Patent Publication Number: US-6217016-B1

Title: Sheet processing apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a sheet processing apparatus for fetching sequentially sheets on which images are formed by an image forming apparatus such as, e.g., photocopiers, printers, and so on, stapling the plural sheets with a stapling means, and folding the sheets in folio and an image forming apparatus. 
     2. Description of Related Art 
     Some image forming apparatus such as photocopiers or the like today can do bookbinding by stapling plural sheets and folding the sheets in folio where connected with a sheet processing apparatus. Such a sheet processing apparatus capable of bookbinding fetches sequentially the sheets on which images are formed at the image forming apparatus body, staples the sheets at about the center of the sheet bundle upon driving the stapler unit, and folds the sheets in folio where the sheets are conveyed to a folding means. 
     As a structure for folding the sheets in folio, it is structured by a folding roller means made of a roller pair, and a projecting means made of a projecting plate and so on. The projecting plate folds the sheet bundle upon projecting the stapled position of the sheet bundle between the nip portions, and the folded sheet bundle is pressed and conveyed by the roller pair to fold the bundle in folio. Therefore, the sheet bundles delivered to the delivery tray are delivered in a state that the sheet bundles are folded in folio and bound in a book form at the center stapled position of the sheets. 
     However, in the folding roller means, at least one roller is movable to change the distance to the nip position according to the thickness of the sheet bundle. If the projecting position by the projecting plate is constant, the plate cannot hit the center of the distance between the nips of the roller pair when the thickness of the sheets is deviated. That is, if the sheet bundle becomes thick, the projecting plate hits the center between the nips until the sheet bundle is nipped by the folding roller, but when the folding roller nips the sheet bundle to change the distance between the nips, the plate may hit a position shifted from the center of the distance between the nips, and therefore, the folding position may be shifted due to differentials in loads given to upstream and downstream of the sheet bundles, or wrinkles may occur on the sheet and may cause tears in sheets. 
     This invention is made with respect to the above viewpoint, and it is an object to provide a sheet processing apparatus in which the projecting means can always hit the center of the distance between the nips as the projecting position even where the distance between the nips of the folding roller is changed. 
     SUMMARY OF THE INVENTION 
     A representative structure of the invention to accomplish the above object is characterized in a sheet processing apparatus for stapling multiple sheets with a stapling means and delivering the sheets in folio, including: two rollers for folding the sheets in folio; and projecting means for projecting between the two rollers to fold the sheets in sandwiching the sheets between the two rollers, wherein at least one roller of the two rollers is made movable, and a cam member is further provided capable of correcting a projecting position by the projecting means according to traveling of the roller. 
     In another representative structure of the invention, a sheet processing means for folding sheets in folio includes: a first roller having a fixed rotational position; a second roller having a rotational position movable; projecting means for projecting between the first roller and the second roller to fold the sheets in sandwiching the sheets between the two rollers; and a cam member for moving the projecting position by pushing the projecting means in the traveling direction of the second roller in association with travel of the second roller. 
     With the above structure, even where the distance between the nips of the two rollers is changed, the cam member operative according to changes of the distance between the nips of the two rollers corrects the projecting position of the projecting means, and therefore, the projecting means always hits the center of the distance between the nips of the two rollers, so that work for folding sheets in folio can be done with high quality. 
     In the above sheet processing apparatus, the movable roller (the second roller) may have a structure to travel according to the thickness of plural sheets, thereby performing work for folding sheets in folio according to the number of sheets to be processed. The cam member may be pivotally mounted around a roller shaft, as a center, of one roller between the two rollers, and include a cam groove capable of engaging with the other roller shaft, and a guide portion capable of pressing the projecting means. From this simple structure, the projecting position of the projecting means can be moved surely in accordance with the travel of the roller. 
     Moreover, in the above sheet processing apparatus, the projecting means, after projecting operation, returns to a home position, thereby being capable of preventing wrinkles from occurring inside due to frictional force between the projecting means and the folded sheets. Particularly, the projecting means returns to the home position after at least one roller rotates in a prescribed amount, so that the sheets are surely nipped by two rollers. 
     The above sheet processing apparatus can have an image forming means body to form images on sheets to structure an image forming apparatus having substantially the same advantages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration showing the whole structure of an image forming apparatus; 
     FIG. 2 is an illustration showing a cross-sectional structure of a finisher unit; 
     FIG. 3 is an illustration showing a cross-sectional structure of a stitcher unit; 
     FIG. 4 is a perspective illustration showing a sheet status of a sheet delivered by an offset operation; 
     FIG. 5 is an illustration showing squeezing for a sheet delivered by the offset operation and a status of the delivered sheet; 
     FIG. 6 is an illustration showing a status in which a proceeding sheet is left over in a buffer path in a double sheet delivery control; 
     FIG. 7 is an illustration showing a status in which two sheets are conveyed at the same time 
     FIG. 8 is a flowchart showing a delivery signal transmission timing for the proceeding sheet in the double sheet delivery control; 
     FIG. 9 is a flowchart showing a delivery signal transmission timing for the proceeding sheet in the double sheet delivery control according to this embodiment; 
     FIG. 10 is an illustration showing a status of a sheet in a case where a side guide top is not used as a guide for sheet delivery; 
     FIG. 11 is an illustration showing a status of a sheet in a case where a side guide top is used as a guide for sheet delivery according to this embodiment; 
     FIG. 12 is an illustration showing a status for escaping the side guide after the front end of the sheet is nipped by a downstream delivery roller pair; 
     FIG. 13 is a cross section showing a position of the stack tray as the essential portion; 
     FIG. 14 is an enlarged view showing as the essential portion a status of a rocking guide and a paddle when the sheets are pulled back; 
     FIG. 15 is an enlarged view showing as the essential portion a status of a rocking guide and a paddle when the sheets are pulled back; 
     FIG. 16 is a view showing an example of a paddle shape; 
     FIG. 17 is a diagram exemplifying drive times of the paddle, moving speed of the side guide, and alignment control according to the sheet size; 
     FIG. 18 is an illustration showing a rear end stopper; 
     FIG. 19 is a diagram showing a waiting position when the stapler serves as a rear end stopper; 
     FIG. 20 is a diagram showing a width alignment status by the side guide; 
     FIG. 21 is a diagram showing a width alignment status by the side guide; 
     FIG. 22 is a diagram showing a status of a knurled belt during the sheet width alignment by the side guide; 
     FIG. 23 is a view showing an example of a knurled belt shape; 
     FIG. 24 is a diagram showing a width alignment status by the side guide; 
     FIG. 25 is an illustration showing an operation status of a rocking guide and a drive mechanism of a downstream delivery roller; 
     FIG. 26 is an illustration showing an operation status of the rocking guide and the drive mechanism of the downstream delivery roller; 
     FIG. 27 is an illustration showing an operation status of the rocking guide and the drive mechanism of the downstream delivery roller; 
     FIG. 28 is a flowchart showing a flow of position control when the rocking guide is made closed; 
     FIG. 29 is a flowchart showing a flow of an extraordinary completion processing when the rocking guide is made closed; 
     FIG. 30 is an illustration showing a low speed drive control when the rotational direction of a drive motor is switched; 
     FIG. 31 is an illustration showing a staple operation according to this embodiment; 
     FIG. 32 is an illustration showing a stapler and a staple cartridge. 
     FIG. 33 is a flowchart illustrating staple cartridge exchange processing; 
     FIG. 34 is a flowchart illustrating stple initialization processing; 
     FIG. 35 is an illustration of a conventional control when staple jamming occurs; 
     FIG. 36 is a diagram showing a control when staple jamming occurs according to this embodiment; 
     FIG. 37 is an illustration showing a stapler initialization when a stapler door is closed; 
     FIG. 38 is an illustration showing a setting up speed for delivery motor; 
     FIG. 39 is an illustration showing a status that the sheet is delivered to the stack tray and a perspective view showing a schematic structure of an essential portion of a tray unit portion; 
     FIG. 40 is a flowchart showing a control when full stacking is detected; 
     FIG. 41 is a flowchart showing a full stacking detection for special sheets; 
     FIG. 42 is a block diagram showing an outline of a control system for finisher unit; 
     FIG. 43 is a partially enlarged view showing a structure of a pickup roller; 
     FIG. 44 is an illustration showing operation of the pickup roller; 
     FIG. 45 is a view showing a shape of a vertical path; 
     FIG. 46 is a diagram showing a shape of staked sheets; 
     FIG. 47 is an illustration showing a drive mechanism for stopper; 
     FIG. 48 is an illustration showing a structure of the stapler unit; 
     FIG. 49 is an illustration showing staple filling to the staple cartridge; 
     FIG. 50 is an illustration showing a motor drive current waveform during the staple initialization; 
     FIG. 51 is an illustration showing a status that a staple fitted in a groove on the anvil is released by the pickup roller; 
     FIG. 52 is a flowchart in a case where the shifting amount of the stopper to the staple stopping position and folding stopping position is automatically adjustable; 
     FIG. 53 is an illustration showing a movable structure of a movable roller; 
     FIG. 54 is a side illustration showing a drive structure of the folding unit; 
     FIG. 55 is a plan illustration showing the drive structure of the folding unit; 
     FIG. 56 is an illustration showing a stopper structure of a projecting plate; 
     FIG. 57 is an illustration showing a structure of a cam member for adjusting the center of the projecting plate; 
     FIG. 58 is a state illustration when the projecting plate is pulled back; 
     FIG. 59 is an illustration showing mechanisms of occurrences of tears and wrinkles in sheets during the folding operation; 
     FIG. 60 is an illustration showin an image forming area and a margin for folding on a sheet; 
     FIG. 61 is a diagram showing a layout of an intermediate sensor, a stopper sensor, and a delivery sensor which detect tears in sheets; 
     FIG. 62 is an illustration showing a shiftingly stacking state of sheet bundles on a stack tray; 
     FIG. 63 is an illustration showing a stacking plate of sheet bundles on a stack tray; and 
     FIG. 64 is a block diagram showing an outline of a control system for stitcher unit. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, an embodiment according to the invention is shown. 
