Patent Publication Number: US-6985679-B2

Title: Image forming apparatus with cleaning operation for special sheets

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus capable of forming an image, which has a function for forming an image on a special sheet such as a tab sheet by an electrostatic system or an electrophotographic recording system. 
   2. Related Background Art 
   A conventional color image forming apparatus employs a transfer mechanism in which an image developed on a photosensitive drum is transferred to an intermediate transfer belt, and the image is further transferred from the intermediate transfer belt to a recording sheet. In this transfer mechanism, the image on the intermediate transfer belt is pulled (or attracted) by a transfer roller provided on a back surface of the recording sheet, thereby transferring the image on the recording sheet. According to this transfer mechanism, since images can stably be transferred irrespective states of sheets such as warpage or end portions of sheets, the transfer mechanisms are widely employed in color image forming apparatuses. 
   According to such a transfer mechanism, however, when an image is transferred from the intermediate transfer belt by a transfer roller, if the recording sheet does not exist on a transfer portion, the image is adversely transferred onto the transfer roller and as a result, there is a problem that a back surface of a recording sheet which is fed to the transfer portion next is contaminated. To solve such a problem, in the case of recording sheets having regular sizes, it is possible to prevent the transfer roller from pulling the image when the recording sheet does not exist on the transfer portion by controlling voltage of the transfer roller. When an image is to be formed on a tab portion of a tab sheet (tab is located at a rear end of the tab sheet), it is necessary to maintain the voltage of the transfer roller so that the image is attracted toward the transfer roller at least until the rear end of the tab portion passes through the transfer portion. When the tab portion of the tab sheet exist on the transfer portion, however, the tab portion is sandwiched between the intermediate transfer belt and the transfer roller, but in the portion of the sheet other than the tab, there is nothing between the intermediate transfer belt and the transfer roller. Therefore, when an image whose tab position is not correct is placed on the intermediate transfer belt, there is a problem that the transfer roller is contaminated. When an original image is copied on a tab sheet, if the original is not a tab sheet but a sheet having a regular size, such a problem is prone to be generated. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide the following image forming apparatus. An image forming apparatus comprising: an image forming unit for forming an image on a recording sheet including a special sheet; and a controller for allowing the image forming unit to carry out an image forming sequence which forms an image on the recording sheet, wherein the controller allows the image forming unit to control a cleaning operation of the image forming unit in response to image formation on the special sheet by the image forming unit. 
   It is another object of the invention to provide the following image forming apparatus. An image forming apparatus comprising: an image forming unit for forming an image on a recording sheet including a special sheet; and a controller for sending the recording sheets at predetermined recording sheet distances from one another to the image forming unit, wherein the controller sends the recording sheets at longer recording sheet distances than the predetermined recording sheet distances to the image forming unit in response to image formation on the special sheet by the image forming unit. 
   It is another object of the invention to provide the following image forming apparatus. An image forming apparatus comprising: an image forming unit for forming an image on a recording sheet including a special sheet by an image-carrier; and a controller for controlling timing for allowing the image-carrier to carry images such that the image-carrier carries the images at predetermined image distances from one another, wherein the controller controls the timing for allowing the image-carrier to carry the images such that the image-carrier carries the images at longer image distances from one another than the predetermined image distances in response to image formation on the special sheet by the image forming unit. 
   It is another object of the invention to provide the following image forming apparatus. An image forming apparatus comprising: an image-carrier for carrying a developer; a transferring device for transferring the developer carried by the image-carrier to a recording sheet; and a controller which selectively carries out first control for controlling the transfer device such that the developer carried by the image-carrier is transferred to the recording sheet, and second control for controlling the transfer device such that a developer which adheres to the transfer device is transferred to the image-carrier, wherein when the developer is transferred to a special sheet, the controller controls the transfer device by the first control and then, controls the transfer device by the second control. 
   It is another object of the invention to provide the following image forming apparatus. An image forming apparatus comprising: an image-carrier for carrying images; a transfer device for transferring the images carried by the image-carrier to a recording sheet; and a controller for controlling a distance between a rear end of the image carried on the image-carrier and a leading end of a next image in accordance with a kind of the recording sheet. 
   Other features and advantages of the present invention will become apparent in the following description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view for explaining an image forming apparatus of a first embodiment of the present invention. 
       FIG. 2  is a block diagram for showing a structure of a control unit which controls the image forming apparatus. 
       FIGS. 3A ,  3 B and  3 C are views for explaining a liquid crystal screen provided on an operating unit of the image forming apparatus. 
       FIGS. 4A ,  4 B and  4 C are views for explaining the liquid crystal screen provided on the operating unit of the image forming apparatus. 
       FIGS. 5A and 5B  are views showing examples of originals and recording sheets on which images are to be copied in accordance with tab sheet insert settings. 
       FIGS. 6A and 6B  are views showing examples of originals and recording sheets on which images are to be copied in accordance with tab sheet preparing mode settings. 
       FIG. 7  is a flowchart for explaining control of a secondary transfer roller of the image forming apparatus. 
       FIG. 8  is a view for explaining an image forming apparatus of a second embodiment of the present invention. 
       FIG. 9  is a view for explaining a relation between a reflection tape of an intermediate transfer belt of the image forming apparatus and a toner image. 
       FIG. 10  is a view for explaining a state in which the intermediate transfer belt is developed and vertically arranged for each image formation. 
       FIG. 11  is a flowchart for explaining control of the secondary transfer roller of the image forming apparatus. 
       FIG. 12  is a flowchart for explaining image forming control steps of an image forming apparatus of a third embodiment of the present invention. 
       FIGS. 13A ,  13 B and  13 C are views for explaining a relation of original images and print positions of tab sheets in the image forming apparatus. 
       FIG. 14  is a flowchart for explaining control of image formation in an image forming apparatus of a fourth embodiment of the present invention. 
       FIG. 15  is a view for explaining a structure of an image forming apparatus of another embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be explained with reference to the drawings below. 
   (First Embodiment) 
     FIG. 1  is a sectional view for explaining an example of a structure of an image of a first embodiment of the present invention. In  FIG. 1 , the image forming apparatus is explained as an electrophotographic image forming apparatus. Reference numeral  1  represents the image forming apparatus, and the image forming apparatus generally comprises an image forming unit (four stations a, b, c and d are arranged in parallel, and structure thereof are the same), a sheet feeding unit, an intermediate transfer unit, a conveying unit, a fixing unit, an operating unit and a control unit. 
   These units will be explained in detail. First, the image forming unit has the following structure. That is, photosensitive drums  11   a ,  11   b ,  11   c  and  11   d  as image-carriers are rotatably supported at their axes, and they are rotated by a driving motor (not shown) in a direction shown with an arrow in  FIG. 1 . Roller chargers  12   a ,  12   b ,  12   c ,  12   d , scanners  13   a ,  13   b ,  13   c ,  13   d , developing devices  14   a ,  14   b ,  14   c ,  14   d  are disposed to be opposed to outer peripheries of the photosensitive drums  11   a  to  11   d  in the rotation direction of the photosensitive drums. Electric charges having uniform charge amount are given to surfaces of the photosensitive drums  11   a  to  11   d  by the roller charger  12   a  to  12   d  and then, the photosensitive drums  11   a  to  11   d  are exposed with light beams such as laser beams which are modulated according to a recording image signal, by the. scanners  13   a  to  13   d , thereby forming an electrostatic latent image there. 
   This electrostatic latent image is visualized by the developing devices  14   a  to  14   d  which accommodate four color developers (toner) such as yellow, cyan, magenta and black. This visualized visible image is transferred to an intermediate transfer body  30 . With such a process, images are formed by each toner in succession. 
   The sheet feeding unit comprises a section for accommodating recording sheets P, a roller for conveying the recording sheet P, a sensor for detecting the passage of the recording sheet P, a sensor for detecting the presence or absence of the recording sheet P, and a guide (not shown) for conveying the recording sheets P along a conveying path. In the drawing, reference numerals  21   a ,  21   b ,  21   c ,  21   d  represent cassettes, reference numeral  27  represents a manual feeding tray, and a deck  28  accommodates the recording sheets P. Reference numbers  22   a ,  22   b ,  22   c ,  22   d  represent pickup rollers for sending the recording sheets P one sheet by one sheet from the cassettes  21   a  to  21   d.    
   A plurality of recording sheets P may be sent out from the pickup rollers  22   a  to  22   d , but the recording sheets P are reliably sent one sheet by one sheet by BC rollers  23   a ,  23   b ,  23   c ,  23   d . One recording sheet P which was separated by the BC rollers  23   a ,  23   b ,  23   c ,  23   d  is further conveyed to a regist roller  25  by pulling-out rollers  24   a  to  24   d  and a regist-front roller  26 . Recording sheets P accommodated in the manual feeding tray  27  are separated one sheet by one sheet by a BC roller  29 , and are conveyed to the regist roller  25  by the regist-front roller  26 . A plurality of recording sheets P accommodated in the deck  28  are transferred to a sheet feed roller  61  by a pickup roller  60 , and they are reliably separated one sheet by one sheet by the sheet feed roller  61 , and are conveyed to a pulling-out roller  62 . The recording sheets P are further conveyed the regist roller  25  by the regist-front roller  26 . 
   Next, the intermediate transfer unit will be explained in detail. Reference numeral  30  represents an intermediate transfer belt, and polyethylene terephthalate (PET) or polyvinylidene fluoride (PVdF) is used as a material thereof. Reference numeral  32  represents a driving roller for transmitting the driving force to the intermediate transfer belt  30 . The driving roller  32  is supported by a tension roller  33  and driven rollers  34 . The tension roller  33  gives an appropriate tension to the intermediate transfer belt  30  by a biasing force of a spring (not shown). The driven rollers  34  sandwich the intermediate transfer belt  30  to form a secondary transfer area. The driving roller  32  has a metal roller surface coated with rubber (urethane or chloroprene) having several millimeters. The driving roller  32  prevents slip with respect to the belt. The driving roller  32  is rotated by a stepping motor (not shown). 
