Patent Publication Number: US-6904261-B2

Title: Insert sheet transporting apparatus an insert sheet transporting method and an image forming apparatus

Description:
This application claims priority from Japanese Patent Application No. 2002-199265 filed Jul. 8, 2002, which is incorporated hereinto by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a system for forming images on the sheets and outputting the sheet whereon the images are formed, more particularly, to an insert sheet transporting apparatus, an insert sheet transporting method and an image forming apparatus for transporting, loading the sheets from a plurality of sheet feeding stages and preparing the bundles of the sheets that the sheets supplied from a plurality of sheet feeding stages are mixed. 
   2. Description of the Related Art 
   The conventional image forming apparatus, such as the copying machine, is designed for being capable of operating in the modes such as the cover placing mode, the insert sheet mode of the like. These modes are designed and controlled for permitting the sheets supplied from the cassette or the sheet feeding tray provided with the image forming apparatus to be placed on the first page and after the last page or inserted between pages. Therefore, even the sheets supplied from sheet feeding stage other than that of the present bundle of the sheets can be added as “a front cover” or “a back cover” or inserted as “an insert sheet”. Further, in the similar fashion, the sheet supplied from a special tray for loading “the insert sheets” can also be inserted. In such a system, the insert sheet is required to undergo a mere transport process, so that the order of insertion (place of insertion), the number of sheet to be inserted can be set freely. Thus, the bundle of the sheets including the inserted sheets can undergo the post-processing such as the processing as a bundle of the sheets, i.e., the discharge in the form of a bundle, processing for binding, processing for folding, processing for bookbinding by the finisher or the like, which is provided with the main unit of the image forming apparatus. 
   Hereinafter, the operation modes, which insert the sheets supplied from the insert sheet stage as the “front cover”, “insert sheet” and “back cover”, are generally called “the insert sheet mode.” 
   In the method for supplying the insert sheet from the cassette, when the timing for the insertion of the sheet comes, the insert sheet is fed into the transport pass of the sheet whereon the image is to be formed, and the fed insert sheet is discharged by way of the transport pass. In this system, a fixing stage is provided on the course of the previously mentioned transport path, and the insert sheet passes the fixing stage similarly to the non-fixed sheet. 
   Where the original having the color image printed thereon is fed as the insert sheet, there happens sometimes that the quality of the printed image is damaged due to the effect of the heat and pressure while the insert sheet passes the fixing stage. Further, as the personal computers spread the use of the color images has also increased, thereby causing the increase in the use of the color copied papers/color printed papers as the insert sheets. When such color copied papers are supplied from the cassette, the transport efficiency of such color copy papers tends to fall due to the influence of the oil or the like deposited on the surfaces thereof, sometimes causing a marked fall of the reliability of the non-fixed sheet transport operation. 
   Further, there has been developed an apparatus comprising the finisher provided with the sheet feeder for supplying the insert sheets so that the insert sheets can be supplied from the finisher. As the examples of this type of apparatus, there are those published under Japanese Patent Application Laid-open Nos. 60-180894(1985), 60-191932(1985), 60-204564(1985) and the like. More particularly, in the cases of those apparatuses recited in said publications, the insert sheets are supplied to the finisher from the insert sheet feeder at predetermined time and transported to and stored in the intermediate tray of the finisher in the loaded state. The printed sheets discharged from the main unit of the image forming apparatus are also guided into the finisher and transported to the intermediate tray for being loaded thereon. In order to let the apparatus perform such an operation, it is necessary for the printed sheets to have supplied in the number required, arranged in the order corresponding to the contents of the images and loaded in advance on the storage device of the insert sheet feeder. 
   When selecting the sheet insert mode in combination with the insert sheet feeder, it is necessary for the insert sheets supplied from the insert sheet feeder to be fed individually and accurately into the finisher. In this stage, since the insert sheets are to be used in a variety of quality and for a variety of images, when the insert sheets undergo the automatic separation/transport stage, the stability in operation tend to differ from that in the case of the blank sheets for recording. For instance, it gives rise to a problem such that two insert sheets in overlapped condition are fed from the insert sheet feeder causing the so-called “overlapped transport”, which put the sequence of the sheets in subsequent bundle out of order. 
   In a conventional image forming apparatus, the necessary number of sheets to be outputted is set by the operating section thereof, and the image forming operation is continued until coming to an end thereof; in such an apparatus, when the overlapped transport of the insert sheets has occurred, the insert sheets will be inserted at wrong places in all bundles of the sheets after the point at which the overlapped transport has occurred, thereby causing the disadvantages such as the wastes of paper, time, power and the like. 
   Further, in the case of a system designed for enabling to stop its operation once to permit the examination of what has happened during the operation up to the point where the supply of sheet bundle is discontinued, it is possible for the user to detect the occurrence of the overlapped transport, if any, earlier than in the case of the system wherein the detection is made without stopping all the operations of the system. However, in the case where a bundle consists of a large number of sheets, even if the user has visually detected the overlapped transport, the operation of the system will continue until the system is stopped, so that even such a system is disadvantageous for the user, since the waste supply of the sheets still occurs. 
   Thus, the object of the present invention lies in providing an apparatus capable of making the recovery of its normal state without discontinuing the operation of the system, even if the overlapped transport of the insert sheets has occurred, by properly controlling the image forming apparatus and the finisher, thereby improving the utility of the image forming apparatus for the users. 
   SUMMARY OF THE INVENTION 
   In order to attain the above-mentioned object, the insert sheet transporting apparatus according to the present invention is designed for being capable of supplying one set of insert sheets to at least one set of sheet bundle and inserting each of the set of insert sheets at predetermined places among the set of the sheet bundle. The insert sheet transporting apparatus comprises a sheet feeding device for loading a plurality of sets of insert sheets and sequentially separating the insert sheets into individual sheet to feed, an overlapped transport detecting device for determining whether or not the insert sheets, which have been fed from the sheet feeding device, have been transported in overlapped condition, and a controller for discharging all the insert sheets of the set involved in the overlapped transport onto a predetermined place and finding the first insert sheet of the next set of insert sheets when the overlapped transport is determined. 
   The image forming apparatus according to the present invention is designed for being capable of forming the images on the predetermined blank sheets, supplying one set of insert sheets to at least one set of sheet bundle that the image is formed thereon, and inserting each of the set of insert sheets at predetermined places among the set of blank sheet bundle. The image forming apparatus comprises a first sheet feeding device for sequentially separating the blank sheets for recording into individual sheet to feed, an image forming device for forming the images on the blank sheets for recording, a transport device for transporting the blank sheets for recording, which have been kept waiting after having been fed from the first sheet feeding device, to be supplied to the image forming device at a predetermined timing, a second sheet feeding device for loading a plurality of sets of insert sheets and sequentially separating the insert sheets into individual sheet to feed, an overlapped transport detecting device for determining whether or not the insert sheets fed from the second sheet feeding device have been transported in overlapped condition, and a controller for discharging all the insert sheets of the set involved in the overlapped transport onto a predetermined place and finding the first sheet of the next set of insert sheet when the overlapped transport is determined by the overlapped transport detecting device, simultaneously with discontinuing the drive of the transport device. 