     FIG. 1 is an illustration showing an inner structure of a photocopier as an example of an image forming apparatus to which this invention is applicable. This photocopier is structured having an image forming apparatus body A combined with a sheet processing apparatus B. The sheet processing apparatus B includes a finisher unit C capable of sorting the sheets on which images are recorded at the image forming apparatus body A according to the number of copies and a stitcher unit D capable of bookbinding the multiple sheets upon stapling the sheets. 
     Herein, the whole structure of the image forming apparatus is generally described, and subsequently, concerning the structure of the sheet processing apparatus, the finisher unit C and the stitcher unit D are described in detail. 
     [The Whole Structure of the Image Forming Apparatus] 
     The image forming apparatus body A reads, in an optical way with an optical means  2 , original documents automatically fed from the original document feeding apparatus  1  mounted on the top of the apparatus and transmits the read information to an image forming means  3  as digital signals for recording the information on recording sheets such as plain papers or OHP sheets. 
     Multiple sheet cassettes  4  in which sheets of various types are contained are set at the lower portion of the image forming apparatus body A, and the image forming means  3  records images in an electrophotographic method on the sheets fed by the conveyance rollers  5  from the sheet cassettes  4 . That is, latent images are formed on a photosensitive drum  3   b  by radiating a laser beam on the photosensitive drum  3   b  from a light radiating means  3   a  based on the information read through the optical means  2 . The latent images are developed with toners and transferred onto the sheets, and the sheets are conveyed to a fixing means  6  to fix the images permanently in application or heat and pressure. 
     In a case of a single side recording mode, the sheet is sent to a sheet processing apparatus B, and in a case of a double side recording mode, the sheet is conveyed to a re-feeding path  7  in a switchback way, thereby sending the sheet to the sheet processing apparatus B after forming images on the other side where the sheet whose one side is recorded is conveyed to the image forming means  3  again. 
     It is to be noted that the sheets can be fed not only from the sheet cassettes  4  but also from the multi-tray  8 . 
     The finisher unit C in the sheet processing apparatus B is structured as shown in FIG.  2 . To deliver the sheets, the finisher unit C can do delivery operations according to respective modes such as, in addition to a normal delivery mode, an offset mode, a staple mode, and so on. 
     The offset mode here is the operation mode in which, when the sheets are delivered upon sorting them by respective copies, the first sheet of each copy is positionally shifted in a sheet width direction (a direction perpendicular to the sheet conveyance direction) by a prescribed amount by actuating a side guide  11  when delivered, and the sheets of the second or latter are delivered in the normal fashion, thereby distinctively showing the boundaries of respective copies. 
     It is to be noted that, where no space to which the sheets are shifted is available with respect to size in the sheet without direction, a reference guide  37  is made to escape lower than the level of a staple tray  12  serving as a temporally stacking means, thereby ensuring the sheet shifting amount adequately. 
     The staple mode is an operation mode in which, when the sheets are delivered upon sorting them with respect to each copy, the sheets are stacked and aligned on the staple tray  12  and stapled with a stapler  13  to deliver them upon stapling the sheets with respect to each copy. 
     It is to be noted that to deliver the sheets, this apparatus can perform a double sheet delivery control capable of delivering two sheets at the same time, in addition to the normal delivery control for delivering the sheets sheet by sheet. In this double sheet delivery control, a sheet sent to the sheet processing apparatus B from the image forming apparatus body A is stored in a buffer path  14  arranged in the finisher unit C, and the sheet is overlapped with another sheet subsequently delivered to deliver the two sheets at the same time. 
     Meanwhile, the stitcher unit D in the sheet processing apparatus B is structured as shown in FIG.  3 . The sheets delivered from the image forming apparatus body A are aligned with respect to each copy and stapled by means of the staple unit, and the stapled sheets are folded in folio and bound into books. In briefly speaking, the sheets delivered from the image forming apparatus body A are conveyed to a vertical path  60  of the stitcher unit D; the sheets are stacked and aligned on a copy basis so that the lower end of the sheet hits a stopper  62 ; the stacked sheets are bound upon stapling the sheets with a stapling unit  61  at two locations of a center in the sheet lengthwise direction (sheet conveyance direction). 
     The stopper  62  is moved downward to move the sheet bundle so that the bound portion reaches a nip position of folding rollers  78 , and where the bound position is struck by a striking plate  79 , the sheet bundle is nipped and conveyed by the nip rollers  78  as to be folded in folio at the bound position. This operation makes the sheet bundle bound at the center in the sheet lengthwise direction as well as delivered on a stacking tray  106  in a folio bound form. 
     [Finisher Unit] 
     The sheet P delivered to the finisher unit C from the image forming apparatus body A according to this embodiment is, in the normal mode, conveyed to a conveyance roller  15  and delivered to the stack trays  18  by means of an upstream delivery roller pair  16  and a downstream delivery roller pair  17 . The stack trays  18  are provided in a plural number as movable in a vertical direction by a driver installed at a lower side of the trays. When sheets are delivered upon sorting, the plural stack trays  18  are moved in a shifting manner step by step at the delivery opening, thereby delivering the sheets P where the sheets P are sorted with respect to each copy. In a case of the offset mode and the staple mode, sheets P can be delivered to the sole stack tray  18  in a sorted state upon offset operation or staple operation. Moreover, in a case of an interruption mode, the sheets P can be delivered on an upper tray  19  without delivered to the stack tray  18 . 
     [Offset Delivery Processing] 
     The finisher unit C according to this embodiment is capable of sorting the sheets in the offset mode as described above. In this mode, as shown in FIG. 4, all copies are delivered on the single stack tray  18 , and where the sheets are delivered on the copy basis, the first sheet P 1  is positionally shifted in the sheet width direction with respect to the sheets P of the second and latter, thereby rendering the boundaries of the copies clear. 
     The downstream delivery roller pair  17  is structured having a downstream delivery roller  17   a  formed at a unit body and a movable delivery roller  17   b  attached to a rocking guide  20  which is capable of rocking around a shaft with respect to the unit body. When the rocking guide  20  moves up as to open, each roller of the downstream delivery pair  17  becomes separated from one another as shown in FIG.  4 . The side guide  11  is provided movably in the sheet width direction between the upstream delivery roller pair  16  and the downstream delivery roller pair  17 , serving as an aligning means guiding one edge of the sheets P in the width direction. During the offset mode, when the first sheet of each copy to be sorted is delivered, the rocking guide  20  is moved up as to open at a time that the rear end of the sheet falls onto the staple tray  12  upon conveyed between the upstream delivery roller pair  16  and the downstream delivery roller pair  17 , and subsequently, the side guide  11  is moved in the arrow direction, thereby shiftin the first sheet P 1  for a prescribed amount. The sheet P 1  is then delivered onto the stack tray  18  upon closing the rocking guide  20 . Subsequently, the sheets P of the second and latter are delivered in the normal fashion, and the sheets are delivered in a form that the first sheet P 1  of each copy is positionally shifted as shown in FIG.  4  and FIG.  5 . As described above, where no space in which the sheet is moved by the prescribed amount in the sheet width direction is available, the reference guide  37  is made to escape lower than the staple tray  12 , thereby ensuring the sheet shifting amount adequately. 
     [Double Sheet Delivery Control During Offset] 
     In the offset delivery, when the first sheet P 1  of each copy is delivered, the offset processing of the sheet P 1  as described above is required, and therefore, the sheets P of the second and latter cannot be delivered until that the processing finishes. Accordingly, the delivery of the second sheet has to be suspended, and the processing time is required to be longer. 
     The second sheet P is temporarily stored in the buffer path  14  during the offset processing in this embodiment, and the second sheet and the third sheet are delivered at the same time, thereby shortening the processing time by delivering the sheets without suspending the sheet delivery even in the offset mode. 
     The double sheet delivery control for such operation is described. While the first sheet is subjecting to the offset processing, when the second sheet is conveyed to the finisher unit C from the image forming apparatus body A, the second sheet is sent to the buffer path  14  by positioning upstream side ends of a first flapper  21  and a second flapper  22  downward as shown in FIG.  6 . The proceeding sheet P 2  sent to the buffer path  14  (in the case of the offset mode, the second sheet) is transferred in the shown arrow direction as to wind a buffer roller  23  by means of the buffer roller  23  which is rotatively driving and a buffer roller  24  driven rotatively in pushing the sheet to the buffer roller  23 . A third flapper  25  is driven as to send the proceeding sheet p 2  in a direction that the sheet is wound around the buffer roller  23 . 
     When a buffer sensor  26  detects the front end of the proceeding sheet P 2 , and when the front end of the proceeding sheet P 2  reaches the prescribed position, the buffer roller  23  is stopped to rotate, and the sheet is stopped in the buffer path  14 . As shown in FIG. 6, when the subsequent sheet P 3  (in the case of the offset mode, the third sheet) enters, the buffer roller  23  begins to rotate, and as shown in FIG. 7, this unit conveys the proceeding sheet P 2  and the subsequent sheet P 3  in an overlapped manner. When the rear end of the proceeding sheet P 2  passes by the position of the third flapper  25 , the third flapper  25  rotates the two sheets P 2 , P 3  in the direction toward the upstream delivery roller pair  16 , thereby delivering the sheets P 2 , P 3  as they are stacked onto the stack tray  18 . 