   Primary transfer rollers  35   a  to  35   d  for applying high voltage to transfer a toner image to the intermediate transfer belt  30  are disposed on a back side of the intermediate transfer belt  30  at a position where the photosensitive drums  11   a  to  11   d  and the intermediate transfer belt  30  are opposed to each other. A secondary transfer roller  36  is opposed to the driven roller  34 , and a secondary transfer area is formed by a nip between the roller  36  and the intermediate transfer belt  30 . 
   The secondary transfer roller  36  is pressed against the intermediate transfer body under an appropriate pressure. A cleaning device  50  for cleaning an image forming surface of the intermediate transfer belt  30  is disposed on the intermediate transfer belt  30  downstream of the secondary transfer area. The cleaning device  50  comprises a cleaner blade  51  (polyurethane rubber or the like is used as its material) and a waste toner box  52  for accommodating waste toner. 
   A fixing unit  40  comprises a fixing roller  41   a  having a heat source such as a halogen heater therein, a roller  41   b  pressed by the fixing roller  41   a  (the roller  41   b  includes a heat source in some cases), and a sheet discharge roller  44  for conveying recording sheets P discharged from the above pair of rollers. 
   The recording sheets P conveyed to the regist roller once stop the rotation of rollers existing upstream of the regist roller, and rotation of the upstream rollers including a regist roller  25  is started again with image forming timing of the image forming unit. The recording sheets P are sent to the secondary transfer area and the image is transferred to the secondary transfer area, the recording sheets P on which the image was fixed in the fixing unit  40  pass through the sheet discharge roller  44  and then, the sending destination is switched by a flapper  73 . 
   When the switching flapper  73  is on a face up sheet discharge side, the recording sheets P are discharged to a face up sheet discharge tray  2  by a sheet discharge roller  45 . When the switching flapper  73  is on a face down sheet discharge side, the recording sheets P are sent toward inversion rollers  72   a ,  72   b ,  72   c , and discharged to a face down sheet discharge tray  3 . A plurality of sensors for detecting the passage of the recording sheets P are disposed on a conveying path of the recording sheets P. Examples of such sensors are sheet feed retry sensors  64   a ,  64   b ,  64   c ,  64   d , a deck sheet feed sensor  65 , a deck pulling-out sensor  66 , regist sensor  67 , a sheet discharge sensor  68 , a face down sheet discharge sensor  69 , a dual side pre-registration sensor  70 , a dual side sheet refeed sensor  71  and the like. 
   Cassette sheet-presence or absence sensors  63   a ,  63   b ,  63   c ,  63   d  for detecting the presence or absence of the recording sheets P are disposed on the cassettes  21   a  to  21   d  which accommodate the recording sheets P. A manual feeding tray sheet-presence or absence sensor  74  for detecting the presence or absence of the recording sheets P on the multi-manual feeding tray  27  is disposed in the multi-manual feeding tray  27 . A deck sheet-presence or absence sensor  75  for detecting the presence or absence of the recording sheets P in the deck  28  is disposed in the deck  28 . 
   The control unit comprises a control substrate (not shown) for controlling the operation of mechanisms in the above-described units and a motor drive substrate (not shown). 
   An operating unit  4  is disposed on an upper surface of the image forming apparatus  1 . An operating unit  4  can select sheet feeding units (the sheet feed cassette  21   a  to  21   d , the manual feeding tray  27 , the deck  28 ), and sheet discharge trays (the face up tray  2 , the face down tray  3 ), and can designate tab sheets. 
   Next, the operation of the apparatus will be explained. As one example thereof, a case in which recording sheets P are conveyed from the cassette  21   a  will be explained. 
   After a predetermined time is elapsed after an image forming operation starting signal is sent, recording sheets P are first sent out one sheet by one sheet from the cassette  21   a  by the pickup roller  22   a . The recording sheets P are conveyed to the regist roller  25  through the pulling-out roller  24   a  and the regist-front roller  26  by the sheet feed roller  23   a . At that time, the regist roller  25  is stopped, and a leading end of the sheet abuts against the nip portion. Thereafter, the regist roller  25  starts rotating when the image forming unit starts forming an image. This rotation timing is set such that the recording sheet P and a toner image which is primary transferred onto the intermediate transfer belt  30  by the image forming unit just coincide with each other in the secondary transfer area. 
   In the image forming unit, if the image forming operation starting signal is sent, a toner image formed on the photosensitive drum  11   d  which is located most upstream in the rotation direction of the intermediate transfer belt  30  is primary transferred, by the above-described process, to the intermediate transfer belt  30  in a primary transfer area by the primary transfer roller  35   d  to which high voltage is applied. The primary transferred toner image is conveyed to a next primary transfer area. In the next primary transfer area, image formation is executed with a delay corresponding to time for conveyance of the toner image between the image forming units, and the leading end of the image is aligned on a former image and a next toner image is transferred. Thereafter, the same steps are repeated, and a toner image having four colors is finally primary transferred onto the intermediate transfer belt  30 . 
   Then, if the recording sheets P approach the secondary transfer area and come into contact with the intermediate transfer belt  30 , high voltage is applied to the secondary transfer roller  36  in accordance with a passing timing of the recording sheets P. Then, the four color toner image formed on the intermediate transfer belt  30  by the above process is transferred to a surface of the recording sheet P. Thereafter, the recording sheets P are guided to the nip portion of a pair of fixing rollers  41   a ,  41   b , the toner image is fixed on the sheet surface by heat of the pair of rollers  41   a  and  41   b  and a pressure of the nip, and the recording sheets P are discharged to the face up sheet discharge tray  2  or the face down tray  3  in a switching direction of the switching flapper. 
     FIG. 2  is a block diagram showing a structure of the control unit which controls the image forming apparatus  1 . The control unit comprises a CPU  201 , an image reader control unit  202 , an image signal control unit  203 , a printer control unit  204 , a ROM  205 , a RAM  206 , and an operation panel control unit  207 . 
   By executing a program stored in the ROM  205 , the CPU  201  controls an original reading unit (not shown) through the image reader control unit  202 . Reference numeral  203  represents an image signal control unit. The image signal control unit  203  accumulates image data of an original read by the original reading unit (not shown) or image data input to the image signal control unit through a network, and outputs print data to the printer control unit  204 . 
   Next, when an image from the image reading unit is formed (copied) by the image forming apparatus  1  shown in  FIG. 1  and a tab sheet insert mode and a tab sheet preparing mode using tab sheets, setting in the operating unit  4  is conducted as hereinbelow. In this embodiment, the image forming apparatus  1  is explained as a copier, but the image forming operation can be carried out in the same manner even if the operation is a printing operation from a network. 
     FIGS. 3A to 3C  and  4 A to  4 C are views for explaining liquid crystal screen provided on the operating unit  4 .  FIG. 3A  shows a standard screen on which basic settings of copy function are carried out. 
   If an icon  1201  of an application mode is pressed down, a sub-window for carrying out various mode settings is opened. A trial copy icon  1203  is used when there exist a plurality of copy images and only the first one is to be copied on a trial basis. 
   A key group  1204  for determining an image mode, selects image modes such as “auto density correction”, “char mode”, “char/picture mode”, “picture mode” and the like. When the “char mode”, “char/picture mode” or “picture mode” is selected, density can be changed by “light” or “dense” key. 
   In a key group  1205  for changing magnification of an original, a 100% “equal size” key, a “reduce” key, an “enlarge” key, a 1% by 1% “zoom” key, and a “smallish” key used for copying the entire original are included. In any cases, the key group  1205  are for changing magnification of the original, and after setting the magnification, the set magnification is indicated. 
   A “sorter” key  1206  is for setting the finishing after a sheet is output. For example, a sort mode used for changing one-volume output for a plurality of originals to plural-volume output, a group sort for outputting the set number of volumes for each page of the originals, and a staple mode for stapling the sheets output in the sort mode on volume basis are included. 
   A “dual side” key  1207  sets a dual side mode for reading or outputting the original or sheet on one side. A “one side/dual side” mode for reading one side of an original and outputting the information on dual side, a “dual side/dual side” mode for reading dual side of the original and outputting the information on dual side, a “dual side/one side” mode for reading dual side of the original and outputting the information on one side, and a page spread copy dual side mode for dividing one side of the original into two and outputting the information on dual side of a reading sheet, are included. 
     FIG. 3A  shows an indication example in which the one side/dual side mode in the dual side mode is set, and balloons of “one side/dual side” on an upper portion of the dual side icon are indicated. 
   As a favorite function key  1208  which can use an arbitrary function like a shortcut, in  FIG. 3A , the setting when one side copy is carried out is indicated, and if this key is pressed down, a one side copy mode is set. 
     FIG. 3B  shows a sub-window indicated when a “sheet type select” key  1202  shown in  FIG. 3A  is pressed down. In the sub-window, a sheet feed stage to be selected from the cassette decks  21   a ,  21   b ,  21   c ,  21   d , the deck  28  and the multi-manual feeding tray  27  of the image forming apparatus body  1  is set. 