   Further, when the transporting of the blank sheets for recording to the image forming apparatus had already started at the point where the overlapped transport of the insert sheets was determined by the overlapped transport detecting device, the controller discontinues the drive of the transport device after the transporting has been completed. 
   Further, when the overlapped transport of the insert sheets has been determined by the overlapped transport detecting device, the controller discharges all the insert sheets of the set involved in the overlapped transport and the blank sheets for recording on which the images have formed by the image forming apparatus onto predetermined place, resumes the drive of the transport device and starts forming the images from the first blank sheet of the set of blank sheet bundle. 
   The insert sheet transporting method according to the present invention is characterized in that one set of insert sheets is supplied to at least one set of blank sheet bundle, and each of the set of insert sheets are inserted at predetermined places among the set of the blank sheets. The insert sheet transporting method comprises a sheet feeding step for loading a plurality of sets of insert sheets and sequentially separating the insert sheets into individual sheet to feed, an overlapped transport detecting step for determining whether or not the insert sheets fed in the sheet feeding step have been transported in overlapped condition, and a control step for discharging all the insert sheets of the set involved in the overlapped transport onto predetermined place and finding the first insert sheet of next set of insert sheets when the overlapped transport of the insert sheets is determined in the overlapped transport detecting step. 
   The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating the construction of the image forming apparatus according to the present invention; 
       FIG. 2  is a block diagram illustrating the composition of the control device of the image forming apparatus given in  FIG. 1 ; 
       FIG. 3  is a schematic block diagram illustrating the constructions of the folding device and the finisher respectively incorporated into the image forming apparatus given in  FIG. 1 ; 
       FIG. 4  is a block diagram illustrating the composition of the controller for finisher to control the finisher given in  FIG. 3 ; 
       FIGS. 5A ,  5 B and  5 C are schematic illustrations of the examples of the setting screen respectively to be outputted through the display unit of the operating section; 
       FIGS. 6A and 6B  are the first schematic diagram for illustrating the procedures for transporting the sheets to the processing tray of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 7  is the second schematic diagram for illustrating the procedure for transporting the sheets to the processing tray of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 8  is the third schematic diagram for illustrating the procedure for transporting the sheets to the processing tray of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 9  is the fourth schematic diagram for illustrating the procedure for transporting the blank sheets to the processing tray of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 10  is the fifth schematic diagram for illustrating the procedure for transporting the blank sheets to the processing tray of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 11  is the sixth schematic diagram illustrating the procedure for transporting the blank sheets to the processing tray of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIGS. 12A ,  12 B,  12 C and  12 D are schematic diagrams for respectively illustrating examples of the image forming procedure in the book binding mode in the image forming system shown in  FIG. 1 ; 
       FIGS. 13A and 13B  are the first schematic diagram for illustrating and the procedure for transporting the sheets to the storage guide of the finisher from the main unit of the image forming apparatus the inserter; 
       FIG. 14  is the second schematic diagram for illustrating the procedure for transporting the sheets to the storage guide of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 15  is the third schematic diagram for illustrating the procedure for transporting the sheets to the storage guide of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 16  is the fourth schematic diagram for illustrating the procedure for transporting the sheets to the storage guide of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 17  is the fifth schematic diagram for illustrating the procedure for transporting the sheets to the storage guide of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 18  is the sixth schematic diagram for illustrating the procedure for transporting the insert sheets to the storage guide of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIG. 19  is the seventh schematic diagram for illustrating the procedure for transporting the insert sheets to the storage guide of the finisher from the main unit of the image forming apparatus and the inserter; 
       FIGS. 20A and 20B  are the diagrams respectively showing an example of folding process and an example of the binding process in the finisher; 
       FIG. 21  is a flowchart for illustrating the sheet inserting process applied throughout the image forming system as an embodiment of the present invention; 
       FIG. 22A  is a schematic diagram illustrating the flow of the control signal between the main unit of the image forming apparatus and the finisher according to the present invention in the normal state, while  FIG. 22B  is a schematic diagram illustrating the flow of the control signal between the main unit of the image forming apparatus and the finisher according to the present invention at the time when the overlapped transport has occurred; 
       FIG. 23  is a schematic diagram illustrating the originals loaded on the original feeding device in the sheet insertion mode; and 
       FIG. 24  is a schematic diagram illustrating the insert sheets loaded on the inserters in the sheet insertion mode. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The embodiments of the present invention will be described referring to the accompanying drawings. 
   (General Composition) 
     FIG. 1  is a longitudinal sectional view showing the composition of the main part of the image forming apparatus as an embodiment of the present invention. 
   The image forming apparatus, as shown in  FIG. 1 , comprises a main unit  10  of the image forming apparatus, a folding device  400  and a finisher  500 , while the main unit  10  of the image forming apparatus comprises an image reader  200  for reading the original image and a printer  300 . 
   The image reader  200  is mounted with an original feeding device  100 . The original feeding device  100 , as a first sheet feeding device, lets the originals, which are loaded on a original sheet tray with their surfaces up, be fed leftward one by one from the first sheet and further be transported rightward by passing a curved path, a platen glass  102  and a scanning point from the left-hand side until being finally discharged into the external sheet discharging tray  112 . When each of the originals passes through the moving original scanning point on the platen glass  102  from the left to the right, the image on the original is scanned by a scanner unit  104  located at the point corresponding to the moving original scanning point. This scanning method is generally called the moving original scanning method. More specifically, when the original passes through the moving original scanning point, the side of the original to be scanned is irradiated by the light from a lamp  103  of the scanner unit  104 , and the light reflected by the original is guided to a lens  108  by way of mirrors  105 ,  106  and  107 . The light passing the lens  108  is focused on the image pickup surface of the image sensor  109 . 
   Thus, when the original is transported so as to pass through the moving original scanning point from the left to the right, the original can be scanned in the main scanning direction, which is orthogonal to the transport direction of the original, and in the sub-scanning direction, which is in the transport direction of the original. In other words, when the original passes through the moving original scanning point, each line of the image on the original is scanned in the main scanning direction by the image sensor  109 , while the original is transported in the sub-scanning direction to complete the scanning of the whole image of the original. The optically picked up image is converted into the image data by means of the image sensor  109  in order to be outputted. The image data outputted from the image sensor  109  undergoes the predetermined processing by a controller  202  for image signal processing, which will be described later, to be inputted, as a video signal, to an exposure controller  110  of a printer  300 . 