     The double sheet delivery control as described above prevents sheets from being delivered from the upstream delivery roller pair  16  during the offset processing operation, and therefore, the operation of the image forming apparatus body is not necessarily stopped. Accordingly, in the offset mode, the processing time is made not longer, and the sheets can be quickly delivered in the offset manner. 
     It is to be noted that in this embodiment, the buffer roller  23  winds a single sheet P on the roller, but the roller can wind two or more sheets to deliver three or more sheets at the same time in order to compensate more time for offset processing operation. The sheet wound on the buffer roller  23  is solely delivered or stacked even without any subsequent sheet. 
     Although in this embodiment, an example in which the sheet subjecting to the offset is the first sheet of each copy is exemplified, this operation is not limited to such a manner, and this invention is effective even where the last sheet of each copy is positionally shifted. The sheet number subjecting to the offset is not limited to a single number and can be a plural number of sheets. 
     The double sheet delivery control can be executed not only in the offset processing but also in the staple processing in the staple mode as described below, so that time for the staple processing can be used in an advantageous way. 
     [Sheet Waiting Position of Double Sheet Delivery Control] 
     In the double sheet delivery control described above, the conveyance has to be made so that a positionally shifting amount between the front ends of the proceeding sheet P 2  waiting in the buffer path  14  and the subsequent sheet P 3  delivered from the image forming apparatus body A becomes constant. To accomplish this, the buffer roller  23  begins to rotate after the subsequent sheet P 3  passes the position of an entry sensor  27  shown in FIG. 6 or after predetermined clocks are counted up upon passage of the subsequent sheet P 3  at the position of the entry sensor  27 , thereby rendering constant the shifting amount between the front ends of the proceeding sheet P 2  and the subsequent sheet P 3 . 
     However, the conveyance speed of the subsequent sheet P 3  delivered from the image forming apparatus body A is changed according to the image formation mode, kinds of sheets, and the like. If the conveyance speed is different between the proceeding sheet P 2  and the subsequent sheet P 3 , positional deviations may occur at the front ends of the proceeding sheet P 2  and the subsequent sheet P 3  because the starting timing of the buffer roller  23  is the same. 
     This embodiment is therefore structured in which the front end position of the proceeding sheet P 2  to be stored in the buffer path  14  is changed according to the conveyance speed of the subsequent sheet P 3  because the conveyance speed of the subsequent sheet P 3  depending on the image formation mode, kinds of sheets is retrievable from the image forming apparatus body. More specifically, in FIG. 6, it is set that a conveyance amount of sheets from time when the front end of the proceeding sheet passes the position of the buffer sensor to time when the sheet stops becomes much when the conveyance speed of the subsequent sheet P 3  is high and conversely, less when the speed is low. According to this operation, a period from the beginning of rotation of the buffer roller  23  to a time which the front end of the proceeding sheet P 2  reaches a meeting point with the subsequent sheet P 3  is short when the conveyance speed of the subsequent sheet P 3  is fast and long when the conveyance speed is slow. Therefore, even where the starting timing of the buffer roller  23  is constant, the proceeding sheet P 2  and the subsequent sheet P 3  can be always conveyed with constant shifting amounts between the front ends of the proceeding sheet P 2  and the subsequent sheet P 3 . 
     It is to be noted that, as a structure for making constant the positions of the front ends of the proceeding sheet P 2  and the subsequent sheet P 3 , this operation can be performed by changing the stating timing of the rotation of the buffer roller  23 , in addition to the method for changing the waiting position of the proceeding sheet P 2  as described above. For example, while the proceeding sheet P 2  is held at a fixed position in the buffer path  14 , the buffer roller  23  starts rotating right after the subsequent sheet P 3  passes by the entry sensor  27  where the conveyance speed of the subsequent sheet P 3  is fast and after predetermined time passes after the subsequent sheet P 3  passes by the entry sensor  27  where the conveyance speed of the subsequent sheet P 3  is slow. The positionally deviated amount between the front ends of the proceeding sheet P 2  and the subsequent sheet P 3  can be made constant by changing the conveyance starting timing of the proceeding sheet P 2  according to the conveyance speed of the subsequent sheet P 3 . 
     It is to be noted that although the conveyance speed of the subsequent sheet P 3  can be retrieved from the image forming apparatus body A as described above, it is retrievable by detecting the conveyance speed of the proceeding sheet P 2  because the subsequent sheet is generally conveyed with the same speed as that of the proceeding sheet P 2 . 
     [Delivery Signal Transmission Timing in the Double Sheet Delivery Control] 
     A sheet is transferred from the image forming apparatus body A to the sheet processing apparatus B, and the sheet delivery signal is transmitted when a prescribed processing is done. As shown by a flowchart in FIG. 8, in the double sheet delivery control, however, if the apparatus is structured in a way in which a sheet is detected by a loading sensor  28  (see, FIG.  2  and FIG. 6) (S 1 ), in which a delivery signal of the proceeding sheet P 2  is transmitted at a time when the proceeding sheet P 2  is conveyed in the buffer path  14  (S 2 ), in which the subsequent sheet P 3  is then conveyed to a predetermined position, and in which the delivery signal of the subsequent sheet P 3  is then transmitted after the double sheet delivery is made (S 3  to S 5 ), the proceeding sheet P 2  in the buffer path  14  is removed together with the subsequent sheet P 3  by paper jam recovering process where the apparatus stops due to paper jamming or the like of the subsequent sheet P 3  after the delivery signal is transmitted (between S 2  and S 3  in FIG.  8 ). Therefore, when image formation is resumed in a successive manner upon recovery from the paper jamming, though the proceeding sheet P 2  is not actually delivered in the stack tray  18  or the like, the apparatus itself recognizes it as the already delivered paper due to the transmission of the delivery signal. Therefore, the processing is made by skipping the page. 
     In this embodiment, as shown by a flowchart in FIG. 9, the delivery signal is not transmitted at a time when the proceeding sheet P 2  is brought to the buffer path  14 , the proceeding sheet P 2  is placed as to be overlapped with the subsequent sheet P 3  and delivered together from the buffer path  14 . The delivery signal of the proceeding sheet P 2  and subsequent sheet S 3  is transmitted at a time when a delivery sensor  29  (see, FIG.  2  and FIG. 6) located right before the upstream delivery roller pair  16  detects the front ends of both sheets P 2 , P 3  (S 11  to S 15 ). 
     With such a structure, even where the subsequent sheet P 3  is jammed to stop the apparatus while the proceeding sheet P 2  is waiting in the buffer path  14  (between S 11  and S 12  in FIG.  9 ), the delivery signal has not been transmitted yet at that time, if the image formation is resumed upon recovery from the paper jamming after the proceeding sheet P 2  is removed from the buffer path  14  during the paper jamming recovery, the images can be formed in restarting with the proceeding sheet P 2 , so that this apparatus can prevent skipping pages from occurring. 
     It is to be noted that the double sheet delivery control described above is effective during the offset processing and the staple processing as described below, this double sheet delivery control can be done at processings other than the above. Processings without the double sheet delivery control can be executed as a matter of course. 
     [Shape of the Side Guide] 
     In the double sheet delivery control thus described, or in the normal delivery mode, the sheet is delivered on the stack tray  18  by the upstream delivery roller pair  16  and the downstream delivery roller pair  17  shown in FIG. 2, and the staple tray  12  located between both roller pairs  16 ,  17  is moved downward (see, FIG.  2 ). Therefore, the front end of the sheet P to be delivered may hang over the staple tray  12  if curled downward as shown in FiG.  10 ( a ), and if the sheet is continuously conveyed as it is, the front end of the sheet may be pulled upon being folded as shown in FIG.  10 ( b ) when nipped by the downstream delivery roller pair  17 . 
     In this embodiment, as shown in FIG. 11, the shape of the side guide  11  is structured in an approximately triangle shape so that the top does not fall in the staple tray  12 . This side guide  11  is made to wait at a position (sheet delivery region) more inside than the width of the sheet to be delivered, thereby conveying the delivered sheet P to the downstream delivery roller pair  17  through guided at a top of the side guide  11  without hanging over the staple tray  12  as shown in FIG.  11 . Therefore, the sheet is delivered where the front end of the sheet is without being folded by the downstream delivery roller pair  17  as described above. 
     It is to be noted that if the side guide is in a shape with a cut in front of the downstream delivery roller pair  17 , the sheet P may hang over the staple tray  12  after guiding the sheet P ends at the side guide  500  even where the sheet P is guided at the top of the side guide  500 . It is therefore desirable to make the guide, like the side guide  11  in this embodiment, in a shape capable of guiding the sheets P from the upstream delivery roller pair  16  to the downstream delivery roller pair  17  (see, FIG.  11 ). 
     It is also to be noted that if an auxiliary guide  30  is provided for guiding the sheet P between the upstream delivery roller pair  16  and the side guide  11 , it is effective for preventing sheets from hanging. 
     It is to be noted that although the side guide  11  as described above guides the sheets P by positioning itself at the delivery region of the sheets P, the sheets P are required to be dropped in the staple tray  12  and aligned by pushing the edges in the sheet width direction in the offset mode and the staple mode. Therefore, in the offset mode and the staple mode, the side guide  11  is moved to escape more outside than the sheet width (outside the sheet delivery region) as shown in FIG. 12 right after the front end of the first sheet is nipped by the downstream delivery roller pair  17  (state shown in FIG.  11 ). This operation also prevents the front end of the sheet from being folded and pulled because the front end of the first sheet has already passed by the downstream delivery roller pair  17 , and the side guide  11  can be placed at a position for waiting and aligning the subsequent sheet. 