   At that time, an icon  1211  shows the multi-manual feeding tray  27 , an icon  1212  shows the cassette  21   a , and A4 sheets are inserted currently. An icon  1213  shows the cassette  21   b , and A5 sheets are currently inserted. An icon  1214  shows that the cassette  21   c  is set as a tab sheet feeding stage, and this indicates that A4 tab sheets are currently inserted. According to such indication, a user can recognize that if he or she should set the tab sheets to which cassette, or the tab sheets are set to which cassette. An icon  1215  shows the cassette  21   d , and this indicates that A3 sheet is currently set but there is no sheet. An icon  1216  indicates that A4 sheets are currently inserted to the deck  28 . An icon  1217  is an auto feed selection key, and this key automatically detects appropriate sheets based on an original size and a set copy mode when the auto sheet is selected, and if such appropriate sheets could not be detected, a second candidate sheet size is selected. 
     FIG. 3C  shows a sub-window which appears when the application mode key  1201  is pressed down, and function icons for setting various copy modes are indicated. A “page spread copy” key  1221  executes a mode in which an original is recognized as a double-page spread, the spread is divided into two, and the information is read as two originals. 
   A “cover/partition sheet” key  1222  selects a sheet feed stage for a front sheet, a back sheet, or a partition sheet, and outputting one copy. 
   A “bookbind mode” key  1223  executes a mode for folding a recording sheet at a center thereof in a case of copying an original and for outputting the information as a double-page spread book. 
   A mode memory key  1225  is a key for collectively storing various mode settings such as the application mode, the sort mode set in the key  1206 , variable magnification mode set in the key  1205 , and the image mode set in the key  1204 . 
   An “OHP inset” key  1226  is a key for setting a mode in which a transparency (OHP sheet) for OHP from the multi-manual feeding tray  27  is fed, printed, or the same image is printed on a normal sheet from another sheet feed stage when OHP sheets are set by the multi-manual feeding tray  27  or the like, or the OHP transparency inset sheet which is not printed and output is inserted between OHP transparencies. 
   A “mixed original” key  1227  executes a mode in which a plurality of kinds of originals are mounted in an original auto feeder (not shown) which automatically feeds originals to the image reading unit, and A4 sheets and A3 sheets are mixed. 
   An “image synthesizing” key  1228  registers a background image, or superpose a previously registered background image on an original and outputting the same. 
   A “tab sheet insert” key  1229  executes a tab sheet insert mode in which when there exists a tab sheet feeding stage as in the icon  1214  in the cassette  21   c  shown in  FIG. 3B , a tab sheet is used as a dividing sheet such as the partition sheet, and an image is moved and printed on a tab based on a copy original corresponding to a tab portion. 
   A “tab sheet print” key  1230  executes a mode in which a tab sheet is fed, an image on an end of an original without tab is moved in a direction opposite from a sheet conveying direction, and printed on a tab portion of the tab sheet when there exists a tab sheet feeding stage like the icon  1214  of the cassette  21   c  shown in  FIG. 3B . 
   A “call” key  1231  executes a mode for calling the setting at the time of last copy. 
   A “shift” key  1232  executes a mode for setting how a sheet on which an original image is to be output should be shifted. 
   A “binding margin” key  1233  executes a mode for moving an image for preparing a margin on a sheet on which an original image is to be output. 
   A “frame erase” key  1234  executes a mode for erasing an outer frame of an original image or erasing an image on an outer frame of a sheet at the time of printing. 
   A “reduction layout” key  1235  executes a mode which is set when a plurality of originals are laid out and printed on one sheet. 
   An “enlargement layout” key  1236  executes a mode for dividing and printing a plurality of images on one sheet prepared by the reduction layout or the like. 
   A “close” key  1237  is a key for closing the sub-window in the application mode and for returning to the standard screen shown in  FIG. 3A . 
     FIG. 4A  shows a sub-window which is opened when the tab sheet insert key  1229  or the tab sheet copy key  1230  is pressed down. In the sub-window, the number of tabs (or the divisional number by tabs) is input. In general, a 5-tab-sheet is set, that is five tab sheets form one set. But in the above embodiment, from 2-tab-sheet in which two tab sheets form one set to 12-tab-sheet in which 12-tab-sheet form one set can be handled. 
   Here, “the number of tabs” is the number of tab sheets included in one set. In an area  1301 , the number of tabs which is to be set currently is shown. The number of tabs can be increased or decreased by a “minus” key  1302  or a “plus” key  1303 . 
   An “OK” key  1304  is a key for setting the input number of tabs. 
   A “cancel” key  1305  is pressed down when the sub-window for inputting the number of tabs to the sub-window of the application mode without setting the number of tabs. 
     FIG. 4B  shows a sub-window for setting a shift amount of a tab image after the “OK” key  1304  shown in  FIG. 4A  is pressed down. That is, the shift amount is an amount for shifting the tab image so as to print an image to be printed in a tab portion, included in an original, on the tab portion. 
   Usually, on an A4 size tab sheet, it is enough to shift a tab image by about 12 mm, but in the above embodiment, since the shift amount is varied in accordance with a size of characters to be printed on the tab or a position on the original, it is possible to set such that the tab image can be shifted by a width from 0 to 25 mm. 
   A “minus” key  1311  is a key for reducing 12 (mm) which is a normal shifting amount of a tab image, and a “plus” key  1312  is a key for increasing 12 (mm) which is the normal shifting amount of the tab image. 
   An indication section  1313  is a section where a shift amount to be set currently is indicated. 
   A “cancel” key  1314  is a key for canceling the setting of the shift amount and for returning to the input screen ( FIG. 4A ) for setting the number of tabs. 
   An “OK” key  1315  is a key for reflecting a value indicated in the indication section  1313  at the time of printing. 
   Next, as shown in  FIG. 4C , a page (tab sheet inserting page) which indicates a position where a tab sheet is inserted is set. 
   In an insert page window  1321  showing a page where the tab sheet is inserted, when a first tab sheet is selected, “first sheet” is pressed down, and a page number in which the tab sheet is to be inserted is input using a numeric keypad (not shown) on the operating unit. 
   In the above embodiment, as shown in  FIG. 4C , an inserting position of the first tab sheet is set before page  2 . Similarly, an inserting position of a second tab sheet is set before page  3 , an inserting position of a third tab sheet is set before page  4 , an inserting position of a fourth tab sheet is set before page  5 , and an inserting position of a fifth tab sheet is set before page  8 . 
   A “cancel” key  1322  is a key for canceling an inserting position where a tab sheet is inserted, and returning to the sub-window ( FIG. 4B ) for setting the shift amount of an image of the tab image. 
   A scroll up key  1323  is for scrolling up the inserting page window  1321  when the number of pages where tab sheets are inserted is seven or more. 
   A scroll down key  1324  is for scrolling down the inserting page window  1321  when the number of pages where tab sheets are inserted is seven or more. 
   An “OK” key  1325  is an “OK” key for reflecting the inserting page of a tab sheet set by the inserting page window on a copy job, and with this, the setting of the tab sheet insert mode is completed. 
   With this setting, in a copy job, it is determined to which portion of one set of sheets a tab sheet should be inserted, how much the image shift amount should be set when information is printed on a tab portion, and on which page the tab sheet should be fed, based on the number of tabs, the tab image shift amount and the inserting page of the tab sheet which are set in  FIGS. 4A to 4C . 
     FIGS. 5A and 5B  show examples of an original ( FIG. 5A ) and a recording sheet ( FIG. 5B ) which are copied in accordance with the tab sheet insert key  1229  and the tab sheet insert setting which is set in  FIGS. 4A to 4C . At the time, although a size of the original and a size of a sheet coincide with each other, a size of a tab original sheet and a size of a tab sheet are different only in a tab portion. 
   A tab original sheet  1401  is an original which is sandwiched between originals  1402  and which is printed (image is formed) on a tab sheet. A recording sheet which is output in corresponding to this tab original is shown in  FIG. 5B . An image which was printed on the tab original sheet  1401  is shifted by a shift amount set in  FIG. 4B  and is printed on a tab portion of a tab sheet  1411 . 
     FIGS. 6A and 6B  show examples of originals (tab original sheets:  FIG. 6A ) which are to be copied by the tab sheet preparing by  1230  and the tab sheet preparing mode setting which is set in  FIGS. 4A and 4B , and recording sheets (output tab sheets:  FIG. 6B ) which are copied. Although a size of the original and a size of a sheet coincide with each other, a size of a tab original sheet and a size of a tab sheet are different only in a tab portion. Recording sheets which are output in corresponding to the different size are shown in  FIG. 6B . An image printed on the tab original sheet shown in  FIG. 6A  is moved by a shift amount set in  FIG. 4B  and is printed on a tab portion of the tab sheet. 
   Next, control of the secondary transfer roller  36  for transferring an image of the intermediate transfer belt  30  onto a recording sheet according to the embodiment will be explained. 
     FIG. 7  is a flowchart for explaining the control of the secondary transfer roller  36 . The processing is called at regular time intervals or if necessary by main sequence. A program for executing this flowchart is stored in the ROM  205 , and is read out and executed by the CPU  201 . 
   First, in step S 1000 , it is judged whether or not an image forming operation is started. If it is judged that the image forming operation is not started in step S 1000  (S 1000 : No), the procedure is returned to step S 1000  to wait until the image forming operation is started. If it is judged that the image forming operation is started in step S 1000  (S 1000 : Yes), the procedure is proceeded to step S 1001 . 
   In step S 1001 , a pre-processing for the image forming operation is carried out. This pre-processing establishes a state in which an image can be transferred to a recording sheet by the secondary transfer roller  36 . For example, predetermined voltage is applied to the secondary transfer roller  36 , flowing current is measured to determine transfer voltage, cleaning sequence of the secondary transfer roller  36  which will be described later is executed, and standby voltage before the image is transferred is output. Then, the procedure is proceeded to step S 1002 . 
