steps of the stepping motor or drive means. In the illustrative embodiment, the reverse movement of the belt  10  is effected at a speed two times as high as the speed of the forward movement or usual speed. On reaching the reference position, the belt  10  is again brought into contact with the belt  10  and then moved counterclockwise, i.e., in the forward direction. 
   A toner image to be transferred to the second side of one sheet P, i.e., a second toner image is formed on the drum  1  in the same manner as the first toner image. At this instant, the top sheet P on the sheet cassette  26  or the manual sheet feed tray  35  is paid out by the pickup roller  27  or  36 , respectively, and conveyed to the nip between the registration rollers  28 . The registration roller pair  28  conveys the sheet P to the nip between the drum  1  and the belt  10  at a timing that matches the position of the toner image and that of the sheet P. The first image transferring means  21  transfers the second toner image from the drum  1  to the second side of the sheet P. 
   While the toner or second toner image is being transferred from the drum  1  to the second side of the sheet P, the other side or first side of the sheet P moves together with the toner existing on the belt  10 , i.e., with the first side contacting the first image. When the sheet P reaches the second image transferring means  22 , the transferring means  22  transfers the toner from the belt  10  to the sheet P by being applied with a voltage. 
   The belt  10  in movement conveys the sheet P carrying the toner images on both sides thereof to a fixing position where the fixing device  30  is located. At this instant, the fixing device  30  is angularly moved to press the fixing roller  19  against the fixing roller  18  via the belt  10 , so that the fixing rollers  18  and  19  cooperate to fix the. toner images on both sides of the sheet P. In this manner, the toner images are fixed on the sheet P with the sheet P contacting the belt  10 , so that the toner images are prevented from being disturbed. The sheet P coming out of the fixing device  30  is separated from the belt  10  at the position where the roller  11  is located. Subsequently, the path selector  42  steers the sheet P toward the stack portion  40  or the print tray  44 . 
   As shown in  FIG. 1 , when the path selector  42  steers the sheet P toward the stack portion  40 , the sheet P is laid on the stack portion  40  with its surface to which the toner image is directly transferred from the drum  1  facing downward. Therefore, to stack consecutive prints on the stack portion  40  in order of page, it suffices to form a toner image corresponding to the second page first, transfer it to the belt  10 , form a toner image corresponding to the second page, and then directly transfer the toner image of the second page to the sheet P. In this respect, the first and second images correspond to the second and first pages, respectively. This is also true with the third page and successive pages. The crux is that when an image is present on an even page, it is formed first and transferred to the belt  10 , and then the image of an odd page preceding the even page is formed and directly transferred from the drum  1  to the sheet P. 
   On the other hand, when the path selector  42  steers the sheet P toward the print tray  44 , the sheet P is laid on the print tray  44  with its surface to which the toner image is directly transferred from the drum  1  facing upward. Therefore, when consecutive prints should be stacked on the print tray  44  in order of page, the first and second images correspond to the first and second pages, respectively. This is also true with the third page and successive pages. The crux is that when an image is present on an odd page, it is formed first and transferred to the belt  10 , and then the image of an even page following the odd page is formed and directly transferred from the drum  1  to the sheet P. 
   Usually, a reversed image or mirror image is formed on the drum  1  and then directly transferred from the drum  1  to the sheet P as a non-reversed image. However, as for image transfer from the belt  10  to the sheet P, a mirror image formed on the drum  1  would also be a mirror image on the sheet P. In light of this, the writing unit  7  scans the drum  7  such that an image to be transferred from the belt  10  to the sheet P is a non-reversed image on the drum  1  while an image to be directly transferred from the drum  1  to the sheet P is a mirror image on the drum  1 . Such an image forming sequence for page arrangement can be implemented by a conventional technology using a memory for storing image data. Also, exposure that selectively forms a reversed image or a non-reversed image can be implemented by a conventional image processing technology. 
   After the image transfer from the belt  10  to the sheet P, the belt cleaner  10  is angularly moved to bring the cleaning roller  25   a  into contact with the belt  10  and cause the roller  25   a  to remove toner left on the belt  10 . Subsequently, the blade  25   b  wipes off the toner deposited on the cleaning roller  25   a . The toner collected by the blade  25   b  is conveyed to the previously mentioned container by the toner conveying means  25   c.    
   The belt  10  moved away from the cleaning position is cooled off by the cooling means  16  that may use any conventional heat radiation scheme. For example, as for a scheme producing an air stream, it is preferable to cause air to flow after the image transfer from the belt  10  to the sheet P, thereby preventing the toner image carried on the belt  10  from being disturbed. Use may also be made of a heat pipe directly contacting the inner surface of the belt  10 . In any case, a fan F 1  discharges heat radiated from the belt  10  to the outside of the printer body. 
   A simplex print mode available with the illustrative embodiment for forming an image on one side of the sheet P will be described hereinafter. First, when the sheet or print P carrying an image on one side thereof, i.e., a simplex print should be driven out to the stack portion  40 , the image transfer from the drum  1  to the belt  10  is not necessary, i.e., a monochromatic or a full-color toner image is directly transferred from the drum  1  to the sheet P. In this case, a reversed image or mirror image is formed on the drum  1  and then transferred to the sheet P as a non-reversed image. 
   More specifically, as shown in  FIG. 1 , the sheet P is conveyed to the nip between the drum  1  and the belt  10  in synchronism with the rotation of the drum  1 . The first image transferring means  21  transfers a toner image formed on the drum  1  to one side or upper surface of the sheet P facing the drum  1 . At this instant, the second image transferring means  22  does not operate. The sheet P with the toner image is conveyed by the belt  10  to the fixing device  30 , separated from the belt  10 , and then driven out to the stack portion  40  face down via the guides  31   a  and  31   b  and roller pair  32 , as indicated by an arrow A 1 . Consequently, even when several pages of documents are dealt with, the first page being first, the resulting prints are stacked on the stack portion  40  in order of page. Next, when the sheet or simplex print P should be driven out to the print tray  44 , the toner image formed on the drum  1  is transferred to the belt  10  by the first image transferring means  21 . After the transfer of the entire page, the belt  10  carrying the toner image is moved in the reverse direction, i.e., clockwise in  FIG. 1  to the reference position. At this instant, the belt  10  is spaced from the drum  1 . On reaching the reference position, the belt  10  is again brought into contact with the drum  1  and then turned in the forward direction, i.e., counterclockwise in  FIG. 1 . Subsequently, the second image transferring means  22  transfers the toner image from the belt  10  to the side or lower surface of the sheet P facing the belt  10 . Again, even when several pages of documents are dealt with, the first page being first, the resulting prints are stacked on the print tray  44  in order of page. 
   Even when an image is to be formed on a thick sheet, OHP (OverHead Projector) film or similar relatively hard sheet in the simplex print mode, the sheet can be substantially linearly conveyed if the manual sheet tray  35  and print tray  44  are designated. Therefore, simplex prints are achievable in order of page even with relatively thick, rigid sheets without degrading conveyance. 
   As stated above, after the transfer of a toner image from the drum  1  to the belt  10 , the illustrative embodiment moves the belt  10  in the reverse direction to the reference position and therefore does not have to wait until the belt  10  completes one full turn, thereby saving time. The reverse movement of the belt  10  is effective not only in the duplex print mode but also in the simplex print mode. Particularly, productivity is noticeably enhanced because the reverse movement of the belt  10  occurs at a speed two times as high as the speed of the forward movement. Stated another way, the illustrative embodiment improves productivity by varying the running condition of the belt or second image carrier  10 . 
     FIG. 2  shows another specific configuration of the fixing device. As shown, the fixing device, labeled  30 B, differs from the fixing device  30 ,  FIG. 1 , in that it does not contact the belt  10 . The fixing device  30 B fixes a toner image or toner images on the sheet with an infrared lamp, xenon lamp or similar lamp. The fixing device  30 , which does not contact the belt  10 , does not have to be angularly movable, but should only be fixed in place. 