     [Stacking Operation on the Staple Tray] 
     In the staple mode, as shown in FIG. 14, after the rocking guide  20  is made open to deliver the sheet P to the staple tray  12  by the upstream delivery roller pair  16 , the sheet P is moved back until the rear end of the sheet P hits a rear end stopper  33  by rotating a knurled teething or knurled belt  32  in the arrow direction which is rotated by drive of a paddle  31  formed on the rocking guide  20  and drive of the upstream delivery roller pair  16 . The side guide  11  then pushes the sheet P toward one side to align the sheet P, and the stapler  13  makes the stapling operation. 
     When the sheet P is delivered to the staple tray  12 , if the delivery speed of the upstream delivery roller pair  16  is high, the sheet P may be delivered as projecting after passing by the upstream delivery roller pair  16  because the rocking guide  20  is open, may excessively proceed forward, and may take time for coming back. If the sheet proceeds forward overly, the sheet may not move back to the knurled belt  32  even by pulling the sheet through hitting with the paddle  31 , and the sheets may not be aligned on the staple tray  12 . 
     To solve this problem, in this embodiment, the rotation speed of the upstream delivery roller pair  16  is controlled to be a low speed while the rear end of the sheet passes by the upstream delivery roller pair  16  in the staple mode. This operation makes the rear end of the sheet delivered on the staple tray  12  fall near the knurled belt  32 , thereby ensuring the sheet P to be pulled by drives of the paddle  31  and the knurled belt  32 , and performing the alignment of the rear ends. 
     It is to be noted that whether the rear end of the sheet passes by the upstream delivery roller pair  16  can be distinguished by detecting predetermined time after the sheet passes by a prescribed sensor or the motor rotation speed. 
     After the rear end of the sheet falls in the staple tray  12 , the upstream delivery roller pair  16 , which has been switched to drive with a low speed, is changed to rotatively drive with a high speed. because this upstream delivery roller pair  16  is also a drive source for rotating the knurled belt  32 , the sheet P fallen on the staple tray  12  is promptly pulled back by the knurled belt  32 , and the rear end of the sheet is made to hit the rear end stopper  33 . 
     In the staple mode, as described above, this apparatus can align sheets quickly as a whole by rendering the conveyance speed slower only when the rear end of the sheet passes by the upstream delivery roller pair  16 . 
     [Rocking Guide] 
     Referring to FIG. 13, the rocking guide  20  is described briefly. The rocking guide  20  rotatively holds the movable delivery roller  17   b,  rocks around a rocking shaft  20   a  as a center by means of a drive mechanism  39  as described below during delivery of the sheet, and pushes the movable delivery roller  17   b  to press the downstream delivery roller  17   a.  In the staple mode, the rocking guide  20  is swingingly moved up around the rocking shaft  20  as the center, thereby moving the movable delivery roller  17   b  away from the downstream delivery roller pair  17 . That is, the rocking guide  20  serves as means for switching states for allowing sheet delivery and for inhibiting sheet delivery with the downstream delivery roller pair  17  composed of the movable delivery roller  17   b  and the downstream delivery roller  17   a.    
     In FIG. 13, numeral  34  is a stopper having a shutter portion  34   a.  The shutter portion  34   a  formed on the edge is lifted up by a link  35  which is moved pivotally upward around the pivotal shaft  35   a  as a pivotal center during transfer of the stack tray, and thereby, the sheets (sheet bundle) stacked on the stack tray  18  are prevented from going reversely into a delivery opening  36  by covering the delivery opening  36  when the stack tray  18  passes by the delivery opening  36 . This stopper  34  opens the delivery opening  36  by moving the link  35  pivotally downward around the pivotal shaft  35   a  as the pivotal center during the delivery of the sheets. 
     [Operation of stack tray during the double sheet delivery control] 
     Referring to FIG. 13, operation of the stack tray  18  when only two sheets P are stapled is described next. FIG. 13 is a cross section showing a position of the stack tray  18  as the essential portion. 
     When the staple processing is performed, the plural sheets S delivered sheet by sheet onto the staple tray  12  are normally moved back by the paddle described below and the knurled belt  32  in the reverse direction to the delivery direction and are aligned by the rear end stopper  33  described below upon hit by the stopper  33 . The stack tray  18  is lifted up at that time so that the front end side of the sheets P come above the rear end side of the sheets on the staple tray  12  (broken line position in FIG.  13 ), thereby easily making the sheets pulled back in the aid of the gravity force. 
     However, if only two sheets P are to be stapled (including the situation of the double sheet delivery control), the lower sheet is pulled back in a direction opposite to the delivery direction on the staple tray  12  by the downstream delivery roller  17   a  which is rotated reversely together with rocking movement of the rocking guide  20  by the drive mechanism  39  described below, and the upper sheet is pulled back similarly in the direction opposite to the delivery directin by the paddle  31  described below and the knurled belt  32 . Accordingly, a sole sheet or double sheets can be pulled back and aligned without aid of the gravity force, so that the front end of the sheet is not necessarily lifted up by moving the stack tray  18  up. 
     Therefore, in this embodiment, if only two sheets P are to be stapled (including the situation of the double sheet delivery control), the stack tray  18  is not moved up. That is, if the three or more sheets are to be stapled, the stack tray  18  is moved up from a solid line position to a broken line position in FIG. 13, but if the sheets P are only two, the stack tray  18  is not lifted up and remains in the solid line position in FIG. 13 to perform the pulling back operation described above. 
     This apparatus thus structured does not have to move up and down the stack tray  18  when the bundle of two sheets are to be stapled, and therefore, can save time for moving up the stack tray  18  and reduce the processing time greatly. 
     [Rocking amount of the rocking guide and paddle shape] 
     Referring to FIGS. 14 to  16 , the paddle  31  for pulling back the sheet P delivered on the staple tray  12  in a direction opposed to the delivery direction, and a rocking amount of the rocking guide  20  supporting the paddle pivotally are described. FIG.  14  and FIG. 15 are enlarged views showing states of the rocking guide and paddle as essential portions in the sheet pulling back operation. FIG. 16 is an illustration showing a shape of the paddle. 
     The rocking guide  20  has the paddle  31  mounted rotatively for pulling back the sheet P delivered on the staple tray  12  in a direction opposite to the delivery direction. The paddle  31  rotates in the direction opposite to the delivery direction at each delivery of a sheet P on the staple tray  12  where the rocking guide  20  is open, transforms elastically upon contacting to the rear end of the sheet P placed on the tray  12 , and pulls back the sheet P by frictional force created between the sheet P and itself. 
     If the paddle  31  pulls back each sheet at every delivery while the rocking guide  20  is swung up and held at a prescribed position, the sheet P may be excessively returned since the contact area and contact pressure of the paddle  31  in contact with the topmost sheet P may change according to the height (level) of the sheets P on the staple tray  12  because the sheets are successively delivered on the staple tray  12 . 
     In this embodiment, to solve this problem, the paddle  31  is structured to keep the contact pressure to the topmost sheet delivered on the staple tray  12  approximately constant. More specifically, the shape of the paddle  31  is formed or molded in a tapered shape whose tip  31   a  is narrowed as shown in FIG.  16 . FIG.  16 ( a ) indicates a case where the paddle  31  is in a tapered shape with stepwise portions on opposite sides of the tip  31   a  of the paddle  31 ; FIG.  16 ( b ) indicates a case where the paddle  31  is in a tapered shape with a stepwise portion on one surface (sheet contact surface) of the tip  31   a  of the paddle  31 ; FIG.  16 ( c ) indicates a case where the paddle  31  is in a tapered shape with a stepwise portion on the other surface (sheet non-contact surface) of the tip  31   a  of the paddle  31 . It is to be noted that the tapered shape of the tip  31   a  of the paddle  31  is not limited to those shown in FIGS.  16 ( a ) to  16 ( c ). 
     Where the paddle  31  is thus formed, the paddle tip serving as a portion contacting to the sheet becomes easily elastically transformed when contacting with the sheet, so that the apparatus can obtain stable returning force notwithstanding the number of the accumulated sheets and have an improved durability. 
     In this embodiment, the plural paddles  31  are provided in the rotational direction, and the paddles  31  come in contact with the sole sheet multiple times per rotation. This structure allows one time rotation of the paddles  31 , when the paddles  31  pull back a relatively large sheet by contacting to the sheet twice, to pull back adequately the sheet, and therefore, the processing time can be shortened in comparison with two time rotation of a single paddle  31 . It is to be noted that in FIG.  16 ( a ), a case where two paddles  31  are arranged in the rotational direction or a case of a twin paddle, the feature is not limited to this. The paddle  31  can be formed in shapes shown in FIG.  16 ( d ), FIG.  16 ( e ), FIG.  16 ( f ), and FIG.  16 ( g ) to obtain substantially the same effects. 
     The paddle  31  can be so structured that the contact area of the paddle  31  with the sheet P delivered on the staple tray  12  is kept constant. More specifically, the apparatus is structured so as to change the swinging amount when the rocking guide  20  is opened (swung upward) according to the height (level) change of the sheets P on the staple tray  12 . Further specifically, for example, according to increase of the number of the delivered sheets P on the staple tray  12 , the rocking guide  20  is swung upward to keep the contact area of the paddle  31  to the topmost sheet P constant. 