   Further, it is also possible that the original is transported to a predetermined stop position on the platen glass  102  where scanner unit  104  is made to scan from the left to the right to scan the original. This method is so-called standing original scanning method. 
   When reading the original without using the original feeding device  100 , first the user is required to put up the original feeding device  100  to place the original onto the platen glass  102  and to let the scanner unit  104  scan the original from the left to the right to scan the original. In other words, the standing original scanning method is employed when scanning the original without using the original feeding device  100 . 
   The exposure controller  110  of the printer  300  modulates the laser beam to be outputted according to the inputted video signal, and the laser beam is irradiated on a photosensitive drum  111  while being scanned by a polygon mirror  110   a . The electrostatic latent image is formed on the photosensitive drum  111  corresponding to the scanning laser beam. In this case, as described later, when scanning the standing original, the exposure controller  110  outputs the laser beam so that an authentic image (not a reflected image) is formed. 
   The electrostatic latent image on the photosensitive drum  111  is turned into the visible image formed by the developer supplied from a processor  113 . On the other hand, the blank sheets supplied from an upper cassette  114  or a lower cassette  115  by means of pickup rollers  127  and  128  are transported to a resist roller  126  by means of the sheet feeding rollers  129  and  130 . When the front end of the blank sheet has reached the resist roller  126 , the resist roller  126  is driven at any timing to transport the blank sheet between the photosensitive drum  111  and a transcriber  116  so that the image formed on the photosensitive drum  111  by means of the developer is transcribed onto the blank sheet supplied from the transcriber  116 . 
   The sheet, whereon the image developed by the developer is transcribed, is transported to a fixing device  117 , where the transported sheet is pressed while being heated to fix the image formed on the surface of the sheet. The sheet that has passed the fixing device  117  is discharged to outside (the folding device  400 ) by way of a flapper  121  and a discharging roller  118 . 
   Then, when the sheet with its surface whereon the images are formed is discharged in face-down state from the main unit, the sheet, which has passed the fixing device  117 , is once guided into a reversing path  122  by means of the switching action of the flapper  121 , and, when the rear end of the sheet has passed the flapper  121 , the sheet is switched back to be discharged from a printer  300  by means of a discharging roller  118 . Hereinafter, this paper discharge mode is called the reverse paper discharge mode. This reverse sheet discharge mode is applied in a situation such that where the images scanned by the original feeding device  100  or the images outputted from the computer need to be formed sequentially from the first page. When this reverse sheet discharge mode is applied, the discharged sheets are arranged in normal order. 
   Further, when a hard sheet such as an OHP sheet is fed from the manual sheet feeding device  125  for forming the images thereon, such a sheet is not guided to a reversing pass  122  and discharged, with the surface on which the image is formed set face-up state, by means of the discharging roller  118 . 
   When the bifacial recording mode for forming the image on both the face and the back of a sheet is set, the sheet is guided to the reversing pass  122  by means of the switching action of the flapper  121  and then to a bifacial transport path  124 , where the guided sheet is re-fed between the photosensitive drum  111  and the transcribing device  116  synchronously with the previously described timing. 
   The sheet discharged from the printer  300  is transported to the folding device  400 . This folding device  400  is designed for folding the sheet in zigzag. For instance, when the sheets of the A3 size or B4 size are to be used, and the setting for folding operation has been made, the folding operation is made by the folding device  400 . When other operation mode is set, the sheet discharged from the printer  300  is transported to a finisher  500  passing through the folding device  400 . The finisher  500  is provided with an inserter  900 , which is designed for feeding the special sheets to be used for the cover or the insert sheets or the like. The finisher  500  performs the functions such as the bookbinding, binding, perforating or the like. 
   (Block Diagram of System) 
   Next, the composition of the controller, designed for controlling the whole of the present image forming apparatus, will be described referring to FIG.  2 .  FIG. 2  is a block diagram showing the composition of the controller for controlling the whole of the image forming apparatus given in FIG.  1 . 
   The controller, as shown in  FIG. 2 , comprises a CPU circuit section  150 . The CPU circuit section  150  incorporates the CPU (not shown), ROM  151  and RAM  152  and generally controls the blocks  101 ,  153 ,  201 ,  202 ,  209 ,  301 ,  401  and  501  by means of a control program stored in the ROM  151 . The RAM  152  not only temporarily stores the control data but also is used as an operation area for the operation processing accompanying the control operation. 
   The controller  101  for original sheet feeding device controls the drive of the original feeding device  100  according to the command from the CPU circuit section  150 . The controller  201  for image reader controls the drive of the scanner unit  104 , the image sensor  109  or the like and transports the analog image signal, outputted from the image sensor  109 , to the image signal controller  202 . 
   The controller  202  for image signal processing converts the analog signal from the image sensor  109  into the corresponding digital signal and performs the subsequent processing to convert the digital signal into the video signal to be outputted to the printer controller  301 . Further, the image signal, inputted from the computer  210  by way of the external I/F  209 , is made to undergo various processing to convert the digital image signal into the video signal to be outputted to the printer controller  301 . The processing operation of the controller  202  for image signal processing is controlled by the CPU circuit section  150 . The controller  301  for printer drives the exposure controller  110  according to the inputted video signal. 
   The operating section  153  comprises a plurality of keys for setting various functions relating to the image formation, the display section for displaying the information concerning the setting conditions or the like not only to output the key signals corresponding to the operations of various keys to the CPU circuit section  150  but also to display the information corresponding to the signal from the CPU circuit section  150  on the display section. 
   The controller  401  for folding device is mounted on the folding device  400  and controls the drive of the whole folding device  400  by exchanging the information with the CPU circuit section  150 . 
   The controller  501  for finisher is mounted on the finisher  500  and controls the drive of the whole finisher by exchanging the information with the CPU circuit section  150 . The content of the control will be described later. 
   (Folding Device) 
   Next, the compositions of the folding device  400  and the finisher  500  will be described respectively referring to FIG.  3 .  FIG. 3  shows the compositions of the folding device  400  and the finisher  500 , which are given in  FIG. 1 , respectively. 
   The folding device  400 , as shown in  FIG. 3 , is provided with the folding and horizontal transport pass  402  for introducing the sheet discharged from the printer  300  and guiding the sheet to the side of the finisher  500 . The pair of transport rollers  403  and the pair of transport rollers  404  are respectively provided on the folding and horizontal transport pass  402 . Further, the exit (on the side of the finisher  500 ) of the folding and horizontal transport pass  402  is provided with the folding pass selecting flapper  410 . This folding pass selecting flapper  410  performs the switching operation for guiding the sheet to the side of the folding and transport horizontal pass  402  or to the side of the finisher  500 . 
   At this stage, when executing the folding operation, the folding pass selecting flapper  410  is turned on to guide the sheet to the folding pass  420 . The sheet guided to the folding pass  420  is transported to the folding roller  421  to be folded in zigzag. In contrast, when the folding operation is not needed, the folding path selecting flapper  410  is turned off to transport the sheet directly to the finisher  500  from the printer  300  by way of the folding and transport horizontal pass  402 . 
   (Finisher Main Section) 
   The finisher  500  sequentially takes in the discharged sheets by way of the folding device  400  and subsequently performs the post-processing for the taken-in sheets such as the processing for orderly collecting a plurality of taken-in sheets into a bundle, the processing for stapling the rear end of the collected sheet bundle, the processing for punching the area near the rear end of the taken-in sheets, the processing for sorting, the processing for non-sorting, processing for bookbinding or the like. 