   In step S 1002 , it is judged whether or not an image formed by the photosensitive drums  12   a  to  12   d  and placed on the intermediate transfer belt  30  by the primary transfer rollers  35   a  to  35   d  has reached a position of the secondary transfer roller  36 . In step S 1002 , if it is judged that the image on the intermediate transfer belt  30  has not yet reached the position of the secondary transfer roller  36  (step S 1002 : No), the procedure is returned to step S 1002  to wait until the image reaches. If it is judged that the images has reached the position of the secondary transfer roller  36  in step S 1002  (S 1002 : Yes), the procedure is proceeded to step S 1003 . 
   In step S 1003 , transfer voltage is output as voltage of the secondary transfer roller  36  for transferring an image from the intermediate transfer belt  30  to a recording sheet. When it is judged whether or not the image has reached the position of the secondary transfer roller  36  in step S 1002 , it is possible to fast judge,the response time of the transfer voltage output in step S 1003 . After the transfer voltage of the secondary transfer roller  36  is brought into image transfer voltage in step S 1003 , procedure is proceeded to step S 1004 . 
   In step S 1004 , it is judged whether or not an image length transferred to the intermediate transfer belt  30  has passed through the secondary transfer roller  36 . If it is judged that the length has not yet passed in step S 1004  (S 1004 : No), the procedure is returned to step S 1004  to wait until the length passes. If it is judged that the length has passed in step S 1004  (S 1004 : Yes), the procedure is proceeded to step S 1005 . 
   In step S 1005 , it is judged whether or not the recording sheet on which the image was transferred by the secondary transfer roller  36  is a tab sheet. If it is judged that the recording sheet is not the tab sheet in step S 1005  (S 1005 : No), the procedure is proceeded to step S 1008 . If it is judged that the recording sheet is the tab sheet in step S 1005  (S 1005 : Yes), the procedure is proceeded to step S 1006 . 
   In step S 1006 , it is judged whether or not it was set to form the image on a tab portion of the tab sheet on the basis of the setting of the shift amount of the image as explained in  FIGS. 4A to 4C , the size of the original image and the size of the tab sheet. If it is judged that it was set not to form the image on the tab portion in step S 1006  (S 1006 : No), the procedure is proceeded to step S 1008 . If it was set to form the image on the tab portion in step S 1006  (S 1006 : Yes), the procedure is proceeded to step S 1007 . 
   In step S 1007 , cleaning processing of the secondary transfer roller  36  is carried out. In the cleaning processing of the secondary transfer roller  36 , voltage having polarity opposite from that when the image of the intermediate transfer belt  30  is transferred to the recording sheet is applied to the secondary transfer roller  36  and in this state, if it is held for a predetermined time (e.g., time required for the secondary transfer roller  36  to rotate at least one revolution), toner is transferred from the secondary transfer roller  36  to the intermediate transfer belt  30 . Toner adhered to the intermediate transfer belt  30  is cleaned by the cleaning device  50 . 
   A time period during which voltage having the opposite polarity is applied to the secondary transfer roller  36  may be changed in accordance with a length of the image to be placed on the tab portion, i.e., the shift amount explained in  FIGS. 4A to 4C , the size of the original and the size of the tab sheet. 
   In a case the cleaning processing of the secondary transfer roller  36  is carried out, when an image is formed on a tab portion of the tab sheet, a space in which the cleaning processing can be carried out is left between an image to be formed on the tab sheet and an image to be formed on the recording sheet which is supplied after the tab sheet, and the next image is formed. In other words, in a case the cleaning processing of the secondary transfer roller  36  is carried out, when an image is formed on a tab portion of the tab sheet, an enough distance for carrying out the cleaning processing is left between a rear end of the tab sheet and a leading end of a recording sheet which is supplied after the tab sheet, and a next recording sheet is supplied. 
   If the cleaning processing of the secondary transfer roller  36  is completed in step S 1007 , the procedure is proceeded to step S 1008 . 
   In step S 1008 , voltage of the secondary transfer roller  36  is changed to voltage (voltage which does not attract toner from the intermediate transfer belt  30  to the secondary transfer roller  36 ) for sheet spacing (a state in which transferring unit has not recording sheets), and the procedure is proceeded to step S 1009 . 
   In step S 1009 , it is judged whether or not an image transferred to the recording sheet is a final image of image forming. If it is judged that the image is not final in step S 1009  (S 1009 : No), the processing is returned to step S 1002  where the following steps are repeated. If it is judged that the image is final (S 1009 : Yes), the procedure is proceeded to step S 1010 . 
   In step S 1010 , post-processing for completing the image forming operation is carried out. In this post-processing, the cleaning sequence of the secondary transfer roller  36  is carried out, voltage of the secondary transfer roller  36  is turned off and the flow is proceeded to the exit. 
   In this embodiment, control is switched, when the sheet is a tab sheet and further an image is formed on the tab sheet. But the same effect can be obtained even if the control is switched when the sheet is the tab sheet. 
   Although the procedure is proceeded to step S 1007  if it is judged that it is a tab sheet in step S 1005 , and that an image is set to be formed on the tab portion, if it is judged that it is a tab sheet in step S 1005 , the procedure may be proceeded to step S 1007  (without through step S 1006 ), and if it is judged that it is not the tab sheet, the procedure may be proceeded to step S 1008 . In this case, the same effect can be obtained with easier judgment. 
   Further, in this embodiment, when images are formed on tab sheets, a distance between images is set larger than a distance between images when the images are formed on recording sheets other than tab sheets, a distance between recording sheets is set larger than a distance between recording sheets when the images are formed on the recording sheets other than the tab sheets, and the cleaning operation is carried out. However, the present invention is not limited to the tab sheets, and the invention can also be applied to a case in which an image is formed on a special sheet such as a pre-punch sheet which is previously formed with a hole and a non-rectangular sheet other than a rectangular sheet. That is, the present invention also includes such a structure that when images are formed on special sheets such as the pre-punch sheets or non-rectangular sheets, a distance between images is set larger than a distance between images when the images are formed on recording sheets other than tab sheets, a distance between recording sheets is set larger than a distance between recording sheets when the images are formed on the recording sheets other than the tab sheets and the cleaning operation is carried out. 
   Although the cleaning of the secondary transfer roller  36  is carried out by applying voltage having opposite polarity to the secondary transfer roller  36  in this embodiment, the cleaning may be carried out by a mechanical cleaning operation. 
   In the embodiment, an image formed by the photosensitive drum  35   a  to  35   d  is once transferred to the intermediate transfer belt  30  and is again transferred to the recording sheet. Alternatively, even if the image is directly transferred to the recording sheet from the photosensitive drum, the same effect can be obtained by carrying out the cleaning operation of the transferring roller  36  which presses the recording sheet against the photosensitive drum. 
   (Second Embodiment) 
     FIG. 8  is a view for explaining an image forming apparatus of a second embodiment of the present invention.  FIG. 8  shows only a structure portion (portion surrounded by a broken line in  FIG. 1 ) which is different from the image forming apparatus  1  having a structure explained in the first embodiment. 
   In this image forming apparatus, a photosensitive drum  101  as an image-carrier is provided so that the photosensitive drum  101  can be rotated by a motor (not shown) in a direction shown with an arrow A in  FIG. 8 . A primary charger  107 , an exposure device  108 , a developing unit  113 , a transfer charger  110  and a cleaner device  112  are disposed around the photosensitive drum  101 . A cleaning charger  111  is disposed in front of the cleaner device  112 . 
   The developing unit  113  comprises four developing devices  113 Y,  113 M,  113 C,  113 K for developing in full color. The developing devices  113 Y,  113 M,  113 C,  113 K develop a latent image on the photosensitive drum  101  using toners of Y, M, C and K. When the image is developed using toner of respective colors, the developing unit  113  is rotated by a motor (not shown) in a direction of an arrow R in  FIG. 8 , and developing devices having these colors are positioned such that they abut against the photosensitive drum  101 . 
   Respective colored toner images developed on the photosensitive drum  101  are transferred on an intermediate transfer belt  102  as an intermediate transfer body by the transfer device  110  in succession, and four color toner images are superposed on the intermediate transfer belt  102 . 
   The intermediate transfer belt  102  is wound around rollers  117 ,  118  and  119 . Of the rollers, the roller  117  is coupled to a driving source (not shown) and functions as a driving roller which drives the intermediate transfer belt  102 . The roller  118  functions as a tension roller for adjusting tension of the intermediate transfer belt  102 . The roller  119  functions as a backup roller of a transfer roller  121  as a secondary transfer device. 
   The belt cleaner  112  is provided at a position opposed to the roller  117  with the intermediate transfer belt  102  interposed therebetween. Toner residue on the intermediate transfer belt  102  is scraped off by a blade  122   a.    
   The recording sheets are supplied to a nip portion, i.e., an abutting portion between the secondary transfer device  121  and the intermediate transfer belt  102  by a pair of rollers  125  and  126 . A toner image formed on the intermediate transfer belt  102  is transferred onto a recording sheet by the nip portion of the secondary transfer device  121 . 
   In the color image forming apparatus having such a structure, an image is formed in the following manner. First, voltage is applied to the charging device  107  so that a surface of the photosensitive drum  101  is negatively charged uniformly with predetermined potential at charging portion. Then, exposure is carried out by an exposure device  108  comprising a laser scanner so that the image portion on the charged photosensitive drum  101  becomes exposure portion potential, thereby forming a latent image. The exposure device  108  is turned ON and OFF based on an image signal to form a latent image corresponding to an image. 