     As shown in FIG. 15, a thickness  t  of the bundle of the sheets P is expressed by “t=τsin θ” wherein: “τ” denotes the distance between the rocking center (rocking shaft  20   a ) of the rocking guide  20  and the rotary center of the paddle  31 ; “θ” denotes the rocking angle of the rocking guide  20 ; “t” denotes the thickness of the bundle of the sheets P. Based on this formula, it is suitable that the rocking amount (rocking angle θ) of the rocking guide  20  is changed according to the change of the thickness t of the bundle of the sheets P. 
     This structure keeps the contact area between the topmost sheet P on the staple tray  12  and the paddle  31  always constant notwithstanding the number of stacked sheets P, so that the apparatus can gain stable returning force, and so that the apparatus can prevent the sheets P from being excessively returned due to changes of the contact area of the paddle  31  to the sheet P. 
     [Operation Timing of the Paddle] 
     The operation timing of the paddle  31  starts as shown in FIG. 15 after the rear end of the sheet P gets settled as shown in FIG. 14 where as shown in FIG. 14 the upstream delivery roller pair  16  on the upstream side over the staple tray  12  releases the rear end of the sheet P. More specifically, the paddle  31  is rotated in the reverse direction to the sheet delivery direction after a prescribe time passes after the rear end of the sheet P passes by the delivery sensor provided on the upstream side of the upstream delivery roller pair  16 . 
     [Rotation number of the paddle according to the sheet size] 
     The drive number of the paddle  31  is described next. For example, with a structure that the paddle  31  is drive to rotate at a fixed rate regardless the size of the sheets, a large size sheet is not easily pulled back due to its large mass, and therefore in some case, cannot be pulled back to the knurled belt  32  even if hit in the same manner as done for small size sheets, thereby inviting failures in alignment of sheets. 
     In the embodiment, the drive number of the paddle  31  varies according to sheet sizes. More specifically, the drive number of the paddle  31  is made larger when the sheet has a relatively long length in the sheet conveyance direction. That is, for example, as shown in FIG. 17, in the cases of sheets having relatively larger sizes such as A 3 , B 4 , LGL, and LDR, the paddle  31  is driven two times, and in the case of sheets having relatively smaller sizes such as A 4 , LTR, B 5 , A 4 R, and LTRR, the paddle  31  is driven one time. 
     This structure surely pulls back the sheet to the knurled belt  32  even though having a large mass, thereby improving the alignment of the sheets. 
     It is to be noted that although in this embodiment the rotation number is changed according to the sheet size, substantially the same control can be done for designation of thicker papers or special papers (e.g., having a low surface frictional coefficient). 
     [Traveling speed of the side guide according to the sheet size] 
     The traveling speed of the side guide for performing alignment of the sheets in the sheet width direction is described next. Where the sheet P is stacked on a staple tray  12 , the sheets are aligned in the sheet conveyance direction by the paddle  31  and the knurled belt  32  as described above, and concurrently, the sheets P are aligned in the sheet width direction by moving the sheets P in the width direction toward the reference guide  37  located on the opposite side with respect to the sheets by pushing the rear end of the sheets (side edges on a side of the rear end stopper  33 ) by means of the side guide  11 . If the sheets have a larger size, because the center of the gravity is far from the pushing position by the side guide  11  and because the sheets have a high inertial moment where having a large mass, the front ends of the sheets cannot follow travelling of the side guide  11  in the sheet width direction, so that alignment failure of the sheets may be invited. 
     In this embodiment, to solve this problem, the side guide  11  changes the traveling speed in the sheet width direction according to the sheet size. More specifically, the side guide  11  is moved with a low speed where the sheet has a relatively long length in a direction (sheet conveyance direction) perpendicular to the traveling direction (sheet width direction) of the side guide  11 . That is, for example, as shown in FIG. 17, in the cases of sheets having a relatively short length in the sheet conveyance direction such as A 4 , LTR, B 5 , and sheets in which the guide moves in a smaller amount in the sheet width direction such as A 3 , and LDR, the side guide is made to travel with a high speed, and in the case of sheets other than above, having a relatively longer length such as B 4 , LGL, and sheets in which the guide moves in a larger amount in the sheet width direction such as A 4 R, and LTRR, the side guide  11  is made to travel with a low speed. 
     With such a structure, the apparatus can reduce influence of the inertial moment, and can improve alignment operation in the width direction even though it is a sheet having a large size (having a long size in the conveyance direction). This is also effective for sheet size needing a large traveling amount in the sheet width direction. 
     [Rear end stopper ] 
     Referring to FIGS. 18,  19 , the rear end stopper  33  for hitting the rear end of the sheets P when the sheets P are aligned in the conveyance direction is described next. The sheets P delivered on the staple tray  12  are conveyed in the direction opposite to the delivery direction by the paddle  31  and the knurled belt  32  and the like as described above, and the sheets are aligned in the sheet conveyance direction upon it by the rear end stopper  33  arranged with a predetermined space in the sheet width direction. 
     For example, if a sheet hitting surface  501   a  of the rear end stopper  501  is flat as shown in FIGS.  18 ( a ), the sheet P may be bent or go below the surface when the sheet P enters more or less in an oblique manner with respect to the sheet hitting surface  501   a , or the sheet end may be damaged by hit with the corner (edge) in the width direction of the sheet hitting surface  501   a.    
     In this embodiment, as shown in FIG.  18 ( b ), the opposite side portions in the width direction of the sheet hitting surface  33   a  of the rear end stopper  33  are formed in a tapered shape (tapered portion  33   b ). 
     This structure as shown in FIG.  18 ( c ) can prevent the sheets P from bending (or going below) by both tapered portions  33   b  even if the sheets P enter obliquely with respect to the sheet hitting surface  33   a , and further can prevent the sheet ends from sustaining damages. 
     As shown in FIG. 19, although the one rear end stopper  33 L corresponds to the knurled belt  32 L on one side of the sheet width direction, since the other knurled belt  32 R corresponds to the other rear end stopper  33 R with a slight shift in the width direction, the sheet end between the rear end stoppers may be pulled overly b the other knurled belt  32 R where the sheet corner vicinity is hit by the other rear end stopper  33 R (particularly, in the case of the R (Reduction) type sheets, in which sheet longitudinal direction is the sheet conveyance direction), and the sheet corner vicinity may become flexible and be bend. 
     During the sorting processing in the offset mode, though the side guide  11  is needed to travel in the sheet width direction, since the movable area is near the knurled belt  32 R, and as shown in FIG. 19, the other knurled belt  32 R and the rear end stopper  33 R as described above are structured to be shifted to each other in the sheet width direction. 
     With such a structure, the sheets of the B 5 R size having a shorter length in the sheet width direction can be subjecting to the offset processing, and the entire apparatus can be made compact. 
     In this embodiment, as shown in FIG. 19, the stapled  13  is made to wait between the rear end stopper  33 L,  33 R (or the center in this embodiment) for aligning the sheets upon hitting the rear end of the sheet bundle during the stapling operation for sheet bundles (particularly, stapling at a single portion of the R type sheets), thereby functioning the stapler  13  in the same way as the rear end stoppers  33 L,  33 R. More specifically, a rib  38  is formed to limit the rear end of the sheet bundle at a cover member  38  of the stapler  13 . 
     This structure prevents the sheet from being pulled excessively because the sheets are regulated by the stapler  13  (or rib  38   a ) waiting between the rear end stoppers even if the sheet is pulled inside by the other knurled belt  32 R when the sheet corner vicinity hits the other rear end stopper  33 R, so that this apparatus prevents sheets from being bent. 
     [Pressing control of the side guide] 
     Alignment of the sheets P in the sheet width direction b the side guide  11  is described next. The alignment of the sheets P in the sheet width direction is performed as described above by moving the sheets in the width direction in pushing the one edge on a rear end side of the sheets by means of the side guide  11  and by hitting the other side ends of sheets onto the reference guide  37  on the opposite side. At that time, the sheets P moved in the width direction by the side guide  11  are in contact with the knurled belt  32 . Therefore, the knurled belt  32  may be twisted according to the sheets P transferred in the width direction by the side guide  11 , and the sheets P may not reach the reference guide  37  by influence with the knurled belt  32 , thereby possibly causing failure of alignments. 
     In this embodiment, as shown in FIG.  20  and FIG. 21, when the side guide  11  aligns the sheets P in the width direction (particularly in a case of sheets of the R type having a large width alignment amount), the side guide  11  is pushed stepwise, and the sheets are aligned in releasing influences from the knurled belt  32 . That is, where the side guide  11  is pushed stepwise, the twisted width of the knurled belt  32  can be a minimum even the knurled belt  32  is twisted during pressing, and thereby the knurled belt  32  an back easily to the normal position (situation shown in the drawing) as well as with a shorter returning time. 
     Moreover, the stepwise pressing control for sheets by the side guide  11  during the alignment in the sheet width direction is changed according to the sheet size. More specifically, the first sheet of A 4 , LTR, B 4 , and LGL sizes and the sheet of the second or latter of LTR and B 5  sizes are pushed by two-time pressing. 
     The two-time pressing herein means operation in which pressing temporally stops after the first pressing and the second pressing is made subsequently. It is to be noted that the number of pressing times is not limited to this. The side guide  11  after the last pressing, as described below, has a structure functioning as a guide located at the pressing position until when the front end of the subsequent sheet comes over the downstream delivery roller pair  17  or for a prescribed period, and more specifically, the sheets are in a state that the sheets are pressed by the side guide  11 . 
     The side guide  11  temporally stops at each pressing of the sheets when the sheets are pushed multiple times stepwise and starts pressing the subsequent sheets after a prescribed time passes for returning the knurled belt  32  to the normal position (recovery of twists). 
     Therefore, the alignment in the width direction by pressing the sheets stepwise by means of the side guide  11  as described below makes the influence from the knurled belt  32  release quickly and is done quickly and precisely. 