     FIG. 3  shows another specific configuration of the fixing device. As shown, the fixing device, labeled  30 C is positioned outside of the loop of the belt  10  and includes the fixing rollers  18  and  19  each accommodating a respective heater. The fixing device  30 C is also fixed in place and does not have to be moved into or out of contact with the belt  10 . 
     FIG. 4  shows another specific configuration of the developing device. As shown, the developing device differs from the revolver  5 R in that four developing units  5   a  through  5   d  each storing toner of a particular color are arranged around the drum  1 . The developing device of  FIG. 4  is similarly applicable to the specific configuration shown in  FIG. 2  or  3 . 
   Reference will be made to  FIGS. 5A through 5F  for describing a specific image forming sequence that the illustrative embodiment effects in the duplex print mode, taking the configuration of  FIG. 2  as an example. The belt  10  is shown as extending in the up-and-down direction for space reasons. In  FIGS. 5A and 5E , while the drum  1  and belt  10  are shown as being spaced from each other, they are, in practice, held in contact with each other. 
   First, as shown in  FIG. 5A , the charger  4  uniformly charges the surface of the drum  1  to negative polarity. The writing unit scans the charged surface of the drum  1  with the laser beam L to thereby form a latent image. The developing device  5  develops the latent image with negatively charged toner, which is represented by black dots in  FIG. 5A , thereby producing a corresponding toner image. Subsequently, the first image transferring means  21 , which is applied with a positive voltage, transfers the toner image from the drum  1  to the belt  10 . This image transfer will be referred to as primary image transfer hereinafter. 
   As shown in  FIG. 5B , after the primary image transfer, the belt  10  is brought to a stop. Subsequently, as shown in  FIG. 5C , the belt  10  is released from the drum  1  in a direction K 1  and then moved in the reverse direction or clockwise to the reference position at the previously stated speed. 
   As shown in  FIG. 5D , a toner image or second image of negative polarity is formed on the drum  1  while the belt  10  is again moved into contact with the drum  1  in a direction K 2  and then moved in the forward direction or counterclockwise. The sheet P is driven by the registration roller pair  28  at such a timing that the first and second images are accurately positioned on the sheet P. 
   As shown in  FIG. 5E , the first image transferring means  21 , which is applied with a positive voltage, transfers the second image of negative polarity from the drum  1  to the sheet P. This image transfer will be referred to as secondary image transfer. At this instant, the first side of the sheet P is overlaid on the first image carried on the belt  10 . 
   Finally, as shown in  FIG. 5F , the second image transferring means  22 , which is also applied with a positive voltage, transfers the first image of negative polarity from the belt  10  to the sheet P. This image transfer will be referred to as tertiary image transfer hereinafter. The belt  10  in movement conveys the sheet P carrying the first and second images thereon to the fixing position. The fixing means  18  and  30 B are heated, or turned on, to fix the first and second images on the sheet P. At this instant, the belt cleaner  25  is pressed against the belt  10  for removing toner left on the belt  10 . In the specific configuration shown in  FIG. 3 , the sheet P separated from the belt  10  is conveyed to the fixing position. 
   Another specific image forming procedure available with the illustrative embodiment will be described hereinafter with reference to  FIGS. 6A through 6F . Briefly, in the sequence to be described, a single image transferring means transfers the toner image carried on the belt  10  and the toner image formed on the drum  1  to both sides of the sheet P at the same time. More specifically, a charger or polarity switching device inverts the polarity of the toner image carried on the belt  10 , so that the toner image can be transferred to the sheet P at the same time as the toner image formed on the drum  1  by a single image transferring means. As for the rest of the construction, the procedure to be described is identical with the previous procedure. 
   The polarity of the toner image carried on the belt or second image carrier  10  may be inverted during either one of the forward movement and reverse movement of the belt  10 . First, assume that the polarity is inverted while the belt  10  is in reverse movement. The specific procedure uses the non-contact type of fixing device  30 B,  FIG. 2 , by way of example. 
   As shown in  FIGS. 6A through 6F , a polarity switching device  50  is positioned downstream of the image transferring means  21  in the direction of forward movement of the belt  10 , but upstream of the fixing device  30 B. The belt  10  is also angularly movable in the direction K,  FIGS. 1 through 4 , into or out of contact with the drum  1 . The polarity switching device  50  is also movable in accordance with the movement of the belt  10 , so that the relative position of the former and latter does not change. The polarity switching device  50  is essentially identical with the second image transferring means  22  of the previous embodiment and may be implemented thereby so long as the relative position mentioned above does not change. 
   The procedure shown in  FIGS. 6A through 6F  differs from the procedure of  FIGS. 5A through 5F  in that it does not effect the tertiary image transfer. The belt  10  is shown as extending in the up-and-down direction for space reasons. In  FIGS. 6A and 6E , while the drum  1  and belt  10  are shown as being spaced from each other, they are, in practice, held in contact with each other. 
   First, as shown in  FIG. 6A , the charger  4  uniformly charges the surface of the drum  1  to negative polarity. The writing unit scans the charged surface of the drum  1  with the laser beam L to thereby form a latent image. The developing device  5  develops the latent image with negatively charged toner, which is represented by black dots in  FIG. 6A , thereby producing a corresponding toner image. Subsequently, the image transferring means  21 , which is applied with a positive voltage, transfers the toner image from the drum  1  to the belt  10  (primary image transfer). 
   As shown in  FIG. 6B , after the primary image transfer, the belt  10  is brought to a stop. Subsequently, as shown in  FIG. 6C , the belt  10  is released from the belt  10  and then moved in the reverse direction or clockwise to the reference position at the previously stated speed. At this instant, the polarity switching device  50  is applied with a positive voltage, or turned on, to switch the polarity of the toner image on the belt  10  from negative to positive. 
   As shown in  FIG. 6D , a toner image or second image of negative polarity is formed on the drum  1  while the belt  10  is again moved into contact with the drum  1  and then turned in the forward direction or counterclockwise. The sheet P is driven by the registration roller pair  28  at such a timing that the first and second images are accurately positioned on the sheet P. 
   As shown in  FIG. 6E , the image transferring means  21 , which is applied with a positive voltage, transfers the toner image of negative polarity carried on the belt  10  and the second toner image of negative polarity formed on the drum  1  to the sheet P at the same time. 
   Finally, as shown in  FIG. 6F , the belt  10  in movement conveys the sheet P carrying the first and second images thereon to the fixing position. The fixing means  18  and  30 B are heated, or turned on, to fix the first and second images on the sheet P. At this instant, the belt cleaner  25  is pressed against the belt  10  for removing toner left on the belt  10 . In the specific configuration shown in  FIG. 3 , the sheet P separated from the belt  10  is conveyed to the fixing position. 
   Next, how the polarity is inverted while the belt  10  is in forward movement will be described with reference to  FIGS. 7A through 7F . Again, the polarity switching device  50  is positioned downstream of the image transferring means  21  in the direction of forward movement of the belt  10 , but upstream of the fixing device  30 B. Also, the polarity switching device  50  may be fixed in place, if desired. 
   First, as shown in  FIG. 7A , the charger  4  uniformly charges the surface of the drum  1  to negative polarity. The writing unit scans the charged surface of the drum  1  with the laser beam L to thereby form a latent image. The developing device  5  develops the latent image with negatively charged toner, which is represented by black dots in  FIG. 7A , thereby producing a corresponding toner image. Subsequently, the image transferring means  21 , which is applied with a positive voltage, transfers the toner image from the drum  1  to the belt  10  (primary image transfer). While the belt  10  conveys the toner image forward, the polarity switching means  50  is applied with a positive voltage, or turned on, to switch the polarity of the toner image from negative to positive. 