   The finisher  500 , as shown in  FIG. 3 , is provided with a pair of entrance rollers  502  for guiding inside the sheet discharged from the printer  300  by way of the folding device  400 . The switching flapper  551  for guiding the sheet to the finisher path  552  or the first binding path  553  is provided on the downstream side of the pair of entrance rollers  502 . 
   The sheet guided to the finisher path  552  is transported to the buffer roller  505  by way of the pair of the transport rollers  503 . Both the pair of transport rollers  503  and the buffer roller  505  are respectively designed to be able to rotate in reverse direction. 
   The entrance sensor  531  is provided between the pair of entrance rollers  502  comprising rollers  502   a  and  502   b  and the pair of transport rollers  503 . Further, the second binding pass  554  is diverged from the finisher pass  552  on the upstream side of the entrance sensor  531  in the sheet transport direction. Hereinafter, this diverging point is called the divergence A. This divergence A forms the divergence to the transport pass for transporting the sheet to the pair of transport rollers  503  from the pair of entrance rollers  502 , but the divergence A also forms the one-way divergence for transporting only to the side of the second binding pass  554  when the sheet is transported to the side of the entrance sensor  531  from the side of the pair of transport rollers  503  by the reverse rotation of the pair of transport rollers  503 . 
   A punch unit  550  is provided between the pair of transport rollers  503  and the buffer roller  505 . The punch unit  550  operates to punch the area near the rear end the transported sheet when necessary. 
   The buffer roller  505  is designed for permitting, when necessary, the predetermined number of sheets to accumulate and wind around the periphery thereof by means of the downward pressure rolls  512 ,  513  and  514  when the sheets are fed around the periphery thereof. The sheets wound around the buffer roller  505  are transported in the direction of the rotation of the buffer roller  505 . 
   The switching flapper  510  is interposed between the downward pressure rolls  513  and  514 , and the switching flapper  511  is provided on the downstream side of the downward pressure roll  514 . The switching flapper  510  is designed to unwind the sheets, which have been wound around the buffer roller  505 , from the buffer roller  505  and guide the unwound sheets to the non-sort pass  521  or the sort pass  522 , while the switching flapper  511  is designed to unwind the sheets, which have been wound around the buffer roller  505 , from the buffer roller  505  and guide the unwound sheets to the sort pass  522  or guide the sheets wound around the buffer roller  505 , as they are, to the buffer pass  523 . 
   The sheets guided to the non-sort pass  521  by means of the switching flapper  510  are discharged onto the sample tray  701  by way of the pair of discharging rollers  509 . The discharged paper sensor  533  which detect the jam or the like is provided on the way to the non-sort pass  521 . 
   The sheet guided to the sort pass  522  by the switching flapper  510  is loaded on the intermediate tray (hereinafter referred to as processing tray)  630  by way of the pairs of the transport rollers  506  and  507 . The sheets loaded in the form of the bundle on the processing tray  630  undergo the matching process, stapling process or the like, when necessary, and then are discharged onto the stack tray  700  by means of the roller  690  and the discharging rollers  680   a  and  680   b . The stapler  601  is used in stapling process wherein the sheets accumulated in bundle on the processing tray  630  are stapled. The operation of the stapler  601  will be described later. Stack tray  700  is designed so as to be capable of traveling upward and downward directions by itself. 
   (Bookbinding Section) 
   The sheets from a first binding pass  553  and that from a second bookbinding pass  554  are stored in a storage guide  820  by the pair of transport rollers  813  and are further transported until the front end of each sheet comes into contact with a movable sheet-positioning member  823 . A binding stage entrance sensor  817  is provided on the upstream side of the pair of transport rollers  813 . Further, two pairs of staplers  818  are provided on the way to the storage guide  820 . The staplers  818  are designed to cooperating with opposing anvil  819  to bind the bundle of the sheets at the center thereof. 
   The pair of folding rollers  826  is provided on the downstream side of a stapler  826 . A thrusting member  825  is provided opposing to the pair of folding rollers  826 . When this thrusting member  825  is projected towards the bundle of sheets stored in the storage guide  820 , the bundle of the sheets is pushed out between the pair of folding rollers  826  to be folded by the pair of folding rollers  826  and then discharged onto the saddle discharge tray  832  by way of the pair of folded sheet discharge rollers  827 . A sheet-from-binder discharge sensor  830  is provided on the downstream side of the pair of folded sheet discharge rollers  827 . 
   When folding the bundle of sheets which has been bound by the stapler  818 , the positioning member  823  is lowered by a predetermined distance so that the stapled position of the bundle of sheets coincides with the central position of the pair of folding rollers  826 . 
   (Inserter Section) 
   Inserter  900  is mounted on the finisher  500 . A bundle of sheets loaded on the tray  901  used as the covers or for use as a set of insert sheets is sequentially separated into individual sheet and then each sheet is transported to the finisher pass  552  or the binding pass  553 . At this stage, the insert sheets or special sheets are loaded on the tray  901  of the inserter  900  in normal state when viewed from the position of the operator. In other words, the special sheets are loaded on the tray  901  with their faces up. 
   The special sheets loaded on the tray  901  are transported to a separating stage, comprising a transport roller  903  and a separation belt  904 , by means of sheet feeding roller  902  so that the bundle of sheets is separated one by one from the top for being transported. 
   The pair of draw-out rollers  905  is provided on the downstream side of the separating stage. The special sheets separated by the pair of draw-out rollers  905  is stably guided to the transport pass  908 . The feeding sheet sensor  907  is provided on the downstream side of the pair of draw-out rollers  905 , while the pair of transport rollers  906 , for guiding the special sheet on the transport pass  908  to the pair of entrance rollers  502 , is provided between the feeding sheet sensor  907  and the entrance roller pair  502 . 
   The transport pass  908  is provided with the overlapped transport detection sensor  950  for determining whether 2 or more special sheets separated and transported from tray  901  are transported in overlapped condition. 
   (Block Diagram of Finisher) 
   Next, the composition of the controller  501  for the finisher for controlling the drive of the finisher will be described referring to FIG.  4 .  FIG. 4  is a block diagram showing the composition of the finisher controller given in FIG.  2 . 
   The controller  501  for the finisher, as shown in  FIG. 4 , is provided with the CPU circuit section  510 , which comprises CPU  511 , ROM  512 , RAM  513  or the like. The CPU circuit section  510  exchanges the data with the CPU circuit section  150 , which is provided on the side of the main unit of the image forming apparatus, through communication IC  514 , and executes various programs stored in the ROM  512  according to the commands from the CPU circuit section  150  to control the drive of the finisher  500 . 
   In performing the above-mentioned control of the drive, the CPU circuit section  510  receives the signals detected by the various sensors. Such various sensors are the entrance sensor  531 , the binding stage entrance sensor  817 , the binding stage sheet discharge sensor  830 , the feeding sheet sensor  907 , the set sheet sensor  910 , and the overlapped sheets transport sensor  960 . The set sheet sensor  910  is designed for detecting whether or not the special sheets are loaded on the tray  901  of the inserter  900 . The overlapped sheets transport sensor  960  is designed, as described previously, for detecting whether or not the sheets, which have been transported separately from the tray  901 , have been transported in a condition such that two or more special sheets are overlapped. The overlapped sheets transport sensor comprises a stationary electrode and a moving electrode which are arranged opposing each other and designed so that thickness of the sheet passing between the opposing electrodes is measured in terms of the electrostatic capacity of the sheet while the sheet is transported passing through the position of the sensor. Needless to say, any other measuring method may be substituted for the present method as long as the substitutive method is capable of detecting the overlapped transport of sheets. The CPU circuit section  510  is connected with the driver  520 . The driver  520  drives the motor and the solenoid according to the signals from the CPU circuit section  510 . Further, CPU circuit section  510  also drives the clutch. 