     [Shape of the knurled belt] 
     Referring to FIGS. 22 to  24 , the shape of the knurled belt  32  is described. The knurled belt  32  pulls further back the sheets, which are pulled back by the paddle  31  as described above in the opposite direction to the sheet delivery direction, and aligns the sheets in the sheet conveyance direction by hitting the sheets P to the rear end stopper  33 . As shown in FIG. 22, if the contact surface of the knurled belt  502  with the sheets P are molded to be flat, an edge  502   a  of the knurled belt  502  may be trapped at the sheets S traveling in the width direction, thereby possibly causing failures in alignment of sheets. 
     In this embodiment, as shown in FIG.  23 ( a ), an edge portion  32   a  of the knurled belt  32  is molded in a tapered shape, or as shown in FIG.  23 ( b ), an outer round surface of the knurled belt  32  is molded in a shape having a cross section with curvature. 
     Those molded shapes make small resistance of the sheets P travelling in the width direction b the side guide  11  during the alignment, thereby being capable of reducing failures in alignment of the sheets due to trapping at the edge portion. 
     [Recovery of the knurled belt] 
     As described above, the alignment of the sheets in the width direction is performed by moving the sheets in the width direction upon pressing the one side end on the rear end side of the sheets by means of the side guide  11  and by hitting the other side end of the sheets to the reference guide  37  located on the other side. At that time, the sheets P moved in the width direction by the side guide  11  are in contact with the knurled belt  32 . Consequently, the knurled belt  32  may be twisted according to the sheets P moved in the width direction by the side guide  11 , the sheets P may be moved in association with recovery of the twist in the knurled belt  32  when the side guide  11  moves (escapes) in a direction opposite to the sheet pressing direction, thereby possibly causing failures in alignment. 
     With this embodiment, after the sheets are pressed by the side guide  11 , the side guide  11  continuously presses the sheets until the knurled belt  32  returns to the normal position (recovering the twist) and releases the pressing on the sheets after the knurled belt  32  returns to the normal position. 
     The side guide  11  functions as a guide for the subsequent sheet upon continuously pressing the sheets at a position shown in FIG. 21, but the knurled belt  32  returns to the normal position during this continuous pressing even if twisted as described above. After the front end of the sheet P under being guided comes over the downstream delivery roller pair  17 , he side guide  11  escapes to an escape position outside the sheet deliver region. 
     With this structure, failures in alignment of the sheet due to influence from the knurled belt  32  can be prevented. 
     [At a time when the downstream delivery rollers rotates in the reverse direction while the rocking guide is open] 
     The state of the downstream delivery roller  17   a  when the rocking guide  20  is open and the state of the side guide  11  are described. The downstream deliver roller  17   a is structured to rotate in a direction opposite to the sheet delivery direction by the drive mechanism  39  as describe below when the rocking guide  20  is opened. The side guide  11  usually aligns the sheets in the sheet width direction upon finishing the reverse conveyance of the downstream delivery roller  17   a.    
     However, the first sheet (or the second sheet during the double sheet delivery control) is in contact with the downstream delivery roller  17   a  by the weight of itself, and this may become resistance during alignment in the width direction and cause failures in alignment of sheets. The frictional resistance between the sheet and the downstream deliver roller  17   a  is smaller when the roller rotates than that when the roller is still. 
     In this embodiment, the side guide  11  finishes the alignment operation in the sheet width direction by the end of the reverse operation of the downstream delivery roller  17   a  for pulling back the first sheet. 
     This structure allows the influence from the frictional resistance between the first sheet and the downstream delivery roller  17   a  to be reduced at a time that the first sheet is aligned in the width direction by the side guide  11 , thereby improving the alignment property for the sheets. 
     [Lock of the downstream deliver roller while the rocking guide is held] 
     If the downstream delivery roller  17   a is stopped in a free state (rotatale state) when the subsequent sheets are stacked on the staple tray  12  and aligned, the lowermost sheet may be shifted on the staple tray  12  while the sheets are aligned. 
     To solve this problem in this embodiment, the drive mechanism  39  as described below locks the downstream delivery roller  17   a  to render the roller not rotatable wile the sheets are stacked on the staple tray  12  and aligned. This structure can reduce shifts of the sheets due to collisions or the like during the paddle operation while the sheets are stacked on the staple tray  12  and aligned. 
     [Reverse operation of the downstream delivery roller when the rocking guide is closed] 
     When the sheets of a bundle is stacked and aligned on the staple tray  12 , the sheet bundle is sandwiched and fixed where the rocking guide  20  is closed, and then the stapler  13  makes the stapling operation. 
     In such a situation, the lowermost sheet of the sheet bundle may be shifted more or less on the staple tray  12  while the sheets are aligned by the paddle  31 , the side guide  11 , and the like. 
     With this embodiment, while the sheet bundle is sandwiched and fixed upon closing the rocking guide  20 , the downstream delivery roller  17   a  is reversed in a prescribed amount (or more or less) by the drive mechanism  38  as described below, thereby providing conveyance force in a direction opposite to the deliver direction to the lowermost sheet of the sheet bundle on the staple tray  12 . 
     This structure makes possible the alignment in correcting shifts even where the lowermost sheet of the sheet bundle is shifted more or less while the sheets are aligned by the paddle  31 , the side guide  11 , and the like. 
     [Drive mechanism for the rocking guide and the downstream delivery roller] 
     Referring the FIGS. 25,  26 , and  27 , the drive mechanism  39  for the rocking guide  20  and the downstream delivery roller  17   a  are described. In the drawings, the numeral  39  represents the drive mechanism and performs to open and close the rocking guide  20  and to drive in normal and reverse directions the downstream deliver roller  17   a . This drive mechanism  39  is constituted of a drive motor  40  as a drive source and a gear series transmitting the drive force from the motor  40 . 
     The drive motor  40  is formed with an encoder  56  for detecting the rotation number and a drive motor rotation detecting sensor  55 , which detect the rotation speed of respective rollers and traveling amount of the rocking guide. 
     This drive mechanism  39  performs to rotate the downstream delivery roller  17   a  in the normal direction (rotation in the sheet delivery direction) while the drive motor  40  rotates normally, to open the rocking guide  20  as well as to rotate in the reverse direction (rotate in a direction opposite to the sheet deliver direction) the downstream delivery roller  17   a  while the rocking guide  20  is made open where the drive motor  40  rotates in the reverse direction, to close the rocking guide  20  as well as to rotate in the reverse direction the downstream delivery roller  17   a while the rocking guide  20  is made closed, and to hold the rocking guide  20  while the drive motor  40  stops temporarily well as to lock the downstream delivery roller  17   a when the rocking guide  20  is held. Hereinafter, the structure of the drive mechanism is descried in detail along the stream of operation. 
     As shown in FIG. 25, when the drive motor  40  is rotated in the normal direction, drive force is transmitted to a fixed gear  42   a of the pendulum gear unit  42  of the meshing a pinion gear  41  on the motor  40  to rotate the gear. A rocking gear  42   b  is swung to a shown position to mesh a delivery gear  43  of the downstream deliver roller  17   a , and the sheets S are delivered and conveyed where the downstream delivery roller  17   a  rotates in the sheet delivery direction (normal direction: in the arrow direction in the drawing). 
     As shown in FIG. 26, when the drive motor  40  is rotated in the reverse direction, drive force is transmitted to the fixed gear  42   a  of the pendulum gear unit  42  of the meshing the pinion gear  41  on the motor  40  to rotate the gear. The rocking gear  42   b  meshes an intermediate gear  44  upon swung to the shown position, and an operational gear  46  rotates in the arrow direction via an intermediate gear  45  meshing the intermediate gear  44 . The operational gear  46  includes a gear portion  46   a  meshing the intermediate gear  45 , a projection  46   b  to open and close the rocking guide  20  in contact with an opening and closing arm  47  attached to the rocking guide  20  as a united body, a partially toothless gear portion  46  capable of meshing the intermediate gear  48  in mesh with the delivery gear  43 . 
     Therefore, when the operational gear  46  rotates in the arrow direction, the partially toothless gear portion  46   c  meshes the intermediate gear  48  to rotate the deliver gear  43  meshing the intermediate gear  48  in the arrow direction in the drawing, and while the downstream delivery roller  17   a  rotates in the direction (the arrow direction in the drawing) opposite to the sheet delivery direction to start pulling back the sheets P, the rocking guide  20  is pushed up in the arrow direction in the drawing where the projection  46   b  pushes up the opening and closing arm  47  in contacting with the arm  47 . 
     Where the rocking guide  20  reaches the position shown in FIG. 27, the drive of the drive motor  40  is topped temporarily, and the rocking guide  20  is held as in the closed state. At that time, since the partially toothless portion  46   c  of the operational gear  46  is stopped in mesh with the delivery gear  46  via an intermediate gear  48 , the downstream delivery roller  17   a  is locked and not rotatable. 
     It is to be noted that the holding position of the rocking guide  20  is changeable as described above according to the height (level) of the sheets, to keep constant the contact area of the paddle  31  to the sheet delivered on the staple tray  12 . 
     Then, stacking and aligning operation for the sheets on the staple tray finishes, and the drive motor  40  is rotated again in the reverse direction. The delivery gear  43  rotates only for a portion meshing the partially toothless gear  46   c  of the operational gear  46 , and the sheets P are pulled back by rotation of the downstream deliver roller  17   a  in the direction opposite to the sheet delivery direction in a prescribed amount (meshed portion as described above). The rocking guide  20  is made closed at the same time, and after closed, the guide prepares for the subsequent processing. 