   As shown in  FIG. 7B , after the trailing edge of the toner image has moved away from the polarity switching device  50 , the belt  10  is brought to a stop. As a result, the entire toner image carried on the belt  10  is inverted in polarity. 
   Subsequently, as shown in  FIG. 7C , the belt  10  is released from the belt  10  and then reversed in the clockwise direction to the reference position at the previously stated speed. Because the polarity of the toner image on the belt  10  has already been switched in polarity, it is not necessary to move the polarity switching device  50  together with the belt  10 . 
   As shown in  FIG. 7D , a toner image or second image of negative polarity is formed on the drum  1  while the belt  10  is again moved into contact with the drum  1  and then turned in the forward direction or counterclockwise. The sheet P is driven by the registration roller pair  28  at such a timing that the first and second images are accurately positioned on the sheet P. 
   As shown in  FIG. 7E , the image transferring means  21 , which is applied with a positive voltage, transfers the toner image of positive polarity carried on the belt  1  and the second toner image of negative polarity formed on the drum  1  to the sheet P at the same time. 
   Finally, as shown in  FIG. 7F , the belt  10  in movement conveys the sheet P carrying the first and second images thereon to the fixing position. The fixing means  18  and  30 B are heated, or turned on, to fix the first and second images on the sheet P. At this instant, the belt cleaner  25  is pressed against the belt  10  for removing toner left on the belt  10 . In the specific configuration shown in  FIG. 3 , the sheet P separated from the belt  10  is conveyed to the fixing position. 
   In the procedure shown in  FIGS. 6A through 6F  or  7 A through  7 F, in the simplex print mode, a toner image is. directly transferred from the drum  1  to the sheet P without the polarity switching device  50  being operated, i.e., in exactly the same manner as when two image transferring means are used. 
   In the procedure of  FIGS. 6A through 6F  or  7 A through  7 F, when a toner image is transferred from the drum  1  to the sheet P by way of the belt  10  in the simplex print mode, the polarity switching device  50  is operated to invert the polarity of the toner image. Such image transfer is executed in the same manner as in the duplex print mode except that the transfer of a second image to the drum  1  is not effected. 
   As stated above, even in the procedure in which a single image transferring means transfers a toner image carried on the second image carrier and a toner image formed on a first image carrier to both sides of a sheet at the same time, the belt  10  is moved in the reverse direction to the reference position after the transfer of the toner image to the second image carrier. It is therefore not necessary to wait until the belt  10  completes one full turn, thereby saving time. The reverse movement of the belt  10  is effective not only in the duplex print mode but also in the simplex print mode. Particularly, productivity is noticeably enhanced because the reverse movement of the belt  10  occurs at a speed two times as high as the speed of the forward movement. 
   In any one of the specific configurations described above, when a toner image to be transferred to the belt or second image carrier  10  has a large size in the direction of movement of the belt, the reverse movement of the belt  10  sometimes lowers productivity. For example, when the image size in the above direction is close to the circumferential length of the belt  10 , it is rather desirable to cause the belt  10  to simply complete one turn than to reverse it. In this respect, the belt  10  should preferably be selectively reversed or continuously moved forward by one turn in accordance with the image size in the direction of movement of the belt  10 . More specifically, the belt  10  should preferably be continuously moved by one turn when the image size is larger than a preselected size. 
   For example, assume that the maximum image size that can be transferred to the belt  10  is size A 3  in a profile position, i.e., 420 mm in the direction of movement of the belt  10 . Then, the belt  10  is reversed for image sizes smaller than A 4  in a landscape position, i.e., 210 mm in the above direction or continuously moved forward by one turn for the image size of A 4  in a landscape position or above. While the configurations using two image transferring means satisfactorily work without regard to such selective movement of the belt  10 , even the condition with a single image transferring means can cope with the selective movement by inverting the polarity of a toner image while moving the belt  10  forward. In any case, the control over the belt  10  stated above prevents productivity from being lowered when image size is large or improves productivity when image size is small. 
     FIGS. 8A and 8B  are graphs comparing a printing time achievable with the illustrative embodiment that varies the running condition of the belt or second image carrier  10  (reverse movement and acceleration) and a printing time particular to a conventional printer. In  FIGS. 8A and 8B , the maximum size that can be transferred to the belt  10  is assumed to be the A 3  profile size while the belt  10  is assumed to move at a speed of 100 mm/sec. 
   As shown in  FIG. 8A , in the conventional printer, the printing time is fixed because a single print is produced by one full turn of a belt. Therefore, 8 seconds are necessary for images for size A 4  to be formed on both sides of a sheet. More specifically, 6 seconds are necessary even up to the end of transfer of the second image, i.e., 4 seconds for the belt to make one turn and 2 seconds for the formation of the second side. 
   By contrast, as shown in  FIG. 8B , the illustrative embodiment needs only about 5 seconds for forming toner images of size A 4  on both sides of a sheet. More specifically, it takes 2 seconds for the first side to be formed, 1 second for the belt  10  to be moved in the reverse direction, and 2 seconds for the second side to be formed. Further, when toner images of size A 6  are to be formed on both sides of a sheet with the belt  10  being moved in the reverse direction, it takes 1 second for the first side to be formed, 0.5 second for the belt  10  to be reversed, and 1 second for the second side to be formed, i.e., about 2.5 seconds in total. In this respect, in the conventional system, 5 seconds are necessary up to the end of image transfer, i.e., 4 seconds for one turn of the belt and 1 second for the formation of the second side. 
   As stated above, assuming that the maximum size that can be transferred to the belt  10  is the A 3  profile size, then the illustrative embodiment reduces the printing time when the image size is smaller than the A 4  landscape size. When the image size is the A 4  profile size or above, the above-described control that does not reverse the belt  10  should only be executed in accordance with the image size. 
   An alternative embodiment of the present invention will be described hereinafter. The alternative embodiment accelerates, after the transfer of a toner image from the first image carrier to the second image carrier, the second image carrier while moving it forward. This acceleration corresponds to varying of the running condition of the second image carrier. The illustrative embodiment is also practicable with any one of the configurations described with reference to  FIGS. 1 through 4 . Control particular to the illustrative embodiment will be described with reference to  FIGS. 9A through 9F , which correspond to  FIGS. 5A through 5F , respectively. In  FIGS. 9A and 9E , while the drum  1  and belt  10  are shown as being spaced from each other, they are, in practice, held in contact with each other. 
   First, as shown in  FIG. 9A , the charger  4  uniformly charges the surface of the drum  1  to negative polarity. The writing unit scans the charged surface of the drum  1  with the laser beam L to thereby form a latent image. The developing device  5  develops the latent image with negatively charged toner, which is represented by black dots in  FIG. 9A , thereby producing a corresponding toner image. Subsequently, the first image transferring means  21 , which is applied with a positive voltage, transfers the toner image from the drum  1  to the belt  10  (primary image transfer). 
   As shown in  FIG. 9B , the primary image transfer of the first toner image ends. Subsequently, as shown in  FIG. 9C , the belt  10  is released from the drum  1  in the direction Ki and then moved at a speed two times as high as the previous or usual speed. 
   As shown in  FIG. 9D , as soon as the belt  10  reaches the reference position, it is again moved at the usual speed and brought into contact with the drum  1  in the direction K 2 . On the other hand, a second toner image of negative polarity starts being formed on the drum  1 . The sheet P is driven by the registration roller pair  28  at such a timing that the first and second images are accurately positioned on the sheet P. 
   The movement of the belt  10  to the reference position can be sensed on the basis of a period of time to elapse since the exposure for the first toner image or the previously mentioned timing mark provided on the belt  10 . With this kind of scheme, it is possible to vary the belt speed and control belt movement. This can be done in terms of the number of steps in the case of a stepping motor. 
   As shown in  FIG. 9E , the first image transferring means  21 , which is applied with a positive voltage, transfers the second toner image of negative polarity from the drum  1  to the sheet P (secondary image transfer. At this instant, the first side of the sheet P is overlaid on the first, image carried on the belt  10 . 