   In this embodiment, the motors provided are the entrance motor M 1  as being the drive source of the pair of entrance rollers  502 , the pair of transport rollers  503  and the pair of transport rollers  906 ; the buffer motor M 2  as being the driving source of the buffer roller  505 ; the sheet discharging motor M 3  as being the driving source for each pair of the transport rollers  506 , the pair of discharging rollers  507  and the pair of discharging rollers  509 ; the bundle discharging motor M 4  as being the driving source for each of the discharging motor M 4  as being the driving source for each of the discharging rollers  680   a  and  680   b ; the transport motor M 10  as being the driving source of the pair of transport rollers  813 ; the positioning motor M 11  as being the driving source of the sheet positioning member  823 ; the folding motor M 12  as being the driving source of the thrusting member  825 , the pair of folding rollers  826  and the pair of folded sheet discharging rollers  827 ; and sheet feeding motor M 20  as being the driving source of the sheet feeding roller  902 , the transport roller  903 , the separation belt  904  and the pair of draw-out rollers  905  of the inserter  900 . 
   The entrance motor M 1 , the buffer motor M 2 , the sheet discharging motor M 3  and the bundle discharging motor M 4  are the stepping motors, each of which is capable of letting the roller pairs rotate at constant speeds or at different speeds respectively by controlling the exciting pulse rate thereof. Further, the entrance motor M 1  and the buffer motor M 2  can be driven either normal direction or reverse direction by means of the driver  520 . 
   The transport motor M 10  and the positioning motor M 11  are stepping motors, while the folding motor M 12  is a DC motor. Further, the speed of the transport motor  10  can be synchronously equalized to the speed of the entrance motor M 1  in transporting the sheets. 
   The paper feeding motor M 20  is a stepping motor, which is designed to be capable of operating at synchronously equal speed to that of the entrance motor M 1  in transporting the sheets. 
   There are provided the solenoid SL 1  for the switching of the switching flapper  510 , the solenoid SL 2  for the switching of the switching flapper  511 , the solenoid SL 10  for the switching of the switching flapper  551 , the solenoid SL 20  for driving the sheet feeding shutter (not shown in  FIG. 3 ) of the inserter  900 , and the solenoid SL 21  for driving the sheet feeding roller  902  for ascending and descending thereof. 
   There are provided the clutch CL 1  for transmitting the drive of the folding motor M 12  to the thrusting member  825  and the clutch CL 10  for transmitting the drive of the sheet feeding motor M 20  to the feed sheet roller  902 . 
   (Operating Section) 
   Next, the example of the selective operation in the post-processing mode by the operating section will be described referring to  FIGS. 5A-5C .  FIGS. 5A-5C  show the examples of the menu for selecting the post-processing mode of the operating section in the image forming apparatus given in FIG.  1 . 
   In the present image forming apparatus, not only such various processing modes as the non-sort mode, sort mode, staple sort mode (stapling mode), bookbinding mode or the like are available but also the sheet inserting mode for enabling the sheets to be added as the front cover or the back cover or inserted into any places is available. Such processing modes can be set through the input operation from the operating section  153 . For example, in setting the post-processing mode, the menu to be selected shown in  FIG. 5A  is displayed on the operating section  153  so that the processing mode can be set by means of the displayed menu. Further, in setting the sheet inserting mode, the menu to be selected shown in  FIG. 5B  can be displayed on the operating section  153  so that not only whether the insert sheet is to be made by means of the inserter  900  or to be made manually by means of manual sheet feeder  125  can be selected but also the place where the insertion is to be made can also be selected through the menu shown in FIG.  5 C. When feeding the special sheet exclusively for use as the cover, only [1] should be selected. However, when a plurality of sheets are to be inserted, it is possible to selectively specify the desired places where the insertions are to be made. 
   (General Description of Operation of Finisher) 
   Next, the process for transporting the sheet to the processing tray  630  in the finisher  500  from the inserter  900  and the printer  300  during the sort mode will be described referring to  FIGS. 6 through 11 .  FIGS. 6 through 11  illustrate the flow of the sheet from the inserter and the printer to the processing tray in the finisher when the image forming apparatus is set in the sort mode. 
   When inserting the sheet C, for use as a cover, is to be inserted among the sheets whereon the images have been formed, the sheet C is set on the tray  901  of the inserter  900 , as shown in FIG.  6 B. In this case, as shown in  FIG. 6A , the sheet C is set so that the surface whereon the image is formed is set facing up and the place to be bound comes to left side when viewed from the position of the operator, whereby the sheet can be fed in the direction of the arrow in FIG.  6 A. The set condition of the sheet C is similar to the set condition of the original in the original feeding device  100 , thereby improving the efficiency in setting the sheet C. 
   When the sheets C is set on the tray  901 , as shown in  FIG. 7 , the top sheet C 1  starts to be fed while switching flapper  551  is switched to the side of the finisher pass  552 . The sheet C 1  is guided, from the transport pass  908 , into the finisher pass  552  by way of the pair of entrance rollers  502 , and, when the front end of the sheet C 1  is detected by the entrance sensor  531 , the feed of the sheet, whereon the image has been formed, from the printer  300  is started. 
   Subsequently, as shown in  FIG. 8 , not only the sheet P 1  supplied from the printer  300  is guided into the finisher  500  but also the sheet C 1  is guided to the sort pass  522  by way of the buffer roller  505 . During this process, the both the switching flappers  510  and  511  are switched to the side of the sort pass  522  respectively. 
   The sheet C 1  guided to the sort pass  522  is stored in the processing tray  630  as shown in FIG.  9 . In this process, the sheet P 1  from the printer  300  has been guided into the finisher pass  522 . Similarly to the case of the sheet C 1 , the sheet P 1  is, as shown in  FIG. 10 , guided to the sort pass  522  by way of the buffer roller  505  and then transported to the processing tray  630 . Further, the sheet P 2 , which follows the sheet P 1 , is guided into the finisher pass  552 . Then, as shown in  FIG. 11 , the sheet P 1  is stored by being placed on the sheet C 1 , which has already been stored in the processing tray  630 , and the following sheet P 2  is stored by being placed on the sheet P 1 . 
   At this stage, the images, which have undergone the processing for the reflected image, are formed on each of the sheet P 1  and the sheet P 2 ; since the sheet P 1  and the sheet P 2  have been discharged by means of the reversed sheet discharging process, similarly to the case of the sheet C 1 , each of the sheet P 1  and the sheet P 2  is stored in the processing tray  630 , with its image forming surface down and with its side to be bound on the side of the stapler  601 . Further, although not shown in  FIG. 11 , when there are the special sheets to be matched with the next bundle of the sheets, it is arranged that such special sheets are fed to the transport pass  908  for waiting while the sheet P 1  and the sheet P 2 , which constitute the present bundle of sheets, are being transported. With this configuration, the productivity during the processing in the sort mode can be improved. 
   (General Description of Bookbinding Operation) 
   Next, the formation of the image during the bookbinding mode will be described referring to  FIGS. 12A-12D .  FIGS. 12A-12D  contain the diagrams for illustrating the process of the image formation in the image forming apparatus given in  FIG. 1  during the bookbinding mode. 
   When the bookbinding mode is specified, the originals set in the original feeding device  100  are scanned sequentially from the first page; the images scanned out from the originals are sequentially stored in the hard disk (not shown) while simultaneously counting the number of the scanned originals. For reference, the hard disk is an external memory incorporated into the controller  202  for image signal processing. 
   When the scanning of the original is finished, the original images, which have been scanned out, are classified according to the following formula (1) to decide on the sequence and the position of the image formation.
 