     With such a structure, the drive mechanism for driving the rocking guide  20  and the downstream deliver roller  17   a  is not required to be installed individually, so that this apparatus can reduce the costs and be simplified. 
     [Closing operation of the rocking guide] 
     As described above, the rocking guide  20  is pivotable around the rocking shaft  20   a , and as shown in FIG. 27, when the operational gear  46  rotates in the arrow direction, the projection  46   b  formed on the operational gear  46  pushes up the opening and closing arm  47  attached to one end of the rocking guide  20  to open the rocking guide  20 . When the operational gear  46  further rotates in the arrow direction in FIG. 27, the rocking guide  20  begins closing, and when the operational gear  46  more rotates, the rocking guide  20  is closed upon falling by its weight where the projection  46   b  is disengaged with the opening and closing arm  47 . 
     If the operational gear  46  is rotated with a high speed when the rocking guide  20  is closed, and if the initial speed of the closing operation is made faster, the rocking guide  20  falls by its weight with large impacts, and the aligned sheets may be disturbed. Such impacts also adversely affect the durability of the apparatus. 
     In the embodiment, to solve such problems as shown in a flowchart of FIG. 28, during motor control for closing the rocking guide  20 , after the motor starts, the rocking guide  20  is made closed with a high speed until the prescribed position No.  1  (S 21 ), but when the rocking guide  20  is closed up to the prescribed position No.  1  (S 22 , S 23 ), the motor output is changed (S 24 ), and it is structured that the closing operation of the rocking guide  20  becomes slower. When the rocking guide  20  is further closed up to the prescribed position No.  2  (S 28 , S 26 ), them motor output is changed again (S 27 ), and the motor drive is stopped after the closing of the rocking guide  20  is detected (S 28 , S 29 ). 
     This makes the rocking guide  20  rotate slowly right before falling by its weight and makes the initial speed for falling by its weight slow. Therefore, the impacts of the rocking guide  20  falling by its weight become smaller, thereby not disturbing the aligned sheets, making the impact sound smaller, and preserving the durability of the apparatus without affected adversely. 
     It is to be noted that, when the rocking guide  20  is made closed, the rocking guide  20  complete the closing operation after a prescribed time passes after the motor starts, and as shown in FIG. 26, the opening and closing arm  47  moves pivotally a sensor flag  49  to turn on the closing sensor not shown. The apparatus recognizes, by this operation, that the rocking guide  20  is closed. 
     Therefore, if the closing sensor is not turned on even after the prescribed time passes, an error presumably occurs. However, such a situation may be brought by, as a matter of facts, stopping of rotation of the drive motor due to impact resistance between the projection  46   b  of the operational gear  46  and the opening and closing arm  47  and deviations of loads to be coupled gears. In such a case, the operational gear  46  is rotated by transmitting the large rotational force, thereby continuously and smoothly performing the work. 
     That is, in this embodiment, as shown in the flowchart in FIG. 29, where the rocking guide  20  is made closed (S 31  to S 37 ), if the closing sensor is not turned on even where the prescribed time passes after the motor states (S 32 ), the motor output is changed to transmit a further larger rotational force (S 33 ). Then, where the closing sensor is not turned on even if the prescribed time passes, the apparatus displays an error indication of the rocking guide  20  and stops the operation. 
     Where the closing state of the rocking guide  20  cannot be detected in the first closing operation as described above, the apparatus can reduce occurrences of stop due to errors by performing the closing operation again by enlarging the motor output and can do the sheet post processing continuously and smoothly. 
     [Switching control of the rotational direction] 
     To switch the pendulum gear unit  42  upon driving the drive motor  40  in the reverse direction, since the rocking gear  42   b  is rotatively driven in accordance with the rotation of the fixed gear  42   a , the rocking gear  42   b  does not easily mesh the delivery gear  43  and the intermediate gear  44  when rotated rapidly and may skip the teeth to be meshed. This may cause noises and reduce the durability and the reliance of the apparatus upon unnecessarily abrading the gears. 
     In this embodiment, as shown in FIG. 30, the apparatus judges as to whether the rotational direction of the drive motor  40  is switched according to the control of the apparatus (S 41 ), and if the rotational direction is not identical (S 42 ), the drive motor  40  is controlled to drive with a low speed (S 43 ). When an adequate prescribed time passes for switching the direction (S 44 ), the drive motor is driven with a speed of the normal control (S 45 ). 
     With such a structure, the rocking gear  42   b  can be meshed surely with the delivery gear  43  and the intermediate gear  44 , thereby preventing the gear tooth skipping or noises, and the apparatus can have a good durability. 
     [Staple operation] 
     As described above, the bundle of the sheets P staked on the stack tray  12  are nipped by the downstream delivery roller pair  17  secured by the delivery gear  43  and are stapled in this state. The stapled position, though various combinations are conceivable, can be selected as shown in FIG. 31 in this embodiment from a mode that a corner is stapled at a single location or mode that an edge is stapled at two locations. 
     When the stapler  13  is not located at a prescribed staple position, the stapler  13  is required to move, but this may cause the sheet bundle stacked on the staple tray  12  to move. Therefore, when the stapler  13  is made to travel, the side guide  11  presses the end of the sheet bundle. This operation prevents the alignment of the stacked sheets P from becoming disorderd. 
     However, if the staple operation is performed while the sheets are pressed by the side guide  11 , failure of the staple operation may occur because the sheet bundle may be bent in the width direction due to pressing of the side guide  11 . 
     When the sheets are stapled, pressing of the side guide  11  is released as shown by a solid line in FIG. 31 to separate the side guide  11  from the bundle of the sheets P, and the staple operation is performed in a state that the sheets are nipped by the downstream delivery roller pair  17 . This can release bending of the sheet bundle caused by the pressure of the side guide  11 , thereby preventing possible staple failures. 
     [Replacement of the staples] 
     As shown in FIG. 32, the stapler  13  is structured to attach a staple cartridge  50 , and to exchange the staple cartridge  50  is replaced when the staples are supplemented. In the stale cartridge  50 , plural staple plates  50   a  constituted in connecting plural staples with each other can be loaded. 
     Inside the stapler  13 , formed are a staple cartridge sensor  13   a  for detecting the frame of the staple cartridge  50 , a staple detection sensor  13   b  for directly detecting the staple exposed at a lower surface of the staple cartridge  50 , and a starting staple detection sensor  13   c  formed at the tip of the stapler  13 . 
     Control for replacing the staple cartridge  50  is shown in a flowchart in FIG.  33 . When a job accompanied with the staple processing (S 51 ), or when no staple state is detected during continuation (S 52 ), the apparatus informs the user of no staple state and ask replacement of the staple cartridge  50  (S 53 ). The user opens a stapler door  51  (see, FIG. 1) on a front surface of the finisher unit C, and loads the staple cartridge  50  in which staple plates  50   a  are filled to the stapler  13 . 
     Though the stapler  13  detects by the sensor that the staple cartridge  50  has been attached, the staple detection sensor  13   b  judges, at a time when staples are inserted to some extent, that there are staples by detecting the lower surface of the staple cartridge  50 . At his time, the cartridge is not attached or secured to a prescribed position yet, so that the staples may not be able to be fed or hit. 
     Therefore, in this embodiment, the apparatus judges whether both of the staple cartridge sensor  13   a  and the staple detection sensor  13   b  are turned on (S 54 ), and if so, the apparatus recognizes that there are staples. This makes the apparatus capable of not only detecting the existence of the staples but also recognizing a state ready for striking the staples, thereby performing surely the staple replacement work. 
     [Initializing processing for staples] 
     When the apparatus detects that there are staples (S 54 ) and that the staple door  51  is closed (S 55 ), an initializing processing for staples begins (S 56 ). Conventionally, for making staples ready, it was done by empty shots for certain times. However, such a system cannot recognize the staples even if the staple plates  50   a already reaches the front end of the staple cartridge  50 , and those were wasteful shots. 
     In this embodiment, the stapler  13  has a staple head detection sensor  13   c , which is arranged at a position opposing to the front end of the staple cartridge  50 . This staple head detection sensor  13   c  detects the end of the initializing processing, and consequently, the apparatus is not required to blindly make wasteful shots of staples any more. On the other hand, with control that empty shots are made until the staple head sensor detection sensor  13   c  detects the staples, such empty shots may be continued endlessly even where staple jamming occurs in the staple cartridge  50  because there is not limitation to the number of the empty shots. 
     To solve such a problem, as shown in FIG. 34, when the initializing processing (S 56 ) starts, a counter n is first reset (S 61 ). The staple plate  50   a  is fed by a single staple upon doing an empty shot (S 62 ). If the staple head detection sensor  13   c  detects the staple (S 63 ), the processing ends, and if the sensor does not detect, the counter n is counted by the one (S 64 ). It is then judged as to whether the counter n reaches the prescribed number (S 65 ). If it is within he prescribed number, further empty shots are repeated, and if it exceeds the prescribed number, the apparatus informs the user of occurrence of staple jamming (S 66 ). 
     By thus imposing a limitation on the empty shot number with the staple head detection sensor  13   c  when staples are detected, the apparatus can avoid the endless loop of the initializing processing. If staple jamming occurs (S 66 ) during the initializing processing (S 56 ), it is recognized as no staple state in the cartridge replacement processing. 
     [Staple jamming processing] 
     In some case, staple jamming occurs while the staple operation is going on. When the finisher unit C detects staple jamming, in a conventional apparatus, as shown in FIG. 35, it is judged as to whether staple jamming occurs (S 72 ) after the staple processing (S 71 ) is performed. If not staple jamming occurs, the sheet bundle is delivered (S 73 ) to continue the processing, and if the staple jamming occurs, the apparatus informs the user of this occurrence (S 74 ) and interrupts the processing. However, this operation makes the stapler  13  stay at a position where the staple operation is executed, and even if the user wants to clear up the jamming by opening the stapler door  51 , the user&#39;s hand may not reach there. 