   Finally, as shown in  FIG. 9F , the second image transferring means  22 , which is also applied with a positive voltage, transfers the first image of negative polarity from the belt  10  to the sheet P (tertiary image transfer). The belt  10  in movement conveys the sheet P carrying the first and second images thereon to the fixing position. The fixing means  18  and  30 B are heated, or turned on, to fix the first and second images on the sheet′P. At this instant, the belt cleaner  25  is pressed against the belt  10  for removing toner left on the belt  10 . In the specific configuration shown in  FIG. 3 , the sheet P separated from the belt  10  is conveyed to the fixing position. 
     FIGS. 10A through 10F  demonstrate another specific procedure available with the illustrative embodiment and uses the polarity switching device  50  like the procedure of  FIGS. 7A through 7F . The polarity switching device  50  is fixed in place. Again, while the drum  1  and belt  10  are shown as being spaced from each other, they are, in practice, held in contact with each other. 
   First, as shown in  FIG. 10A , the charger  4  uniformly charges the surface of the drum  1  to negative polarity. The writing unit scans the charged surface of the drum  1  with the laser beam L to thereby form a latent image. The developing device  5  develops the latent image with negatively charged toner, which is represented by black dots in  FIG. 10A , thereby producing a corresponding toner image. Subsequently, the image transferring means  21 , which is applied with a positive voltage, transfers the toner image from the drum  1  to the belt  10  (primary image transfer). While the belt  10  conveys the toner image forward, the polarity switching means  50  is applied with a positive voltage, or turned on, to switch the polarity of the toner image from negative to positive. 
   As shown in  FIG. 10B , when the trailing edge of the toner image moves away from the polarity switching device  50 , the entire toner image carried on the belt  10  has been inverted in polarity. Subsequently, as shown in  FIG. 10C , the belt  10  is released from the belt  10  in the direction K 1  and then moved at a speed two times as high as the previous or usual speed. 
   As shown in  FIG. 10D , when the belt  10  reaches the reference position, it is again moved at the usual speed and brought into contact with the drum  1  in the direction K 2 . On the other hand, a second toner image of negative polarity starts being formed on the drum  1 . The sheet P is driven by the registration roller pair  28  at such a timing that the first and second toner images are accurately positioned on the sheet P. 
   As shown in  FIG. 10E , the image transferring means  21 , which is applied with a positive voltage, transfers the toner image of positive polarity carried on the belt  1  and the second toner image of negative polarity formed on the drum  1  to the sheet P at the same time. 
   Finally, as shown in  FIG. 10F , the belt  10  in movement conveys the sheet P carrying the first and second images thereon to the fixing position. The fixing means  18  and  30 B are heated, or turned on, to fix the first and second images on the sheet P. At this instant, the belt cleaner  25  is pressed against the belt  10  for removing toner left on the belt  10 . In the specific configuration shown in  FIG. 3 , the sheet P separated from the belt  10  is conveyed to the fixing position. 
   In the procedure shown in  FIGS. 10A through 10  F, in the simplex print mode, a toner image is directly transferred from the drum  1  to the sheet P without the polarity switching device  50  being operated, i.e., in exactly the same manner as when two image transferring means are used. 
   In the procedure of  FIGS. 10A through 10F , when a toner image is transferred from the drum  1  to the sheet P by way of the belt  10  in the simplex print mode, the polarity switching device  50  is operated to invert the polarity of the toner image. Such image transfer is executed in the same manner as in the duplex print mode except that the second image is not formed on the drum  1 . 
   As stated above, after the transfer of the toner image to the belt or second image carrier  10 , the illustrative embodiment accelerates the movement of the belt  10  up to the reference position. This successfully reduces a period of time necessary for the belt  10  to complete one turn and therefore the image forming time. The acceleration of the belt  10  is effective not only in the duplex print mode but also in the simplex print mode. Stated another way, the illustrative embodiment improves productivity by varying the running condition of the belt  10 . 
   When a toner image of maximum size is to be transferred to the belt  10 , the illustrative embodiment does not accelerate the movement of the belt  10 . This is because when such a toner image is transferred to the belt  10 , the leading edge of the toner image reaches a position adjacent the secondary image transfer position when the trailing edge of the same is transferred from the drum  1  to the belt  10  or when it moves away from the polarity switching device  50 . 
   So long as the image size to be transferred to the belt  10  is smaller than the maximum size, which is the A 3  profile size or 420 mm in the direction of movement of the belt  10 , the illustrative embodiment accelerates the movement of the belt  10  without exception to thereby enhance productivity. For example, the illustrative embodiment reduces the printing time to 85% with the A 4  profile size, to 80% with the B 5  profile size, to 75% with the A 4  landscape size or to 65% with the A 6  landscape size, compared to the conventional apparatus. 
   A specific configuration for moving the belt  10  included in any one of the illustrative embodiments into or out of contact with the drum  1  will be described hereinafter with reference to  FIG. 11 . As shown, the belt unit  20  includes a box-like frame  51  supporting the belt  10  thereinside. The rollers  11  through  13  are journalled to the frame  51  while the belt  10  is passed over the rollers  11  through  13 . A tie bar or reinforcing member  51   b  connects the upper ends of opposite side walls of the frame  51 . The fixing roller  18 , image transfer roller  21  and so forth not relevant to the understanding of the specific configuration are not shown in  FIG. 11 . 
   A pulley  52  is mounted on one end of the roller  11  while a drive belt  54  is passed over the pulley  52  and a pulley mounted on the output shaft of a stepping motor  53 . The stepping motor  53  is selectively driven in the forward or the reverse direction to thereby drive the belt  10  in the forward or the reverse direction. The stepping motor  53  is independent of a motor assigned to the drum or first image carrier  1 . 
   The shaft of the roller  11  is journalled to the printer body or body frame, so that the belt unit  20  is angularly movable about the shaft of the roller  11 . Springs  56  constantly bias the frame  51  upward toward the drum  1  at the bottom of the roller  13 , thereby pressing the belt  10  against the drum  1  with preselected pressure. A member, not shown, included in the frame  51  abuts against a support member, which support the drum  1 , for thereby accurately positioning the belt  10  and drum  1  relative to each other. 
   Bosses  55  protrude sideways from the end of the frame  51  adjacent to the roller  13  and are received in notches  58  formed in a generally U-shaped yoke  57 . A shaft  59  extends throughout the intermediate portions of opposite side walls of the yoke  57  and is journalled to the body frame. A stub  60  protrudes from the end wall of the yoke  57 . A solenoid  61  is mounted on the body frame above the stub  60  and includes a plunger  62 . A spring  63  is anchored to the plunger  62  and stub  60  at opposite ends thereof. 
   In operation, when the solenoid  61  is energized, the plunger  62  thereof is retracted while causing the yoke  57  to angularly move counterclockwise about the shaft  59 , as indicated by an arrow M in  FIG. 11 . Consequently, the bosses  55  of the frame  51  are forced downward against the action of the springs  56  and causes the belt unit  20  to bodily move about the shaft  11  clockwise, as indicated by an arrow N in  FIG. 11 , thereby releasing the belt  10  from the drum  1 . When the solenoid  61  is deenergized, the plunger  62  is projected with the result that the belt unit  20  is moved in the direction opposite to the direction N by the springs  56 , again bringing the belt  10  into contact with the drum  1 . At this instant, the yoke  57  is, of course, moved in the direction opposite to the direction M. 
   Reference will be made to  FIGS. 12 and 13A  through  13 C for describing a specific mechanism for protecting the belt  10  from offset, i.e., preventing it from being dislocated sideways. In  FIG. 12 , structural elements identical with the structural elements of  FIG. 11  are not labeled. 