 M=n× 4− k   (1)
 
where M: Number of sheets of the originals
         n: any integer of 1 or more to represent the number of sheets   k: any one of the values of 0, 1, 2 and 3       

   Here, the detailed descriptions for the control of the sequence and the position of the image formation are omitted. 
   If the image formation process is described taking an example of the case where the number of sheets of the originals to be scanned is 8 sheets, as shown in  FIG. 12A , the original image data for the 8 pages (R 1  through R 8 ) is stored in a hard disk according to the sequence of reading. 
   For each of the image data (R 1  through R 8 ), the sequence and the position of the image formation are decided. By doing so, as shown in  FIG. 12B , after the previously described reflected image processing, the image of R 4  is formed on the left-hand half of the first face (surface) of the sheet P 1 , the first page, while the image of R 5  is formed on the right-hand half of the same, and the sheet P 1  is guided to the two-side transport pass  124 . Then, the sheet P 1  is transported again to the transcribing device  116  to have the image of R 6  formed on the left-hand half of the second face (back face) of the sheet P 1  while having the image of R 3  formed on the right-hand half thereof. Then, the sheet P 1  with the images formed on both the surface and the back thereof is discharged after having the sides thereof reversed in the reversing process and transported to the binding pass  553  of the finisher  500 . As the result of the discharge after reversing the sides, the sheet P 1  is transported in the direction of an arrow as shown in the  FIG. 12C  with its second face up, the second face having the image of the R 6  and the image of R 3  formed thereon, and by being preceded by the image of R 6 . 
   Subsequently, the image of R 2  is formed on the left-hand half of the first face (surface) of the sheet P 2 , second page, while the image of R 7  is formed on the right-hand half thereof, and the sheet P 2  is guided to the two-side transport pass  124 . The sheet P 2  is fed again to the transcribing device  116  to have the image of R 8  formed on the left-hand half of the second face (back face) thereof while having the R 1  image formed on the right-hand half thereof. Then, the sheet P 2  is turned over and discharged to the first binding pass  533  of the finisher  500 . By undergoing this overturned discharge process, the sheet P 2 , as shown in  FIG. 12C , is transported in the direction of an arrow as shown in the figure with its second surface up, the surface having the image of R 8  and the image of R 1  formed thereon, and by being preceded by the image of R 8 . 
   Each of the sheet P 1  and the sheet P 2  is guided into the storage guide  820  for being stored therein by way of the binding pass  553  of the finisher  500 . This storage guide  820  is arranged, as shown in  FIG. 12D , so that the sheet P 1  is stored on the side of the thrusting member  825 , while the sheet P 2  is stored on the side of the folding roller  826  respectively. Further, each of the first face of the sheet P 1  and sheet P 2  is stored facing the side of the thrusting member  825 . 
   The positioning of the each of the sheets P 1  and P 2  is made by means of the positioning member  823 . 
   (General Descriptions of the Operations of Bookbinder and Inserter) 
   The transport of the sheet to the storage guide  820  in the finisher from the inserter  900  and the printer  300  during the bookbinding mode will be described referring to FIG.  13 A through FIG.  20 B. FIG.  13 A through  FIG. 19  illustrate the flow of the sheet from the inserter and the printer to the storage guide in the finisher to the storage guide during the bookbinding mode of the image forming apparatus shown in  FIG. 1 , while  FIGS. 20A and 20B  show an example of the bookbinding process consisting of the folding process and the binding process in the finisher shown in FIG.  3 . 
   When binding the sheets by inserting the sheet C 1  for use as a cover into the bundle of sheets whereon the images have been formed, as shown in  FIG. 13B , the sheet C 1  is set on the tray  901  of the inserter  900 . In this case, as shown in  FIG. 13A , the sheet C 1  is set on the tray  901 , with its surface, whereon the image R and the image F are formed, facing up, and transported by being preceded by the image F. In other words, the sheet C 1  is set so that it can be seen in normal state by the operator or is set in the condition similar to that in which the original is set in the original feeding device  100 . Therefore, efficiency of operation for setting the sheet C 1  can be improved. 
   When the sheet C 1  is set in the tray  901 , as shown in  FIG. 14 , the feed of the sheet C 1  placed on the top of the insert sheets is started, and the switching flapper  551  is switched to the side of the finisher pass  552 . The sheet C 1  is guided into the finisher pass  552  from the transport pass  908  by way of the pair of entrance rollers  502 , and, when the front end of the sheet C 1  is detected by the entrance sensor  531 , the feed of the sheet (the sheet P 1  shown in FIG.  15 ), whereon the image has been formed, from the printer  300  will be started. 
   Next, as shown in  FIG. 15 , not only the sheet P 1  supplied from the printer  300  is guided into the finisher  500  but also the sheet C 1  is guided to the side of the non-sort pass  521  by way of the buffer roller  505 . In this case, the switching flapper  510  has already been switched to the side of the non-sort pass  521 . 
   Further, when the sheet C 1  is guided to the side of the non-sort pass  521  and is further transported until its rear end passes through the entrance sensor  531 , as shown in  FIG. 15 , the transport of the sheet C 1  is stopped once. In this case, the sheet P 1  from the printer  300  is guided into the finisher  500 . Then, while the sheet C 1  is stopped, the sheet P 1  is, as shown in  FIG. 16 , guided to the first binding pass  553  by means of the switching flapper  551  to be stored in the storage guide  820 , and the following sheet P 2  is also guided to the first binding pass  553 . In this case, the sheet C 2  following the sheet C 1  is separated and transported before the pair of transport rollers  906  to be kept waiting there until predetermined number of sheets P are stored in the storage guide  820 . 
   When the predetermined number of sheet P are stored in the storage guide  820 , as shown in  FIG. 17 , the sheet C 1  is transported in reverse direction and guided into the storage guide  820  by way of the diverging point A and the second binding pass  554 . In this case, the sheet C 1  is, as shown in  FIG. 18 , transported from the side of the image R and is stored by being placed on the bundle of the sheets P 1 , P 2 , which have already been stored in the storage guide  820 . When the sheet C 1  is stored in the storage guide  820 , the feed of the following sheet C 2  is started. Similarly to the sheet C 1 , the sheet C 2  is also guided to the side of the non-sort pass  521  and stops once. At this stage, when the sheet C 2  is found not conforming to the predetermined size, the sheet C 2  is discharged into the sample tray  701  without being stopped once in the state as is indicated in FIG.  16 . 