   Developing bias which is previously set for each color is applied to a developing roller of the developing device such as  113 Y, and a latent image is developed by the toner when the latent image passes through the position of the developing roller, and is visualized as a toner image. The toner image is transferred to the intermediate transfer belt  102  and is further transferred to a recording sheet by the secondary transfer device  121 , and is supplied to a fixing device  105 . 
   At the time of full color printing, the intermediate transfer belt  102  is rotated four times, four color toner particles are superposed on the belt, and then the toner is transferred to the recording sheet. Toner remaining on the photosensitive drum  101  establishes a state in which charge of toner can easily be cleaned by an auxiliary cleaning device, the toner is removed and collected by the cleaner device  112  and lastly, the photosensitive drum  101  is uniformly discharged to 0 volt by a discharging device (not shown) to prepare for a next image forming cycle. 
   The above-described color image forming timing is controlled taking a predetermined position on the intermediate transfer belt  102  as a reference. The intermediate transfer belt  102  is wound around the roller group comprising the driving roller  117 , the tension roller  118  and the backup roller  119 , and predetermined tension is applied to the intermediate transfer belt  102  by the tension roller  118 . 
   A reflection type sensor  136  for detecting the reference position is disposed between the driving roller  117  and the roller  119 . The reflection type sensor  136  detects a marking such as a reflection tape provided on an outer peripheral end of the intermediate transfer belt  102 , and outputs an I-top signal. When predetermined time is elapsed after the I-top signal is detected, exposure by the exposure device  108  comprising a laser scanner is started. 
   As described above, the intermediate transfer belt  102  is rotated four times to superpose the image on the intermediate transfer belt  102  when a full color image is formed. Therefore, a peripheral length of the intermediate transfer belt  102  is longer than a maximum image forming size. 
     FIG. 9  is a view for explaining a relation between the toner image and the reflection tape or the like. Reference numeral  102  represents an intermediate transfer belt, and reference numerals  137 A and  137 B represent markings such as reflection tapes provided on an outer peripheral end of the intermediate transfer belt  102 . In this embodiment, a peripheral length of the intermediate transfer belt  102  is set to 502.65 mm (=160×π mm), and this length is equal to a length of a maximum image size of 432 mm plus a length corresponding to time capable of rotating the developing devices ( 113 Y to  113 K in  FIG. 8 ). 
   When a length of an image to be formed and/or a recording sheet is equal to or smaller than a letter size (216 mm) (“small size (A4 or letter)”, hereinafter), images for two sheets are formed on the intermediate transfer belt  102 , and when the length is equal to or larger than that (large size (A3, B4 or legal, hereinafter), image for one sheet is formed. When toner image of two sheets is to be formed on the periphery of the intermediate transfer belt  102 , a reference signal of a first sheet (area A, hereinafter) is obtained by  137 A, and a reference signal of a second sheet (area B, hereinafter) is obtained by  137 B. That is, when there are the area A and area B on the intermediate transfer belt  102  and an image is formed on a recording sheet of small size (A4 or letter), an image of one page is transferred in the area A and an image of another one page is transferred in the area B. With this, image of total two pages can be held on the intermediate transfer belt  102 . In the case of a large size (A3, B4 or legal), a toner image is formed using the  137 A as a reference. That is, when an image is formed on a recording sheet of large size (A3, B4 or legal), an image of one page is formed such as to be spread over the areas A and B. 
     FIG. 10  is a view for explaining a state in which the intermediate transfer belt  102  is spread and arranged in time sequence. As an example,  FIG. 10  shows a case in which four sheets of small size (A4 or letter) and one sheet of large size (A3, B4 or legal) are formed as images. 
   As shown in  FIG. 10 , in the case of small size (A4 or letter), a toner image is formed at a position away from the markings  137 A and  137 B by distances L, and in the case of large size (A3, B4 or legal), a toner image is formed at a position away from the marking  137 A by a distance L. 
   Next, control of the secondary transfer roller  121  for transferring an image of the intermediate transfer belt  102  on a recording sheet will be explained. 
     FIG. 11  shows a flowchart for explaining the control of the secondary transfer roller  121  for transferring an image of the intermediate transfer belt  102  on a recording sheet. The processing shown here is called at regular time intervals or if necessary by main sequence. A program for executing this flowchart is stored in the ROM  205 , and is read out and executed by the CPU  201 . 
   First, in step S 2000 , it is judged whether or not an image forming operation is started. If it is judged that the image forming operation is not started in step S 2000  (S 2000 : No), the procedure is returned to step S 2000  to wait until the image forming operation is started. If it is judged that the image forming operation is started in step S 2000  (S 2000 : Yes), the procedure is proceeded to step S 2001 . 
   In step S 2001 , a pre-processing for the image forming operation is carried out. This pre-processing establishes a state in which an image can be transferred to a recording sheet by the secondary transfer roller  121 . For example, predetermined voltage is applied to the secondary transfer roller  121 , flowing current is measured to determine transfer voltage, cleaning sequence of the secondary transfer roller  121  which will be described later is executed, and standby voltage before the image is transferred is output. Then, the procedure is proceeded to step S 2002 . 
   In step S 2002 , it is judged whether or not an image formed by the photosensitive drum  101  and placed on the intermediate transfer belt  102  by the transfer device  110  has reached a position of the secondary transfer roller  121 . In step S 2002 , if it is judged that the image on the intermediate transfer belt  102  has not yet reached the position of the secondary transfer roller  121  (step S 2002 : No), the procedure is returned to step S 2002  to wait until the image reaches. At that time, as already explained, the intermediate transfer belt  102  is rotated four times, and it is judged whether or not a top of the four color image transferred to the intermediate transfer belt  102  by the transfer device  110  has reached the position of the secondary transfer roller  121 . If it is judged that the images has reached the position of the secondary transfer roller  121  in step S 2002  (S 2002 : Yes), the procedure is proceeded to step S 2003 . 
   In step S 2003 , transfer voltage is output as voltage of the secondary transfer roller  121  for transferring an image from the intermediate transfer belt  102  to a recording sheet. When it is judged whether or not the image has reached the position of the secondary transfer roller  121  in step S 2002 , it is possible to fast judge the response time of the transfer voltage output in step S 2003 . After the transfer voltage of the secondary transfer roller  121  is brought into image transfer voltage in step S 2003 , procedure is proceeded to step S 2004 . 
   In step S 2004 , it is judged whether or not an image length transferred to the intermediate transfer belt  102  has passed through the secondary transfer roller  121 . If it is judged that the length has not yet passed in step S 2004  (S 2004 : No), the procedure is returned to step S 2004  to wait until the length passes. If it is judged that the length has passed in step S 2004  (S 2004 : Yes), the procedure is proceeded to step S 2005 . 
   In step S 2005 , it is judged whether or not the recording sheet on which the image was transferred by the secondary transfer roller  121  is a tab sheet. If it is judged that the recording sheet is not the tab sheet in step S 2005  (S 2005 : No), the procedure is proceeded to step S 2008 . If it is judged that the recording sheet is the tab sheet in step S 2005  (S 2005 : Yes), the procedure is proceeded to step S 2006 . 
   In step S 2006 , it is judged whether or not it was set to form the image on a tab portion of the tab sheet on the basis of the setting of the tab sheet insert mode or the tab sheet preparing mode explained in  FIGS. 3A to 3C  and the setting of the shift amount of the image as explained in  FIGS. 4A to 4C , the size of the original image and the size of the tab sheet. If it is judged that it was set not to form the image on the tab portion in step S 2006  (S 2006 : No), the procedure is proceeded to step S 2008 . If it was set to form the image on the tab portion in step S 2006  (S 2006 : Yes), the procedure is proceeded to step S 2011 . 
   In step S 2011 , it is judged whether or not the image transferred to the recording sheet is small size (A4 or letter) image and whether or not there is employed a mode in which two sheets of images are transferred to the intermediate transfer belt  102  to form a color image. If it is judged that the mode in which two sheets of color image are transferred to the intermediate transfer belt  102  is not employed in step S 2011  (S 2011 : No), the procedure is proceeded to step S 2007 . If it is judged that the mode in which two sheets of color image are transferred to the intermediate transfer belt  102  is employed in step S 2011  (S 2011 : Yes), the procedure is proceeded to step S 2012 . 
   In step S 2012 , it is judged whether or not the transferred image is an image in an area A on the intermediate transfer belt  102 . If it is judged that it is an image in the area A (S 2012 : Yes), the procedure is proceeded to step S 2008 , and since the next image (image in the area B) is already processed at the same interval, the cleaning processing is not carried out. If it is judged that it is the image in area A in step S 2012 , (S 2012 : No), the procedure is proceeded to step S 2007 . 
   In step S 2007 , cleaning processing of the secondary transfer roller  121  is carried out. In the cleaning processing of the secondary transfer roller  121 , voltage having polarity opposite from that when the image of the intermediate transfer belt  102  is transferred to the recording sheet is applied to the secondary transfer roller  121  and in this state, if it is held for a predetermined time (e.g., time required for the secondary transfer roller  121  to rotate one revolution), toner is transferred from the secondary transfer roller  121  to the intermediate transfer belt  102 . Toner adhered to the intermediate transfer belt  102  is cleaned by the belt cleaner  122 . A time period during which voltage having the opposite polarity is applied to the secondary transfer roller  121  may be changed in accordance with a length of the image to be placed on the tab portion, i.e., the shift amount explained in  FIGS. 4A to 4C , the size of the original and the size of the tab sheet. 
   In a case the cleaning processing of the secondary transfer roller  121  is carried out, i.e., when an image is formed on a tab portion of the tab sheet, a space in which the cleaning processing can be carried out is left between an image to be formed on the tab sheet and an image to be formed on the recording sheet which is supplied after the tab sheet, and the next image is formed. In other words, in a case the cleaning processing of the secondary transfer roller  121  is carried out, when an image is formed on a tab portion of the tab sheet, an enough distance for carrying out the cleaning processing is left between a rear end of the tab sheet and a leading end of a recording sheet which is supplied after the tab sheet, and a next recording sheet is supplied. 