   As shown in  FIGS. 13A through 13C , the roller  12  over which the belt  10  is passed is slightly tiltable from the horizontal position. More specifically, a slot  51   a  is formed in the frame  51  through which one shaft  12   a  of the roller  12  extends, allowing the roller  12  to tilt. The other shaft  12   b  of the roller  12  is supported by the frame  51  via a bearing  64 . A lever  66  is connected to the shaft  12   a  via a bearing  65 . As shown in  FIG. 12 , the lever  66  is angularly movably supported by a shaft  67  protruding from the frame  51 . 
   Pins  68  and  69  are studded on opposite surfaces of the lever  66  at the end of the lever  66  remove from the roller  12 . A tension spring  70  is anchored to the pin  69  and frame  51  at its opposite ends, constantly biasing the pin  69  downward, i.e., biasing the lever  66  counterclockwise in  FIG. 12 . A solenoid  72  is mounted on the frame  51  via a bracket  71  and includes a plunger  73 . A hook  74  is connected to the lower end of the plunger  73  and anchored to the pin  69 . 
   When the solenoid  72  is deenergized, the pin  69  of the lever  66  is pulled downward by the tension spring  70  while pulling out the plunger  73 . Consequently, the lever  66  is angularly moved clockwise in  FIG. 12  to thereby lift the shaft  12   a , as shown in  FIG. 13A . In this condition, the roller  12  is slightly tilted from the horizontal position, i.e., raised at the shaft  12   a  side. Therefore, the belt  10  in turn tends to move toward the shaft  12   a  side of the roller  12 , as indicated by an arrow in  FIG. 13A . 
     FIG. 13B  shows the belt  10  shifted to the shaft  12   a  side. 
   As shown in.  FIG. 13C , when the solenoid  72  is energized, the plunger  73  is retracted while lifting the pin  68  against the action of the spring  70 , so that the lever  66  angularly moves clockwise in  FIG. 12 . As a result, the roller  12  is slightly tilted from the horizontal position, i.e., lowered at the shaft  12   a  side. In this condition, the belt  10  in turn tends to move toward the shaft  12   b  side, as indicated by an arrow in  FIG. 13C . 
   Further, a spot  75  is provided on one end portion of the roller  12  adjoining the shaft  12   a . A sensor  76  is mounted on the inner surface of the frame  51  and emits a light beam toward the spot  75 . When the belt  10  is shifted toward the shaft  12   a , the belt  10  hides the spot  75 . The resulting output of the sensor  76  indicates that the belt  10  has been shifted toward the shaft  12   a . In this case, the solenoid  72  is energized to slightly lower the shaft  12   a  side of the roller  12  for thereby correcting the offset of the belt  10 . 
   A spot and a sensor may also be located at the shaft  12   b  side of the roller  12 , in which case, the solenoid  72  will be turned on or turned off in accordance with two sensor outputs. 
   The offset of the belt  10  can be corrected without resorting the mechanism of  FIG. 12  if the belt  10  is moved in the reverse direction at a preselected timing over a preselected period of time. In any case, the offset of the belt  10  can be adequately controlled. 
   Some different configurations to which any one of the illustrative embodiments shown and described is applicable will be described hereinafter. 
     FIG. 14  shows a full-color image forming apparatus including an image forming section PU arranged substantially at the center of the apparatus body. In the image forming section PU, four image forming units SU are arranged side by side along and in contact with the lower run of an inclined, intermediate image transfer belt  60 . An optical writing unit  7  is positioned below the image forming sections SU. Because the image forming units SU are identical in configuration except for the color of toner, only one of them will be described with reference to  FIG. 15 . 
   As shown in  FIG. 15 , each image forming unit SU includes the drum  1  around which the drum cleaner  2 , discharger  3 , charger  4  and developing device  5  are arranged. The developing device  5  stores any one of cyan toner, magenta toner, yellow toner and black toner and develops a latent image formed on the drum  1 . The writing unit  7  scans the charged surface of the drum  1  with the laser beam L at the position between the charger  4  and the developing device  5 . More specifically, using conventional laser optics, the writing unit  7  forms the latent image on the drum  1  in accordance with image data corresponding in color to the toner stored in the developing device  5 . The laser optics may be replaced with an LED (Light Emitting Diode) array and focusing means, if desired. An image transfer roller  65  faces the drum  1  with the intermediary of the intermediate image transfer belt (simply belt hereinafter)  60 . The reference numeral  66  designates a back roller. The image transfer roller  65  transfers the toner image formed on the drum  1  to the belt  60 . 
   Referring again to  FIG. 14 , the belt  60  is passed over a drive roller  61  and a driven roller  62  and caused to turn counterclockwise by the drive roller  61 . Members disposed in the loop of the belt  60  except for the image transferring means are suitably grounded via the apparatus body. The belt cleaner  25  faces the driven roller  62  via the belt  60 . A toner replenishing section TS is positioned above the belt  60  and includes toner cartridges TC, i.e., a through d each storing toner of a particular color. Powder pumps, not shown, replenish the toner of different colors from the toner cartridges a through d to the developing devices. In a full-color print mode, a cyan, a magenta, a yellow and a black toner image formed on the drums  1  by the four image forming units SU, respectively, are sequentially transferred to the belt  60  one above the other, forming a full-color image. In a monochromatic print mode, only the image forming apparatus SU storing the black toner forms a monochromatic toner image; the toner image is transferred to the belt  60 . In the configuration shown in  FIG. 14 , among the four image forming units SU, the most downstream unit d stores the black toner in order to prevent productivity from being lowered in the monochromatic print mode. 
   Another intermediate image transfer belt or body  110  is positioned at the right-hand side of the image forming section PU. The intermediate image transfer belt (simply belt hereinafter)  110  is passed over rollers  111 ,  112 ,  113  and  115 . The roller  111  is a drive roller driven by a stepping motor independent of the motor assigned to the drum  1  and belt  60 , causing the belt  110  to turn. The belt  110  is angularly movable about the drive roller  111 , as indicated by a double-headed arrow K. A moving mechanism, which will be described later, so moves the belt  110  into or out of contact with the belt  60 . 
   The belt  10  is heat-resistant and provided with resistance that allows toner to be transferred to the belt  110 . A mark, not shown, is provided on the belt  110  for controlling the system. In the event of power-up, the mark on the belt  10  is optically sensed to bring the belt  110  to a preselected reference or initial position. 
   The image transfer roller or first image transferring means  21  is positioned between the opposite runs of the belt  110  in the vicinity of the roller  61  supporting the belt  60 . The heat roller  18 , back rollers  114  and  115  and a back plate BP are also arranged inside of the loop of the belt  110 . The roller  112  plays the role of cooling means at the same time. The members inside the loop of the belt  110  except for the image transferring means are suitably grounded via the apparatus body. A belt cleaner  250 , the charger or second image transferring means  22  and so forth are arranged outside of the loop of the belt  110 . The belt cleaner  250  assigned to the belt  110  includes a cleaning roller  250 A, a blade  250 B and toner conveying means  250 C and wipes off toner left on the belt  110  after the transfer of a full-color image to a sheet. The belt cleaner  250  is angularly movable about a fulcrum  250 D into or out of contact with the belt  110 . In  FIG. 14 , the roller  250 A is shown as being released from the belt  110 . More specifically, the belt cleaner  250  is released from the belt  110  when a toner image to be transferred to a sheet is present on the belt  110 , but brought into contact with the belt  110  when cleaning is required. 
   The image transfer roller  21 , back roller  115  and roller  61  supporting the belt  60  cooperate to press the belts  60  and  110  against each other for thereby forming a preselected nip for image transfer. The charger  22  is positioned outside of the loop of the belt  110  and faces the back plate BP, which is positioned above the image transfer roller  21 . 
   Two sheet cassettes  26 - 1  and  26 - 2  are positioned one above the other below the image forming section PU. The pickup roller  27  associated with designated one of the sheet cassettes  26 - 1  and  26 - 2  pays out the sheets P one by one toward the registration roller pair  28  via the guides  29 . 