   After the sheet C 1  is stored in the storage guide  820  by being placed on the bundle of the sheet P 1 , P 2 , the thrusting member  825  is projected to the sheet C 1  and the bundle of the sheets P 1 , P 2  so that the bundle of the sheets is pushed out towards the pair of folding rollers  826 . The bundle of the sheets is folded at the center thereof (corresponding to the boundary between the images formed thereon) by means of the pair of folding rollers  826  and is discharged into the saddle discharge tray  832 . In such a folded state, as shown in  FIG. 20B , the image F on the sheet C 1  is formed on the front cover while the image R is formed on the last page. The images on the sheets P 1 , P 2  are formed on the pages arranged sequentially and the directions of the images on the sheet C 1  and on the sheet P 1 , P 2  coincide with on another. 
   As described previously, the feed of the sheet C 1  from the inserter  900  and the transport of the sheet P 1 , P 2  from the printer  300  can be controlled respectively so that, in the bookbinding mode, not only the image F on the sheet C 1  is made to appear on the cover and the image R on the sheet C 1  is made to appear on the last page while the images on the sheets P 1 , P 2  are made to appear in the order of the page numbers but also the directions of the images can be made to coincide with one another. Thus, the sheets and the special sheets can be combined in the process of bookbinding without spoiling the quality of the print on the special sheets supplied from the inserter  900  and without adversely affecting the sheet transport durability of the printer  300 . Furthermore, during the sort mode, the special sheet to be inserted can be kept waiting in the non-sort pass  521  by means of the finisher  500  and then the sheets from the printer can be guided to be stored in the storage guide  820  and then, the special sheet, which has been kept waiting in the non-sort pass  521 , is guided into the storage guide  820  for being stored. Therefore, the productivity of binding operation including the process for combining the sheet and the special sheet can be improved. 
   Further, when necessary, the sheet C 1  overlapped with the bundle of the sheet P 1 , P 2  in the storage guide  820  can be bound at the center thereof with the stapler. 
   (Description of Operation by Flowchart) 
   The insert processing in the sheet insertion mode as an embodiment of the present invention will be described referring to the Flowchart of FIG.  21 . 
   A bundle of sheets can be prepared by means of the inserter  900 . For example, the inserter  900  can be used in preparing a bundle of sheets consisting of 6 sheets, of which the second, the third and the sixth sheets as being the special sheets are fed from the inserter  900 , and the images on the first, the fourth and the fifth sheets are formed by the image forming apparatus  10 . In the following description, the bundle of sheets is assumed to consist of six sheets. In feeding the special sheets from the inserter  900 , the place for inserting the special sheet among the bundle of the sheets can be set by the operating section  153  of the image forming apparatus  10 , and any desired number of sheets can be inserted. Further, in preparing several bundles of the sheets, the sheets to constitute bundles are set bundle by bundle on the tray  901  in the order of feeding from the inserter  900 . In other words, in the case the above example, the insert sheets are set sequentially as the second, the third and the sixth sheets of the first bundle and as the second, the third and the sixth sheets of the second bundle and so on. In this case, as shown in  FIG. 23 , the first, the fourth and the fifth sheets are loaded on the original tray of the original feeding device  100 . Further, as shown in  FIG. 24 , the number of sheets corresponding to registered or prepared number, i.e., a set of 3 sheets, namely, the second, the third and the sixth sheets, are stacked on the inserter  900 . 
   The sequence for feeding the sheets is specified by means of the control section  153  of the image forming apparatus  10  before turning on the copy start key (S 151 ). Depending on the bundle of the sheets to be prepared, the timing at which the feed of the sheet whereon the image has been formed by the image forming apparatus  10  is to be started and the timing at which the insert sheets are to be fed from the inserter  900  are adjusted by the image forming apparatus  10  (S 152 ). 
   The image forming apparatus  10  determines whether the first sheet is one supplied from the inserter or not (S 153 ). For example, in the above-mentioned case, the first sheet is supplied from the image forming apparatus  10  (S 154 ). At S 154 , the sheet for recording, which has been kept waiting after having been previously transported to the resist roller  126  from the cassettes  114  and  115 , is transported to the transcribing device  116 . 
   Further, for the timing (the second sheet in the above example) by which the sheet is to be fed from the inserter, which is the second sheet feeding device, the command requiring the feed of the insert sheet from the inserter  900  is issued to the finisher  500 . When the insert sheet is supplied from the inserter  900  (S 155 ), whether the overlapped sheets have been supplied or not is determined by the overlapped supply detection sensor  950 , as being a overlapped transport detection device. (S 156 ) 
   Here, how to determine the overlapped transport will be discussed briefly. In determining the occurrence of the overlapped transport in the case of the sheet insertion mode, the thickness (dn) of the sheet to be inserted is measured by the overlapped transport detecting sensor  950  when preparing the first bundle of the sheets, and the thickness of each sheet (d 1 , d 2 , . . . , dn; 1 through n are page numbers) is stored in the RAM 513 . The data of the sheet thickness is used as the source of the reference values in determining the overlapped transport of the sheet for the subsequently prepared bundle of the sheet. Then, when preparing the bundles of sheets from the second bundle on, the thickness (Xn) of each of the insert sheets is measured at the time of the passage of the overlapped transport sensor so that the measured data concerning the thickness of the sheet for n-page can be compared with the data dn concerning the thickness of the sheet stored in the RAM 513 . The determining processing the overlapped transport is processed by the CPU provided on the side of the finisher. 
   When it is found that there has been no overlapped transport (S 157 ), the sheets are transported to the processing tray  630  (S 158 ). The image forming apparatus  10  examine whether or not the sheet concerned is the last sheet of the bundle (S 159 ). When it is found that the sheet concerned is not the last sheet, the step of the processing is put back to the step S 152  for controlling the next sheet. 
   When the sheet is found to be the last sheet of the bundle at the step S 159 , the bundle of the sheets is discharged onto the stack tray  700  from the stack tray  630  (S 160 ). In this stage, the bundle of the sheets discharged onto the processing tray  630  can be bound by means of the stapler  818 . 
   At step S 161 , it is examined whether the discharge of the last bundle is finished or not. If not finished, the processing goes back to S 152 ; if discharge of the last bundle is finished, the processing comes to an end. 
   Next, the processing to be made in step S 157  in the case where the overlapped transport of the insert sheets is detected will be described. At step S 162 , the number of overlapped sheets is determined. Here, the procedure for determining the number of overlapped sheets will be described briefly. 
   In the above example, the number of insert sheets is 3 sheets, namely, the sheets for the second, the third and the sixth pages, and it is assumed that the sheet for the second page is transported overlapping with the sheet for other page. In this case, where the thickness X 2  of the second page satisfies
 