   If the cleaning processing of the secondary transfer roller  121  is completed in step S 2007 , the procedure is proceeded to step S 2008 . 
   In step S 2008 , voltage of the secondary transfer roller  121  is changed to voltage for between sheets, and the procedure is proceeded to step S 2009 . Here, the voltage for sheet spacing may be voltage when an image is transferred from the intermediate transfer belt  102  to the recording sheet. 
   In step S 2009 , it is judged whether or not an image transferred to the recording sheet is a final image of image forming. If it is judged that the image is not final in step S 2009  (S 2009 : No), the processing is returned to step S 2002  where the following steps are repeated. If it is judged that the image is final (S 2009 : Yes), the procedure is proceeded to step S 2010 . 
   In step S 2010 , post-processing for completing the image forming operation is carried out. In this post-processing, the cleaning sequence of the secondary transfer roller  121  is carried out, voltage of the secondary transfer roller  121  is turned off and the flow is proceeded to the exit. 
   In this embodiment, control is switched, when the sheet is a tab sheet and further an image is formed on the tab sheet. But the same effect can be obtained even if the control is switched when the sheet is the tab sheet. 
   Although the procedure is proceeded to step S 2011  if it is judged that it is a tab sheet in step S 2005  and that an image is set to be formed on the tab portion, if it is judged that it is a tab sheet in step S 2005 , the procedure may be proceeded to step S 2011  (without through step S 2006 ), and if it is judged that it is not the tab sheet, the procedure may be proceeded to step S 2008 . In this case, the same effect can be obtained with easier judgment. 
   Further, in this embodiment, when images are formed on tab sheets, a distance between images is set larger than a distance between images when the images are formed on recording sheets other than tab sheets, a distance between recording sheets is set larger than a distance between recording sheets when the images are formed on the recording sheets other than the tab sheets, and the cleaning operation is carried out. However, the present invention is not limited to the tab sheets, and the invention can also be applied to a case in which an image is formed on a special sheet such as a pre-punch sheet which is previously formed with a hole and a non-rectangular sheet other than a rectangular sheet. That is, the present invention also includes such a structure that when images are formed on special sheets such as the pre-punch sheets or non-rectangular sheets, a distance between images is set larger than a distance between images when the images are formed on recording sheets other than tab sheets, a distance between recording sheets is set larger than a distance between recording sheets when the images are formed on the recording sheets other than the tab sheets and the cleaning operation is carried out. 
   As explained above, when a mechanism in which an image is transferred by contacting with a recording sheet such as a transferring roller is employed to form an image on a tab sheet, costs are not increased unlike the image mask processing in which the cleaning sequence of the transferring roller is carried out to process the image in accordance with a tab shape, and it is possible to provide an image forming apparatus capable of preventing from being contaminated a back surface of a recording sheet on which an image is to be formed next. 
   (Third Embodiment) 
   An image forming apparatus of a third embodiment of the present invention has the same structure as that explained using  FIGS. 8 to 10  and  2  to  6 . 
   Next, control for image forming operation of this embodiment will be explained. 
     FIG. 12  is a flowchart for explaining the control step for the image forming operation of the image forming apparatus of this embodiment. The processing is called at regular time intervals or if necessary by main sequence. A program for executing this flowchart is stored in the ROM  205 , and is read out and executed by the CPU  201 . 
   First, in step S 3001 , it is judged whether or not an image forming operation is started. If it is judged that the image forming operation is not started (S 3001 : No), the procedure is returned to step S 3001  to wait until the image forming operation is started. If it is judged that the image forming operation is started in step S 3001  (S 3001 : Yes), the procedure is proceeded to step S 3002 . 
   In step S 3002 , it is judged whether or not an image to be formed is a color image. If it is judged that the image to be formed is the color image (S 3002 : Yes), the procedure is proceeded to step S 3003 . 
   In step S 3003 , it is judged whether or not the marking  137 A is detected by the sensor  136 , and if it is judged that the marking has not yet detected (S 3003 : No), the procedure is returned to step S 3003  to wait until the marking is detected. If it is judged that the marking  137 A is detected in step S 3003  (S 3003 : Yes), the procedure is proceeded to step S 3004 . 
   In step S 3004 , an image of one color is formed in the area A by an I-top signal based on a signal of the sensor  136 , and this image is transferred to the intermediate transfer belt  102 . Then, the procedure is proceeded to step S 3005 , and it is judged whether or not the recording sheet size is small size (A4 or letter). If it is judged that the size is not small size (A4 or letter) in step S 3005  (S 3005 : No), the procedure is proceeded to step S 3008 . Here, when it is judged whether or not the size is small size (A4 or letter), in the case of tab sheet, this judgment is made with size from which the tab portion is removed. If it is judged that the size is small size (A4 or letter) in step S 3005 , the procedure is proceeded to step S 3015 . 
   In step S 3015 , it is judged whether or not the recording sheet is a tab sheet and furthermore it is set to form an image on a tab portion. Here, whether or not it is set to form the image on a tab portion of the tab sheet is judged on the basis of the setting of the tab sheet insert mode or the tab sheet preparing mode explained in  FIGS. 3A to 3C  and the setting of the shift amount of the image as explained in  FIGS. 4A to 4C , the size of the original image and the size of the tab sheet. If it is judged that the sheet is the tab sheet and further it is set to form the image on the tab portion in step S 3015  (S 3015 : Yes), the procedure is proceeded to step S 3008 . That is, if it is judged that it is set to form the image on the tab portion of the tab sheet in step S 3015 , the image is transferred only to the area A of the intermediate transfer belt  102 , and the image is not transferred to the area B, so that the intermediate transfer belt  102  is prohibited to hold images of plurality of pages. 
   On the other hand, it is judged that the sheet is not the tab sheet and it is set not to form the image on the tab portion (S 3015 : No), the procedure is proceeded to step S 3006 . 
   In step S 3006 , it is judged whether or not the marking  137 B has been detected by the sensor  136 , and if the marking  137 B has not yet been detected (S 3006 : No), the procedure is returned to step S 3006  to wait until the marking is detected. If the marking  137 B is detected in step S 3006  (S 3006 : Yes), the procedure is proceeded to step S 3007 . 
   In step S 3007 , an image of one color is formed in the area B by the I-top signal based on a signal of the sensor  136 , and this image is transferred to the intermediate transfer belt  102 . That is, if it is judged that a mode for forming an image on a recording sheet other than a tab sheet, or a mode for not forming an image on a tab portion of a tab sheet is employed, the image is transferred to the area A and area B of the intermediate transfer belt  102 , thereby allowing the intermediate transfer belt  102  to hold the image of plurality of pages. 
   Then, the procedure is proceeded to step S 3008 , and it is judged whether or not the color which forms the image is K (black). If it is judged that the color is not black in step S 3008  (S 3008 : No), the procedure is returned to step S 3003 . If it is judged that the color is black in step S 3008  (S 3008 : Yes), since the image transfer of one page (four colors) is completed, the image is transferred to the recording sheet from the intermediate transfer belt  102  by the secondary transfer roller  121 , and the procedure is proceeded to step S 3011  as will be explained later using  FIG. 7 . 
   In step S 3011 , it is judged whether or not the recording sheet on which the image was transferred is a tab sheet. If it is judged that the recording sheet is not a tab sheet (S 3011 : No), the procedure is proceeded to step S 3014 , and if it is judged that the recording sheet is a tab sheet (S 3011 : Yes), the procedure is proceeded to step S 3012 . 
   In step S 3012 , it is judged whether or not it is set to form an image on a tab portion of a tab sheet. If it is judged that it is set not to form the image on the tab portion in step S 3012  (S 3012 : No), the procedure is proceeded to step S 3014 , and if it is judged that it is set to form the image on the tab portion in step S 3012  (S 3012 : Yes), the procedure is proceeded to step S 3013 . 
   In step S 3013 , it is judged whether or not time required for executing the cleaning processing of the secondary transfer roller  121  (time required for rotating the secondary transfer roller  121  at least one revolution) is elapsed after the image is transferred to the tab sheet from the intermediate transfer belt  102 . If it is judged that the time required for the cleaning processing is not elapsed in step S 3013 , (S 3013 : No), the procedure is returned to step S 3013  to wait until the time is elapsed. If it is judged that the time required for the cleaning processing is elapsed in step S 3013 , (S 3013 : Yes), the procedure is proceeded to step S 3014 . 
   In step S 3014 , it is judged whether or not an image transferred to the recording sheet is a final image of image forming. If it is judged that the image is not final in step S 3014  (S 3014 : No), the processing is returned to step S 3002  where the following steps are repeated. If it is judged that the image is final in step S 3014  (S 3014 : Yes), the flow is proceeded to the exit. 
   If it is judged that the image is not the color image in step S 3002  (S 3002 : No), the procedure is proceeded to step S 3009 . In step S 3009 , it is judged whether or not the marking  137 A or  137 B is detected by the sensor  136 . If it is judged that the marking  137 A or  137 B is not detected in step S 3009  (S 3009 : No), the procedure is returned to step S 3009  to wait until the markings are detected. If it is judged that the marking  137 A or  137 B is detected in step S 3009  (S 3009 : Yes), the procedure is proceeded to step S 3010 . 
   In step S 3010 , a black image is formed by the I-top signal based on a signal of the sensor  136 , the image is transferred to the intermediate transfer belt  102 , the image is transferred from the intermediate transfer belt  102  to the recording sheet by the secondary transfer roller  121  as described above, and the procedure is proceeded to step S 3011 . 