   The fixing device  30  faces the heat roller  18  with the intermediary of the belt  110 . The fixing device  30  is angularly movable as in  FIG. 1  such that the fixing roller  19  selectively moves into or out of contact with the belt  110 .  FIG. 14  shows the fixing roller  19  in a position where it contacts the belt  110 . 
   The operation of the printer shown in  FIG. 14  will be described hereinafter. On the power-up of the printer, the belt  110  is brought to its reference or initial position on the basis of the mark provided thereon. 
   In the duplex print mode, a first toner image to be transferred to the first side of a sheet P is formed by the image forming section PU and then transferred from the belt  60  to the belt  110 , which is turning clockwise or forward. Subsequently, a second toner image is formed by the image forming section PU. At this instant, the second image transferring means  22 , fixing device  30  and belt cleaner  250  are released from the belt  110  or otherwise held inoperative so as not to disturb the toner image. 
   After the entire first toner image has been transferred from the drum  60  to the belt  110 , the belt  110  is reversed in the counterclockwise direction to the preselected position. The distance over which the belt  110  is reversed is controlled in terms of the number of steps of the stepping motor or drive means. In this specific configuration, the belt  110  is reversed at a speed two times as high as the speed of forward movement. The belt  110  is released from the belt  60  before the start of reverse movement. As soon as the belt  110  is returned to the preselected position, it is again brought into contact with the belt  60  and moved forward or clockwise. 
   On the other hand, a second toner image to be transferred to the second side of the same sheet P is formed by the image forming section PU. At the same time, the top sheet of designated one of the sheet cassettes  26 - 1  and  26 - 2  is paid out by the pickup roller  27  and conveyed toward the registration roller pair  28 . 
   The second toner image is transferred from the belt  60  to the second side of the sheet P conveyed by the registration roller pair  28  at the preselected timing. This image transfer is effected by the image transfer roller or first image transferring means  21  positioned inside of the loop of the belt  110 . At this time, the first image present on the belt  110  has been returned to the preselected position and is therefore overlaid on the first side of the sheet P. The sheet P carrying the second toner image on one side or second side and overlaid on the first image at the other side is conveyed by the belt  110  upward. The charger or second image forming means  22  transfers the first toner image from the belt  110  to the first side of the sheet P. 
   When the sheet P carrying the first and second toner images thereon reach the fixing device  30 , the fixing roller  19  and heat roller  18  fix the toner images on the sheet P. For this purpose, the fixing roller  19  is brought into pressing contact with the heat roller  18  via the belt  110 . Subsequently, the sheet P is separated from the belt  110  by curvature at the position where the roller  111  is located, and then driven out to the stack portion  40  by the roller pair  34 . The belt  110  is continuously turned forward even after the separation of the sheet P, so that the belt cleaner  250  cleans the surface of the belt  110 . 
   In the simplex print mode, a toner image formed by the image forming section PU is directly transferred from the belt  60  to a sheet P without the intermediary of the belt  110 . In this case, the belt  110  should only be turned forward in synchronism with the belt  60  without any reverse movement. 
   As stated above, a toner image formed by the image forming section PU is transferred from the belt  60  to either one of the sheet P and belt  110 . In this sense, the belts  60  and  110  play the role of the first and second image carriers, respectively. 
   Again, after the transfer of a toner image to the belt or second image carrier  110 , the belt  110  is reversed to the preselected position. It is therefore not necessary to wait until the belt  110  complements one full turn, promoting rapid image formation. Particularly, productivity is enhanced because the belt  110  is moved at a higher speed during reverse movement than during forward movement. 
   Assume that the maximum image size that can be transferred to the belt  110  is the A 3  profile size or 420 mm in the direction of rotation of the belt  110 . Then, the belt  110  is reversed if the image size is smaller than the A 4  landscape size or 210 mm, but is not done so if the image size is the A 4  landscape size or above. This successfully preserves high productivity when the image size is large or improves productivity when the image size is small. 
   In the specific configuration shown in  FIG. 14 , the image transfer roller or first image transferring means  21  is disposed in the loop of the belt  110  and applied with a charge opposite in polarity to the toner so as to transfer the toner by attraction. Alternatively, the first image transferring means may be disposed in the loop of the belt  60 , e.g., the roller  61  may be implemented as an image transfer roller and applied with a charge of the same polarity as the toner, in which case the toner will be transferred by repulsion. In this alternative arrangement, the roller  21  in the loop of the belt  110  may be implemented as a grounded back roller. 
     FIGS. 16A and 16B  show a specific configuration of the mechanism for moving the belt or second image carrier  110  into or out of contact with the belt  60 . As shown, the rollers over which the belt  110  is passed are journalled to a frame  120 , which is angularly movable about the shaft of the roller  111 . A spring  122  is loaded between the frame  120  and the printer body for constantly biasing the frame  120  clockwise, as viewed in  FIGS. 16A and 16B . A solenoid  121  is mounted on the printer body above the frame  120  and has a plunger connected to the frame  120 . 
   As shown in  FIG. 16A , when the solenoid  121  is deenergized, the belt is pressed against the belt  60  under the action of the spring  122 . As shown in  FIG. 16B , when the solenoid  121  is energized, it causes the frame  120  to angularly move counterclockwise away from the belt  60  against the action of the spring  22 . The belt  110  is held in the position of FIG.  16 B′when reversed at the higher speed. 
     FIGS. 17A and 17B  show another specific configuration of the moving mechanism. As shown, this moving mechanisms does not move the entire frame supporting the belt  110 , but moves only a belt support roller  115  with, e.g., a solenoid for thereby moving the belt  110  into or out of contact with the belt  60 . The image transfer roller  21  may be moved integrally with the belt support roller  115 , if desired. It is preferable to provide an arrangement that maintains the belt  110  under tension when the belt  110  is spaced from the belt  60 . 
     FIGS. 18A through 18C  show a specific mechanism for sensing the position of the belt  110  in a top plan view, a side elevation and a front view, respectively. As shown, timing marks  123   a  and  123   b  are provided on the outer surface of the belt  110  adjacent opposite edges of the belt  110  in the widthwise direction. The distance between the timing marks  123   a  and  123   b  is selected to be one-half of the circumferential length of the belt  110 . Sensors  124   a  and  124   b , which respectively sense the timing marks  123   a  and  123   b , adjoin the opposite edge portions of the belt  110  and face the portion of the belt  110  adjacent the image transfer roller  21 , but slightly above the roller  21 . The timing marks  123   a  and  123   b  are painted in a color different from the color of the surface of the belt  110 . The sensors  124   a  and  124   b  may be implemented as a reflection type photosensor each. 
   The timing marks  123   a  and  123   b  and sensors  124   a  and  124   b  are used to control the position of the belt  110 , i.e., movement to the reference or initial position and variation of the running condition. While the position of the belt  110  can be controlled with a single timing sensor and a single sensor, two timing marks  123   a  and  123   b  and two sensors  124   a  and  124   b  are successful to extend the life of the belt  110 . Particularly, in the configuration that reverses the belt  110  and when images of small sizes are frequently formed, the timing marks  123   a  and  123   b  spaced from each other by the previously stated distance prevent only the same portion of the belt  110  from being repeatedly used for thereby protecting the belt  110  from deterioration 
     FIG. 19  is a timing chart demonstrating the operation of the printer to occur in the duplex print mode. As shown, on the elapse of periods of time T 1   a , t 1   b , t 1   c  and t 1   d  since the sensor  124   a  or  124   b  has sensed the timing mark  123   a  or  123   b , the yellow, magenta, cyan and black developing sections  5   a  through  5   d  of the image forming unit SU, respectively, start development. On the elapse of a period of time t 2  since the sensing of the timing mark, primary image transfer is effected from the drums  1  of the image forming unit SU to the belt or first image carrier  60  by the image transferring means  65 . Further, on the elapse of a period of time t 3  since the sensing of the timing mark, secondary image transfer is effected from the belt  60  to the belt or second image carrier  110  by the image transferring means  21 . 