 d   2 +β 3   d   3 &lt; X   2 &lt; d   2 + d   3 +β d   6 ,
 
it can be concluded that the 2 sheets, i.e., the second page and the third page, were overlapped when transported, where it is assumed that β=0.5. In general, where a set of insert sheets loaded on the inserter consists of n sheets, in determining whether the overlapped number of sheets occurring when m-th sheet is to be transported is t or not, it can be determined by
 
 dm+d ( m+ 1)+ . . . + βd ( m+t− 1)&lt; Xm&lt;dm+d ( m+ 1)+ . . . +β d ( m+t ).
 
   In this way, the transported number of overlapped sheets can be determined by using the reference values of the sheet thickness stored in the RAM  513 . 
   When it is detected that the two sheets, i.e., the second page and the third page, have been transported in overlapped condition as the result of the determination of the transported number of overlapped sheets, the sheets transported in overlapped condition are discharged onto the processing tray  630  (S 163 ). Further, it should be noted that this processing is executed on the side of the finisher. 
   Next, the processing for the void transport of the insert sheet is executed (S 164 ). Here, an explanation about the void transport will be made briefly. 
   Following the transport of the overlapped sheets for the second and third pages, the sheet for the sixth page, which is to be inserted into the same bundle of the sheets, is also discharged onto the processing tray  630 . In other words, when the overlapped transport has occurred with respect to a sheet inserted among n-th bundle (including the case where the insert sheets have been transported overlapping with last sheet of (n−1)-th bundle), all of the insert sheets of n-th bundle are discharged onto the processing tray  630  to bring the first sheet among (n+1)-th bundle to the condition in which the feed of the sheet is possible. By carrying out such void transport processing, the sheet to be inserted as the first page of the next bundle can be selected. In other words, the first sheet of the next bundle or the next set of the insert sheets can be found. 
   The number of sheets for void transport is determined according to the procedure described below. 
   Where it is assumed that the number of insert sheets corresponding to one set of bundled sheets (record sheets and insert sheets) is X; and the insert sheet, which has been overlapped with other sheet when transported, is the Y-th sheet among one set of insert sheets; and the number of sheets transported in overlapped condition is Z, it follows that
 
 Y+ ( Z− 1),
 
so that the number of sheets, which have been fed for insertion among the bundle of the insert sheets (one set of insert sheets to be inserted into one set of bundled sheets) at point in time when the overlapped transport has occurred, can be obtained.
 
   When the sheet, which has been transported in overlapped condition, belongs to the same bundle of the insert sheets, that is, where
 
 Y+ ( Z− 1)≦ X, 
 
   It follows that the number of sheets of void transport X−(Y+(Z−1)). 
   Further, where the sheet, which have been transported in overlapped condition, is involved in the next bundle of sheets and the following bundles of sheets, that is, where
 
 Y +( Z− 1)&gt; X, 
 
It follow that the number of sheets of void transport=X−{(Y+(Z−1))/remainder of X}.
 
   During this process, the formation of the image on the record sheet is suspended. 
   Then, when the sheets transported in overlapped condition (S 163 ) and the insert sheets transported in void condition (S 164 ) are discharged onto the processing tray  630 , the sheets whereon the images have been formed, the sheets transported in overlapped condition and the insert sheets, which are loaded on the processing tray  630 , are discharged onto the stack tray  700  (S 165 ). In this case, even when the post-processing mode, such as the stapling mode, has been selected, the post-processing is skipped for the discharge onto the stack tray  700 . 
   Then, in order to start preparing a new bundle, the feed of insert sheet or the record sheet is started from the first page (S 152 ). In the above case, since the first page is the sheet for recording, the formation of the image is resumed from that on the first page. 
   What is described above is the flow of the processing in the whole system (including the main unit of the image forming apparatus and the finisher) as is illustrated in  FIG. 21 , and, further, in order to clarify the functions of the main unit of the image forming apparatus and the finisher, the flow of the control signal between the main unit of the image forming apparatus and the finisher is shown in  FIGS. 22A and 22B . 
     FIG. 22A  shows the flow of the control signal when the sheet feeding process by the inserter of the finisher is finished normally (without overlapped transport of the sheets), while  FIG. 22B  shows the flow of the control signal when the transport of the sheet in overlapped condition has occurred during the paper feeding process by the inserter. 
   When the command requiring the feed of the sheet by the inserter is outputted from the main unit of the image forming apparatus to the finisher, the finisher measures the thickness of the sheet to determine whether the sheets in overlapped condition have been transported or not. When it is found by the finisher that the feed of the sheet by the inserter has been finished normally, the signal for telling the normal completion of the feed of the sheet is outputted to the main unit from the finisher (FIG.  22 A). 
   When the finisher has detected the transport of the sheets in overlapped condition, the finisher counts the number of sheets transported in overlapped condition on the basis of the data concerning the sheet thickness. Then, the occurrence the overlapped transport of sheets and the number of overlapped sheets is notified to the main unit from the finisher. In receipt of such notice, the commands for the void transport and the number of sheets for void transport are issued to the finisher from the main unit (FIG.  22 B). 
   In the case of another embodiment wherein it is timed that the image is formed on the next record sheet during the feed of the insert sheet, if this method is applied to the above case, during the feed of the insert sheet as the second page, the record sheet, which has been kept waiting at the resist roller  126 , is transported to the transcriber  116  to form the image on the record sheet for the fourth page. When the overlapping of the insert sheets corresponding to the second and the third pages has occurred at the timing of the transport thereof, the feed of the sheet from the resist roller  126  is prohibited at the point when the overlapped transport is detected, and the insert sheets transported in overlapped condition and the following record sheet corresponding to the fourth page are discharged onto the processing tray  630  for void transport processing. When the record sheet for the fourth page in transport is nipped between the resist rollers  126 , the drive is stopped after the record sheet has passed through the resist rollers  126 . In other words, the recording sheet, whose front end had passed the resist rollers  126  (or on the resist rollers) at the point when the overlapped transport occurred, is discharged. 
   Further, when recording sheets for the fifth page have already been supplied from the cassettes  114  and  115 , this recording sheet will not be used as the recording sheet for the fifth page but will be used as the recording sheet for recording the image of the fist page after the recovery from the overlapped transport, and thus such recording sheet is transported to the resist rollers  126  to be kept waiting there. 
   Then, for the recording sheet which has been kept waiting at the resist rollers  126 , the setting for having the image of the fifth page formed thereon will be cancelled, and the resetting will be made for having the image from the first page formed thereon. 
   Owing to the control system described in the foregoing, even if the overlapped transport has occurred with respect to the sheet for insertion, the condition for enabling the printing to be started from the front page of the next bundle can be recovered automatically. 
   As discussed in the foregoing, the system according to the present invention provides only one discharge tray and is designed so that appropriate recovery processing can be made automatically even if the transport of the sheets for insertion in overlapped condition has occurred, thereby liberating the user from the need of the recovery processing that requires the intervention by the user and providing the users with an image forming apparatus having improved utility. 
   Furthermore, (the image forming apparatus) according to the present invention enables the users to re-use the expensive sheets for insertion even after the overlapped transport has occurred. 
   The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications maybe made without departing from the invention in its broader aspect, and it is the intention, therefore, in the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.