   Next, control of the secondary transfer roller  121  possessed by the image forming apparatus of the embodiment will be explained using  FIG. 7 . This  FIG. 7  is the same as the previous embodiment, but the explanation will be made in accordance with the present embodiment. A program for executing this flowchart is stored in the ROM  205 , and is read out and executed by the CPU  201 . 
   First, in step S 1000 , it is judged whether or not an image forming operation is started. If it is judged that the image forming operation is not started in step S 1000  (S 1000 : No), the procedure is returned to step S 1000  to wait until the image forming operation is started. If it is judged that the image forming operation is started in step S 1000  (S 1000 : Yes), the procedure is proceeded to step S 1001 . 
   In step S 1001 , a pre-processing for the image forming operation is carried out. This pre-processing establishes a state in which an image can be transferred to a recording sheet by the secondary transfer roller  121 . For example, predetermined voltage is applied to the secondary transfer roller  121 , flowing current is measured to determine transfer voltage, cleaning sequence of the secondary transfer roller  121  which will be described later is executed, and standby voltage before the image is transferred is output. Then, the procedure is proceeded to step S 1002 . 
   In step S 1002 , it is judged whether or not an image formed by the photosensitive member  101  and placed on the intermediate transfer belt  102  by the primary transfer reached a position of the secondary transfer roller  121 . At that time, in formation of a color image, as already explained, the intermediate transfer belt  102  is rotated four times, and it is judged whether or not a top of an image of which fourth color image is transferred to the intermediate transfer belt  102  by the transfer device  110  has reached the position of the secondary transfer roller  121 . If it is judged that the images on the intermediate transfer belt  102  has not yet reached the position of the secondary transfer roller  121  in step S 1002  (S 1002 : No), the procedure is proceeded to step S 1003 . If it is judged that the images has reached the position of the secondary transfer roller  121  in step S 1002  (S 1002 : Yes), the procedure is proceeded to step S 1003 . 
   In step S 1003 , transfer voltage is output as voltage of the secondary transfer roller  121  for transferring an image from the intermediate transfer belt  102  to a recording sheet. When it is judged whether or not the image has reached the position of the secondary transfer roller  121  in step S 1002 , it is possible to fast judge the response time of the transfer voltage output in step S 1003 . After the transfer voltage of the secondary transfer roller  121  is brought into image transfer voltage in step S 1003 , procedure is proceeded to step S 1004 . 
   In step S 1004 , it is judged whether or not an image length transferred to the intermediate transfer belt  102  has passed through the secondary transfer roller  121 . If it is judged that the length has not yet passed in step S 1004  (S 1004 : No), the procedure is returned to step S 1004  to wait until the length passes. If it is judged that the length has passed in step S 1004  (S 1004 : Yes), the procedure is proceeded to step S 1005 . 
   In step S 1005 , it is judged whether or not the recording sheet on which the image was transferred by the secondary transfer roller  121  is a tab sheet. If it is judged that the recording sheet is not the tab sheet in step S 1005  (S 1005 : No), the procedure is proceeded to step S 1008 . If it is judged that the recording sheet is the tab sheet in step S 1005  (S 1005 : Yes), the procedure is proceeded to step S 1006 . 
   In step S 1006 , it is judged whether or not it was set to form the image on a tab portion of the tab sheet on the basis of the setting of the tab sheet insert mode or the tab sheet preparing mode explained in  FIGS. 3A to 3C  and the setting of the shift amount of the image as explained in  FIGS. 4A to 4C , the size of the original image and the size of the tab sheet. If it is judged that it was set not to form the image on the tab portion in step S 1006  (S 1006 : No), the procedure is proceeded to step S 1008 . If it was set to form the image on the tab portion in step S 1006  (S 1006 : Yes), the procedure is proceeded to step S 1007 . 
   In step S 1007 , cleaning processing of the secondary transfer roller  121  is carried out. In the cleaning processing of the secondary transfer roller  121 , voltage having polarity opposite from that when the image of the intermediate transfer belt  102  is transferred to the recording sheet is applied to the secondary transfer roller  121  and in this state, if it is held for a predetermined time (e.g., time required for the secondary transfer roller  121  to rotate at least one revolution), toner is transferred from the secondary transfer roller  121  to the intermediate transfer belt  102 . Toner adhered to the intermediate transfer belt  102  is cleaned by the belt cleaner  22 . 
   A time period during which voltage having the opposite polarity is applied to the secondary transfer roller  36  may be changed in accordance with a length of the image to be placed on the tab portion, i.e., the shift amount explained in  FIGS. 4A to 4C , the size of the original and the size of the tab sheet. 
   In a case the cleaning processing of the secondary transfer roller  121  is carried out, when an image is formed on a tab portion of the tab sheet, the image is transferred only to the area A of the intermediate transfer belt  102  and not transferred to the area B so that the intermediate transfer belt  102  is prohibited to hold the image of a plurality of pages. In other words, in a case the cleaning processing of the secondary transfer roller  121  is carried out, when an image is formed on a tab portion of the tab sheet, an enough distance for carrying out the cleaning processing is left between a rear end of the tab sheet and a leading end of a recording sheet which is supplied after the tab sheet, and a next recording sheet is supplied. 
   If the cleaning processing of the secondary transfer roller  121  is completed in step S 1007 , the procedure is proceeded to step S 1008 . 
   In step S 1008 , voltage of the secondary transfer roller  121  is changed to voltage for sheet sparing, and the procedure is proceeded to step S 1009 . Here, the voltage between the sheets may be voltage when an image is transferred from the intermediate transfer belt  102  to the recording sheet. 
   In step S 1009 , it is judged whether or not an image transferred to the recording sheet is a final image of image forming. If it is judged that the image is not final (S 1009 : No), the processing is returned to step S 1002  where the following steps are repeated. If it is judged that the image is final (S 1009 : Yes), the procedure is proceeded to step S 1010 . 
   In step S 1010 , post-processing for completing the image forming operation is carried out. In this post-processing, the cleaning sequence of the secondary transfer roller  121  is carried out, voltage of the secondary transfer roller  121  is turned off and the flow is proceeded to the exit. 
   In the above-described control operation, although the procedure is proceeded to step S 1007  if it is judged that it is set to form the image on the tab portion in step S 1006 , it is also possible to control such that the procedure is proceeded to step S 1007  if it is judged that the sheet is a tab sheet irrespective whether or not it is set to form the image on the tab portion, and the procedure is proceeded to step S 1008  if it is judged the sheet is not the tab sheet. 
   In this embodiment, control is switched, when the sheet is a tab sheet and further an image is formed on the tab sheet. But the same effect can be obtained even if the control is switched when the sheet is the tab sheet. 
   Although the procedure is proceeded to step S 1007  if it is judged that it is a tab sheet in step S 1005  and that an image is set to be formed on the tab portion, if it is judged that it is a tab sheet in step S 1005 , the procedure may be proceeded to step S 1007  (without through step S 1006 ), and if it is judged that it is not the tab sheet, the procedure may be proceeded to step S 1008 . In this case, the same effect can be obtained with easier judgment. 
   Further, in this embodiment, when images are formed on tab sheets, a distance between images is set larger than a distance between images when the images are formed on recording sheets other than tab sheets, a distance between recording sheets is set larger than a distance between recording sheets when the images are formed on the recording sheets other than the tab sheets, and the cleaning operation is carried out. However, the present invention is not limited to the tab sheets, and the invention can also be applied to a case in which an image is formed on a special sheet such as a pre-punch sheet which is previously formed with a hole and a non-rectangular sheet other than a rectangular sheet. That is, the present invention also includes such a structure that when images are formed on special sheets such as the pre-punch sheets or non-rectangular sheets, a distance between images is set larger than a distance between images when the images are formed on recording sheets other than tab sheets, a distance between recording sheets is set larger than a distance between recording sheets when the images are formed on the recording sheets other than the tab sheets and the cleaning operation is carried out. 
   (Fourth Embodiment) 
   Next, an image forming apparatus of a fourth embodiment of the present invention will be explained. The image forming apparatus of the fourth embodiment of the invention has the same structure as that explained using  FIGS. 8 to 10  and  FIGS. 2 to 6 . The fourth embodiment has the same arrangement as explained using  FIG. 7  in the third embodiment. 
     FIGS. 13A to 13C  are views for explaining a relation between an original image of the image forming apparatus of the embodiment and a print position of a tab sheet. The original image in  FIG. 13A  has the same size as that of a tab sheet from which a tab portion is removed. When an image formed on the intermediate transfer belt  102  is transferred to the recording sheet, a tab sheet is precedently conveyed to the secondary transfer roller  121  as compared with a case of a normal recording sheet. In this case, a distance between the area B and the area A of the image formed on the intermediate transfer belt  102  (time required for switching the developing device  133 ) is the same as that of a normal recording sheet. In the case of  FIG. 13B , the original image has a size including the tab portion of the tab sheet. In this case, if the image is formed on the intermediate transfer belt  102  as in the same manner as that of a case in which there is no tab portion, an image between the areas B and A becomes narrow, and the developing device  133  can not be switched (switch from cyan to magenta for example) in time. Thereupon, the image is formed by the I-top signal generated by the  137 A as shown in  FIG. 13C . 
   Next, a control method of the image forming operation of this embodiment will be explained. 
     FIG. 14  is a flowchart for explaining the control method of the image forming operation of this embodiment. The processing is called at regular time intervals or if necessary by main sequence. A program for executing this flowchart is stored in the ROM  205 , and is read out and executed by the CPU  201 . 