   After the secondary image transfer, the solenoid  121  of the moving mechanism is energized to release the belt  110  from the belt  60 . At the same time, the motor assigned to the belt  110  is stopped and then reversed at the higher speed. When the belt  110  is returned to the preselected position, as determined by sensing the timing mark  123   a  or  123   b , the above motor is stopped and then driven forward at the lower or usual speed. Such a procedure is repeated up to the last image. On the elapse of a period of time t 4  since the end of return of the belt  110 , the registration roller  28  is driven to convey a sheet. Subsequently, on the elapse of a period of time t 5 , tertiary image transfer is effected by the image transferring means  22 . 
   When the belt  110  is reversed, the same number of pulses as when it is moved forward are fed to the stepping motor, but within half a period of time, thereby doubling the belt speed. Such control over the stepping motor is demonstrated in  FIG. 20 . 
   While the configuration of  FIG. 14  uses the first embodiment that reverses the belt  110 , it may alternatively use the second embodiment that accelerates the belt  110  in the forward direction. In the first embodiment, a single image transferring means and a polarity switching device may be used to transfer images to both sides of a sheet at the same time, as described with reference to  FIGS. 6A through 6F  or  7 A through  7 F. This is also true with the second embodiment, as described with reference to  FIGS. 10A through 10F . Further, the fixing device may have the configuration shown in  FIG. 2  or  3 . 
   As shown in  FIG. 21 , the unit including the belt or second image carrier  110  is configured to be openable away from the printer body. The openable unit additionally includes the members and devices arranged inside of the loop of the belt  110  as well as the belt cleaner  250 . Upper one and lower one of the outlet rollers  34 , respectively labeled  34   a  and  34   b , are mounted on the openable unit and printer body, respectively. When the openable unit is opened away from the printer body, the sheet path extending from the sheet feed section to the outlet roller pair  34  is uncovered to facilitate access in the event of a jam. 
     FIG. 22  shows a modification of the configuration described with reference to  FIG. 14 . As shown, a fixing device  30 C is positioned outside of the loop of the belt  110 . The belt cleaner  250  assigned to the belt  110  differs in configuration and position from the belt cleaner  250  of  FIG. 14 . As shown in  FIG. 23 , the unit including the belt  110  is also configured to be openable away from the printer body. In the modification, the fixing device  30 C is mounted on the printer body and remains thereon when the openable unit is opened. 
     FIG. 24  shows another specific construction identical with the construction of  FIG. 14  or  22  except for the arrangement of the image forming section PU. As shown, the belt or first image carrier  60  is passed over three rollers  61 ,  62  and  63  in a triangular position. Four image forming units SU are arranged side by side along the lower run of the belt  60 . The optical writing unit  7  is located below the image forming units SU in a horizontal position. As for the rest of the configuration,  FIG. 24  is identical with  FIG. 22 . Again, the unit including the belt  110  is openable away from the printer body. 
   Referring to  FIG. 25 , a specific system including two printers connected to a host computer HC by a network will be described. The two printers each may have any one of the specific configurations shown in  FIGS. 14 ,  22  and  24 . The network may be either wired or wireless. Labeled OP in  FIG. 25  is an operation panel. 
   As best shown in  FIG. 14 , the printer of  FIG. 14 ,  22  or  24  includes a cover  40 A constituting the bottom of the stack portion  40  and openable about a shaft  40 B. As shown in  FIG. 25 , when the cover  40 A is opened, toner cartridges can be easily dealt with. Because the shaft  40 B adjoins the outlet roller pair  34 , prints stacked on the stack portion  40  are prevented from dropping even when the cover  40 A is opened. 
   As shown in  FIG. 25 , a door  67  mounted on the front of each printer is openable about its left edge for uncovering the image forming section PU in the event of, e.g., maintenance. The belt  60 , four image forming units SU and members arranged therearound constituting the image forming section PU can be pulled out of the printer body with the writing unit  7  being left on the printer body. Subsequently, the belt  60  and image forming units SU can be dismounted independently of each other. The image forming section PU is guided by guide rails, not shown, so that it can be easily, surely pulled out. The door  67  is hinged to the printer body in the vertical direction, making the members arranged in the lower portion to be easily seen in the event of maintenance. Moreover, sheets can be easily replenished to the sheet cassettes  26 - 1  and  26 - 2  even when the door  67  is open. A seal member, not shown, prevents the structural elements of the writing device  7  from being smeared by toner. A controller, not shown, allows the writing device  7  to selectively form a non-reversed image or a reversed or mirror image, as needed. 
   The sheet cassettes  26 - 1  and  26 - 2  each can be pulled out toward the front of the printer body for the replenishment or the replacement of sheets. In the printer shown in the right part of  FIG. 25 , the door  67  is opened while the sheet cassette  26 - 2  is pulled out. 
   A specific configuration of the printer including a plurality of first image carriers and a second image carrier movable into and out of contact with the first image carriers will be described hereinafter with reference to  FIG. 26 . Either one of the first and second embodiments described above may be applied to the configuration to be described. As shown in  FIG. 26 , the image forming section PU capable of forming a full-color image is located at substantially the center of the printer. Four image forming units SU are arranged side by side along the upper run of the belt  110 . The optical writing unit  7  is positioned above the image forming units SU. The image forming units SU are identical in configuration except for the color of toner. Each image forming unit SU is identical with the image forming unit shown in  FIG. 15  except for the positional relation between the structural elements. In  FIG. 26 , a group of image carriers made up of the four image forming units SU (a through d) constitutes a first image carrier in combination. It is to be noted that the first image carrier, or group of image carriers, may include any desired number of image forming units. For example, the black image forming unit may be omitted or may be combined with the red and blue image forming units. 
   In  FIG. 26 , the developing device  5  of each image forming unit stores one of cyan, magenta, yellow and black toner and develops a latent image formed on the associated drum with the toner. In the monochromatic print mode, only the image forming unit assigned to black forms an image. In the specific configuration shown in  FIG. 26 , the image forming unit SU-d located at the highest level or most downstream position is assigned to black so as to prevent an image from being disturbed by the other image forming units. 
   As shown in  FIG. 27  in detail, the first image transferring means  21  are arranged in the loop of the belt  110  for transferring toner images from the drums  0 . 1  to the belt  110  or transferring them directly to the upper surface of a sheet. The second image transferring unit for transferring a toner image from the belt  110  to the lower surface of the sheet is implemented as the charger  22  located downstream of the image forming unit SU-d. 
   The belt or second image carrier is passed over the rollers  111  through  114  and movable counterclockwise, as viewed in  FIG. 27 . Devices arranged inside of the loop of the belt  110  are suitably grounded via the printer body. A belt cleaner  250  faces the belt  110  at a position where the driven roller  113  is located. A moving mechanism, which will be described later, causes the belt  110  to selectively move about the shaft of the roller  111  into or out of contact with the in a direction K into or out of contact with the image forming units SU or first image carrier. 
   As shown in  FIG. 26 , the two sheet cassettes  261  and  26 - 2  are stacked one above the other in the lower portion of the printer body. The pickup roller  27  associated with designated one of the sheet cassettes  26 - 1  and  26 - 2  pays out the top sheet from the cassette. Electric units E 1  and E 2  are located above the sheet cassette  26 - 1 . A toner container  70  is positioned at the top right corner of the printer body. Toner is replenished from the toner container  70  to corresponding one of developing devices via a powder pump not shown. The top of the printer body constitutes the stack portion or print tray  40 . A fixing device  30 D is located downstream of the image forming unit SU-d assigned to black and uses a belt. As shown in  FIG. 27 , the belt  110  is mounted on a unit frame  67  angularly movable about the shaft of the roller  111 . An eccentric cam  68  is affixed to a shaft  69  and held in contact with the bottom of the frame  67 . When the cam  68  is caused to rotate, it moves the unit frame  67  in the direction K with the result that the belt  110  is angularly moved into or out of contact with the image forming units SU. The belt  110  may be angularly moved about the roller  112 , if desired. 
   More specifically, as shown in  FIG. 28 , two eccentric cams  68  are mounted on opposite ends of a shaft  69 . A joint  71  is affixed to the outside surface of one of the cams  68  located at the rear side of the printer body. The joint  71  is configured to receive projections formed on one end of a shaft  72 . A gear  73  is affixed to the other end of the shaft  72  and provided with a clutch  74 . The clutch  74  is selectively coupled or uncoupled to establish or interrupt, respectively, drive transmission from a motor, not shown, to the gear  73 . A photointerrupter  76  is so positioned as to sense a feeder portion  75  included in the joint  71 . 
   When the motor rotates the gear  73  via the clutch  74 , the shaft  69  and therefore the cams  68  are rotated via the shaft  72  and joint  71 , raising or lowering the unit frame  67 . At this instant, the photo interrupter  76  senses the feeler portion  75  of the joint  71  and therefore the position of the eccentric cams  68 . The position of the belt  110  is controlled in accordance with the output of the photointerrupter  76 . 
   In  FIG. 27 , the cams  68  in rotation cause the unit frame  67  to angularly move about the roller  111  in the direction K. Therefore, when each cam  68  is brought to a position indicated by a phantom line in  FIG. 27 , it raises the unit frame  67  and therefore the belt  110 . Consequently, the upper run of the belt  110  contacts the four image forming units SU-e through SU-d, i.e., the drums  1 , as indicated by a phantom line in  FIG. 27 . When the cam  68  is brought to a position indicated by a solid line in  FIG. 27 , the unit frame  67  and therefore the belt  110  is released from the image forming units SU-a through SU-d, as indicated by a solid line in  FIG. 27 . 
   In operation, in the full-color print mode, toner images formed in cyan, magenta, yellow and black on the drums  1  of the four image forming units or first image carrier SU are sequentially transferred to the belt  110  one above the other, completing a full-color image. In the monochromatic print mode, a black toner image is transferred from the image forming unit SU-d to the belt  110 . In any case, such image transfer is effected by the image transfer rollers or first image transferring means  21 . Of course, the belt or second image carrier  110  is held in contact with the drums  1  during image transfer. 
   In the duplex print mode, after the entire first toner image to be transferred to the first side of a sheet has been transferred to the belt  110 , the belt  110  is released from the image forming units or first image carrier SU and then reversed to a preselected position. The distance of reverse movement is controlled on the basis of the number of steps of the stepping motor assigned to the belt  110 . Again, the belt  110  is reversed at a speed two times as high as the speed of forward or usual movement. When the belt  110  reaches the preselected position, it is again brought into contact with the image forming units SU and caused to rotate forward, i.e., counterclockwise in  FIG. 26  at the usual speed. 
   On the other hand, a second toner image to be transferred to the second side of the same sheet is formed by the image forming units SU. At the same time, a sheet is fed from designated one of the sheet cassettes  26 - 1  and  26 - 2  toward the registration roller pair by the pickup roller  27 . The second toner image is transferred from the image forming units SU to the second side of the sheet. In the monochromatic print mode, a black toner image is transferred from the image forming unit SU-d to the sheet. In any case, the image transfer is effected by the image transfer rollers  21  disposed in the loop of the belt  110 . At this time, the, first toner image on the belt  110  has already been returned to the preselected position and is therefore overlaid on the first side of the sheet. While the sheet carrying the two images on both sides thereof is conveyed upward by the belt  110 , the charger or second image transferring means  22  transfers the first toner image from the belt  110  to the first side of the sheet. 
   As stated above, after one page of toner image has been transferred to the belt  110  in the duplex print mode, the belt  110  is reversed at the higher speed for thereby enhancing productivity. 
   In the simplex print mode, toner images are directly transferred from the image forming units SU to a sheet being conveyed by the belt  110  one above the other. To print an image on the lower side of a sheet, it suffices to transfer a toner image to the lower side of a sheet by way of the belt  110  by use of the charger or second image transferring means  22 . In this case, the reverse movement of the belt  110  effected at high speed enhances productivity. 
   Again, it is rather desirable to cause the belt  10  to simply complete one turn than to move it in the reverse direction, depending on the image size. For example, assume that the maximum image size that can be transferred to the belt  10  is the A 3  profile size. Then, the belt  10  is reversed for an image size smaller than the A 4  landscape size or continuously moved forward by one turn for an image of the A 4  landscape size or above. In any case, such control over the belt  10  prevents productivity from being lowered when the image size is large or improves productivity when the image size is small. 
   The configuration of  FIG. 27  including four image forming units arranged side by side reduces a period of time necessary for forming a full-color image, compared to the configuration that causes a single drum to make four full rotations. This, coupled with enhanced productivity implemented by the first or the second embodiment varying the belt running condition, realizes a printer achieving. a remarkable improvement in productivity in the full-color duplex print mode. 
   The configuration of  FIG. 27  may also include the polarity switching means  50  shown in  FIGS. 6A through 6F ,  7 A through  7 F or  10 A through  10 F. This allows a single image transferring means  21  to transfer images to both sides of a sheet although the image transferring means should be assigned to each image forming unit. 
   Further, the fixing device of  FIG. 3  using a heat roller may be positioned outside of the loop of the belt  110  or the fixing device of  FIG. 1  or  2  may be positioned inside of the loop of the belt  110 . In addition, the first image transferring means  21  may be implemented as a charger, if desired. 
   In any one of the illustrative embodiments shown and described, the speed of reverse movement of the belt is not limited to a speed two times as high as the usual speed, but may be a speed that is any suitable multiple of the usual speed. The distance of reverse movement of the belt may be controlled on the basis of the output of an encoder mounted on, e.g., the output shaft of a servo motor in place of the number of steps of a stepping motor. 
   The reference image sized used to selectively reverse the belt is not limited to A 4 , but may be suitably selected in accordance with the circumferential length, conveyance speed and speed of reverse movement of the belt as well as the configurations of the various devices. The moving mechanism for selectively moving the first and second image carriers into or out of contact with each other is open to choice. This is also true with the mechanism for correcting the offset of the belt. The offset correcting mechanism may be applied to the belt or second image carrier  110  shown in any one of  FIGS. 14 ,  22 ,  24  and  26  as well. 
   The drum may be replaced with a photoconductive belt in any one of the configurations shown in  FIGS. 1 ,  2 ,  3 ,  4  and  26  as well. The polarities of the drum, toner, image transfer voltage and so forth are only illustrative and may be reversed each. 
   The optical writing unit  7  may use an LED array in place of the laser optics or may even use an analog exposing system. In the case of an analog exposing system, a non-reversed image can be formed on the photoconductive element if a mirror is used. 
   Further, the configurations of the charging means, developing device, first and second image transferring devices, polarity switching device and fixing device shown and described are only illustrative. Of course, the present invention may be implemented as a copier or a facsimile apparatus, if desired. 
   In summary, it will be seen that the present invention provides an image forming apparatus having various unprecedented advantages, as enumerated below. 
   (1) Productivity is enhanced in both of the simplex and duplex print modes. Particularly, higher productivity is achievable at low cost in the full-color duplex print mode. 
   Images can be surely transferred to both sides of a sheet at the same time. 
   Drive means assigned to a second image carrier is independent of drive means assigned to a first image carrier, allowing the running condition of the second image carrier to be easily controlled. 
   When the running condition of the second image carrier is varied, the second image carrier can be accurately controlled, enhancing image quality. 
   Productivity is prevented from falling when image size is relatively large. 
   An image is free from disturbance during fixation and therefore high quality. 
   Jam processing and maintenance are easy to perform. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.