   First, in step S 4001 , it is judged whether or not an image forming operation is started. If it is judged that the image forming operation is not started in step S 4001  (S 4001 : No), the procedure is returned to step S 4001  to wait until the image forming operation is started. If it is judged that the image forming operation is started in step S 4001  (S 4001 : Yes), the procedure is proceeded to step S 4002 . 
   In step S 400 , it is judged whether or not an image to be formed is a color image. If it is judged that the image is the color image in step S 4002  (S 4002 : Yes), the procedure is proceeded to step S 4003 . 
   In step S 4003 , it is judged whether or not a marking  137 A is detected by the sensor  136 , and if it is judged that the marking has not yet detected (S 4003 : No), the procedure is returned to step S 4003  to wait until the marking is detected. If it is judged that the marking  137 A is detected (S 4003 : Yes), the procedure is proceeded to step S 4004 . 
   In step S 4004 , an image of one color is formed in the area A by an I-top signal based on a signal of the sensor  136 , and this image is transferred to the intermediate transfer belt  102 . Then, the procedure is proceeded to step S 4005 , and it is judged whether or not the recording sheet size is small size (A4 or letter). 
   If it is judged that the size is not small size (A4 or letter) in step S 4005  (S 4005 : No), the procedure is proceeded to step S 4009 . Here, when it is judged whether or not the size is small size (A4 or letter), in the case of tab sheet, this judgment is made with size from which the tab portion is removed. If it is judged that the size is small size (A4 or letter) in step S 4005  (S 4005 : Yes), the procedure is proceeded to step S 4006 . 
   In step S 4006 , whether or not the recording sheet is a tab sheet and furthermore an image to be formed on the tab sheet has a length including a tab portion as shown in  FIG. 13B , is judged. If it is judged that the recording sheet is the tab sheet and the image to be formed on the tab sheet has a length (a length obtained by adding a length of a small size sheet in its conveying direction to a length of a tab portion) including the tab portion as shown in  FIG. 13B  in step S 4006 , the procedure is proceeded to step S 4009 . If the recording sheet is not a tab sheet and the image to be formed on the tab sheet does not have a length including the tab portion as shown in  FIG. 13B  in step S 4006 , the procedure is proceeded to step S 4007 . 
   In step S 4007 , it is judged whether or not the marking  137 B has been detected by the sensor  136 , and if the marking  137 B has not yet been detected in step S 4007  (S 4007 : No), the procedure is returned to step S 4007  to wait until the marking is detected. If the marking  137 B is detected in step S 4007  (S 4007 : Yes), the procedure is proceeded to step S 4008 . 
   In step S 4008 , an image of one color is formed in the area B by the I-top signal based on a signal of the sensor  136 , and this image is transferred to the intermediate transfer belt  102 . Then, the procedure is proceeded to step S 4009 , and it is judged whether or not the color which forms the image is K (black). If it is judged that the color is not black in step S 4009  (S 4009 : No), the procedure is returned to step S 4003 . If it is judged that the color is black in step S 4009  (S 4009 : Yes), since the image transfer of one page (four color images) is completed, the image is transferred to the recording sheet from the intermediate transfer belt  102  by the secondary transfer roller  121 , and the procedure is proceeded to step S 4010  as explained using  FIG. 7 . 
   In step S 4010 , it is judged whether or not an image transferred to the recording sheet is a final image of image forming. If it is judged that the image is not final in step S 4010  (S 4010 : No), the processing is returned to step S 4002  where the following steps are repeated. If it is judged that the image is final in step S 4010  (S 4010 : Yes), the flow is proceeded to the exit. 
   If it is judged that the image is not the color image in step S 4001  (S 4001 : No), the procedure is proceeded to step S 4011 . 
   In step S 4011 , it is judged whether or not the marking  137 A or  137 B is detected by the sensor  136 . If it is judged that the marking  137 A or  137 B is not detected in step S 4011  (S 4011 : No), the procedure is returned to step S 4011  to wait until the marking is detected. If it is judged that the marking  137 A or  137 B is detected in step S 4011  (S 4011 : Yes), the procedure is proceeded to step S 4012 . 
   In step S 4012 , a black image is formed by the I-top signal based on a signal of the sensor  136 , the image is transferred to the intermediate transfer belt  102 , the image is transferred from the intermediate transfer belt  102  to the recording sheet by the secondary transfer roller  121  as described above, and the procedure is proceeded to step S 4010 . 
   In this embodiment, control is switched when the sheet is a tab sheet and an image is formed on the tab sheet. But the same effect can be obtained even if the control is switched in the case of the tab sheet. 
   In the present embodiment, an image is transferred from the photosensitive body  101  to the intermediate transfer belt  102 , and the intermediate transfer belt  102  is rotated four times, thereby forming a color image. However, the same effect can also be obtained by winding a recording sheet around a transfer drum, the transfer drum around which the recording sheet is attached is rotated four times, thereby directly transferring the image on the recording sheet from the photosensitive body  101 . 
   A structure for winding a recording sheet around a transfer drum will be explained in accordance with  FIG. 15 . In  FIG. 15 , disposed around a photosensitive drum  303  which is a photosensitive body are a primary charger for charging a peripheral surface of the drum, developing devices  1 Y,  1 M,  1 C and  1 BK for visualizing an electrostatic latent image formed by exposure light image from exposure means, and a cleaning device  312  for collecting developer (toner) remained on a drum surface after the transfer of image. If the photosensitive drum  303  is rotated in a clockwise direction, the photosensitive drum  303  is charged by the primary charger. Then, an electrostatic latent image is formed by exposure light image including image information such as reflection light from a surface of an original (not shown) by exposure means such as a scanning optical device. The electrostatic latent image is developed in full color by the developing device and becomes a toner image. The developing apparatus includes developing devices  1 Y,  1 M,  1 C and  1 BK. The developing devices  1 Y,  1 M,  1 C and  1 BK respectively accommodate toner particles of yellow Y, magenta M, cyan C and black BK so that full color recording can be carried out by combination of toner particles having the colors. A transfer drum  309  which is a recording sheet carrier is in contact with a periphery of the photosensitive drum  303 . 
   A transfer charger  310  for charging a recording sheet P which is a transfer medium with polarity opposite from the transferring image formed on the photosensitive drum  303  is provided in the transfer drum  309 . The transfer drum  309  carries a plurality of recording sheets P therearound and transfers the toner image by the photosensitive drum  303 . A sheet feed cassette in which the recording sheets P are mounted is provided upstream of the transfer drum  309  in a conveying direction of the recording sheets. The recording sheets P mounted in the sheet feed cassette are conveyed from its uppermost one along a guide member by a sheet-feed roller. The recording sheet P is conveyed to an image-transfer position in synchronization with the visualized image formed on the photosensitive drum  303 . A separating pawl  315  for separating the recording sheet P from a peripheral surface of the transfer drum  309 , a conveying device  324 , a fixing entrance guide  338  and a fixing device for fixing the transferring image of the recording sheet P are provided downstream of the transfer drum  309 . 
   Toner adhering to the transfer drum  309  is cleaned in the following manner. Bias opposite from that when an image is transferred from the photosensitive drum  303  to a recording sheet is applied to the transfer charger  310  and the photosensitive drum  303 , thereby moving the toner adhering to the transfer drum toward the photosensitive drum  303 , and the cleaning device  312  cleans the toner adhering to the photosensitive drum  303 . 
   Even if the recording sheet is wound around the transfer drum and the transfer drum around which the recording sheet is attached is rotated four times, and an image is directly transferred to the recording sheet from the photosensitive drum in this manner, the same effect as those of the previous embodiments can be obtained if the control is carried out in such a manner that it is judged that the sheet is a tab sheet or that the sheet is a tab sheet and there is an image on the tab sheet, only one sheet is attached to the transfer drum in accordance with a fact that the sheet is a tab sheet or that the sheet is a tab sheet and there is an image on the tab sheet, or a plurality of (two for example) recording sheets are wound around the transfer drum in accordance with a fact that the sheet is not a tab sheet or that the sheet is not a tab sheet and there is no image on the tab sheet. 
   In accordance with a fact that a length of an image to be formed on a tab recording sheet is longer than a length of the tab recording sheet from which a length of a tab is subtracted, if a plurality of (two for example) recording sheets are wound around the transfer drum, the same effect as those of the previous embodiments can be obtained. 
   As explained above, when a recording sheet is a tab recording sheet and an image is formed on a tab of the tab recording sheet, a first mode in which a toner image of a plurality of pages is transferred to the image-carrier is prohibited and thus, it is possible to provide time for cleaning a second transfer roller. 
   Further, when a recording sheet is a tab recording sheet, since the first mode in which the toner image of a plurality of pages is transferred to the image-carrier is prohibited, it is possible to provide time for cleaning the second transfer roller. 
   Further, when a length of an image to be formed on a tab recording sheet is longer than a length of the tab recording sheet from which a length of a tab is subtracted, since the first mode in which the toner image of a plurality of pages is transferred to the image-carrier is prohibited, it is possible to secure time for switching the developing device. 
   Furthermore, when a recording sheet is a tab recording sheet and an image is formed on a tab of the tab recording sheet, since a first grasping mode for grasping a plurality of recording sheets is prohibited, it is possible to provide time for cleaning the second transfer roller. 
   Moreover, when a recording sheet is a tab recording sheet, since the first grasping mode for grasping a plurality of recording sheets is prohibited, it is possible to provide time for cleaning the second transfer roller. 
   In addition, when a length of an image to be formed on a tab recording sheet is longer than a length of the tab recording sheet from which a length of a tab is subtracted, since a first mode for transferring a toner image of a plurality of pages to the image-carrier, it is possible to secure time for switching the developing device.