     In this embodiment, as shown in FIG. 36, the sheet bundle is first delivered (S 83 ) after the staple processing (S 81 ) is executed, and it is judged as to whether the staple jamming occurs (S 83 ). If not staple jamming occurs, the processing is going on as it is, and if the staple jamming occurs, the apparatus informs the user of the jamming after the stapler  13  is moved to an initial position near the stapler door  51  (S 84 ) and then stops the processing. The initial position is the vicinity of the stapler door  51  and the easiest position for clearing the jamming by the user when the use opens the door. The reason that the sheet bundle is delivered is that a remaining bundle may receive damages upon contacting with the stapler  13  while the stapler  13  is moved to the initial position. 
     Moving of the stapler to the initial position, even where stapler jamming occurs, can avoid a situation that the user may not reach the stapler easily, thereby making the maintenance of the apparatus easy. 
     [Stapler initializing operation during stapler jamming] 
     The stapler door  51  of the finisher unit C is made open and closed at a time that the staples are replaced as described above, but if the sheet under carried causes jamming, there is no need for opening and closing the door. However, the user may open and close the door, and at that time, it is forseeable that he user may inadvertently move the stapler. 
     The position control of the stapler is controlled by a travelling amount from the initial position, and the present position is not confirmed by means such as a sensor or the like. Therefore, the position of the stapler is moved during recovery from paper jamming, the apparatus cannot recognize this, and the stapler may make stapling at a wrong place if the staple operation starts as it is. 
     In this embodiment, as shown in FIG. 37, where opening and closing of the stapler door  51  is detected (S 91 ) and where the stapling processing is performed (S 92 ), the stapler  13  is returned once to the initial position before the staple operation is executed (S 93 ), and is then moved again to the stapling position to execute the staple operation (S 95 ). Because the apparatus is structured to execute the staple operation after the position of the stapler  13  is thus confirmed, staples may not be placed at wrong locations even where the user moves the stapler  13 . 
     [Delivery of sheet bundle] 
     As described above, when the staple operation ends, the drive motor  40  rotates in the normal direction to render the pendulum gear unit  42  in mesh with the delivery gear  43 , and the bundle of the sheets P is delivered on the stack tray  18  upon rotation of the upstream deliver roller pair  17  in the conveyance direction. 
     Where a sheet bundle whose one corner is subjecting to the staple operation is delivered, an edge surface on the side opposite to the side where the staple operation is made is easily disordered. This phenomenon occurs with influences according to the size and number of the sheets, and such disorder becomes more remarkable, since the friction between sheets becomes smaller as the sheet size is smaller. 
     In the conventional apparatus, the control is which the downstream delivery roller pair  17  delivers the sheet bundle is unchanged, and as shown in FIG.  38 ( a ), the control of the drive motor  40  is done b the output of 100%. For example, where the delivery speed is set again in reference to the sheet bundle of sheets of a medium number under this circumstance, excessive conveyance force may be given to the sheet bundle having a small number and easily cause such disorders, and on the other hand, the delivery speed may be reduced because the mass of the sheet bundle may be larger where the number of the stacked sheets becomes larger. 
     In this embodiment, to solve this problem, the stack tray  18  is moved up where the sheet bundle whose one location is subject to the staple operation is delivered, and the apparatus delivers the sheets where the stacking surface of the stack tray  18  is made closer to the downstream delivery roller pair  17 . This makes resistance between the sheet bundle and the stacked surface of the stack tray  18  smaller and suppresses occurrences of such disorders. 
     Furthermore, the stack tray  18  located closer to the downstream deliver roller pair  17  is dissented for a fixed amount right before the rear end of the sheet bundle passes by the downstream delivery roller pair  17 . This prevents the sheets from proceeding in the reverse way due to contacts or the like of the rear end of the sheet bundle with the downstream delivery roller pair  17 . 
     As shown in the drawing, the setting up speed of the drive motor  40  during delivery is controlled to be slow according to the sheet size and sheet number, and thereby the apparatus corresponds to the sheet bundles having different sheet sizes and sheet numbers. That is, the apparatus starts with drive force of about 80% to the sheets of a large size and with drive force of about 60% to the sheets of a small size. 
     More specifically, as shown in FIG.  38 ( b ), when the sheet size is the small size, the setting up speed is made slower than that of the large size, thereby preventing disorders which otherwise occurs due to quick acceleration. When the stacked number is large, the drive torque at the setting up time is made lower than the time when the number is smaller, and the torque from the drive roller is controlled as to transmit to the lowermost sheet of the sheet bundle adequately and evenly. The drive shifts to have gradually the normal conveyance speed and the normal drive torque, and finally, any sheet bundles even having the different size and number are delivered with substantially the equal speed. 
     From those operations, this apparatus can improve the stacking property on the stack tray  18  in preventing the disorder in the edge surface on a side where no staple is made even when the sheet bundle whose one location is subjecting to the staple operation is delivered, and can deliver the sheets with the same speed regardless the size and number of the bundle. The drive motor  40  is not drive with the 100% output, so that this apparatus has been effect to reduce the operating sounds generated from the apparatus. 
     By making closer the stack tray  18  to the downstream delivery roller pair  17 , the sheet bundle to be delivered is prevented from being bent, so that the lowermost sheet of the sheet bundle is prevented from bending. 
     [Detection of stacking mixed sheets] 
     Where the stacked sheets on the stacking tray  18  are delivered with control for sheet size or sheet processing mode, which is different from the stacked sheets, the apparatus is required to impose some limitation on the sheet number to be stacked as a special handling for mixed sheets because the stacking property becomes impaired in comparison with the sheet stacking in the same size or sheet stacking for the same processing. 
     A stack sensor  53  is therefore provided at about a center of the stacking tray  18  for detecting whether the sheet is stacked on the tray, and if the sheet P is stacked on the stacking tray  18 , the apparatus executes the handling program from mixed sheets with the following conditions. 
     (1) Where the sheet P stacked on the tray is not a sheet delivered and stacked on the finisher unit C. 
     (2) Where a sheet P having the different sheet size is delivered and stacked on the stack tray  18  by the finisher unit C. 
     (3) Where a sheet is delivered and stacked with the different processing mode by the finisher unit C. 
     In the fisher C according to this embodiment, a detection signal from stack sensor  53  is monitored when image formation starts, and the signal is not monitored after the image formation has already started. This is because the first delivered sheet may be misidentified as a sheet of mixed sheets if the detection signal from the stack sensor  53  is monitored even after the sheet is delivered after the image formation has already started. 
     [Detection of the stacked amount] 
     A measuring sensor  54  arranged on a top of the rocking guide  20  detects the level of the sheets stacked on the stack tray  18  or the topmost surface of the sheet bundle. The measuring sensor  54  includes a light emitting portion radiating light such as infrared ray to sheet bundles and a photo receiving portion for receiving light reflected irregularly at the sheet bundle. The sensor  54  detects the level by measuring the angle of the reflected light. 
     When the sheets are delivered on the stack tray  18 , as shown in FIG.  39 ( a ), the sheet P may not fall because the rear end of the sheet P is trapped at the finisher unit C. If the level detection is implemented in such a circumstance, the level may not be detected accurately. Therefore, this apparatus has a structure that when the sheet P is delivered the stack tray  18  moves down once and up again to render the sheet P settled on the stack tray  18 . 
     It is desirable that the measuring sensor  54  detects the level when the stack tray  18  moves up where the level of the sheets P stacked on the stack tray  18  is detected. However, because the subsequent sheet P is in fact already delivered when the stack tray  18  moves up, the sensor cannot detect the sheet on the stack tray  18  due to interference from the delivered sheet. 
     This apparatus is structured to get the detected result if data within the permissive error range are brought successively where the level is detected twice or more with a prescribed time interval, because the sheets may not be settled yet at a moment where a level detection is performed during dissenting of the stack tray  18 . The apparatus can detect the established actual level and maximizes the productivity of the apparatus. 
     The position of the stack tray  18  after moved up is controlled so that the stacked surface becomes always constant based on the data obtained by the level detection (paper surface level control). 
     Here, a structure for recognizing the sheet stacking amount (the level of the stacked sheets) on the stack tray  18  is briefly described using FIG. 39 b ). It is to be noted that the detailed structure is disclosed in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 9-48549. FIG.  39 ( b ) is a perspective view showing a schematic structure, as the essential portion, of the tray unit, a driver for the tray unit, and a position detecting portion of the tray unit. 
     In this embodiment, a tray unit  58  is structured by securing three stack rays  18  to respective tray frames  57 , and the tree stack trays  18  can move up and down as a united body with respect to the finisher frame  59  of the finisher unit C. Moving up and down of the tray unit  58 , or namely, the stack trays  18 , is structured by moving up and down of the try unit itself with respect to the finisher frame  59  where the normal and reverse rotational drive of a stacker motor  209  is transmitted to a rack portion  58   a formed at a portion of the tray unit  58  via a pinion gear  225 . 
     An encoder  226  is mounted on an output shaft of the stacker motor  209 . Where the pulse amount from the encoder  226  is detected with a stacker motor clock sensor  227 , the apparatus can detect how far pulses the tray unit  58  moves from a home position as the initial position, or the traveling amount of the tray unit  58 . It is to be noted that the detection whether the stack tray  18  is in the home position is made by detection of a tray unit flag  57   a  provided at a lower portion of the tray frame  57  by a tray home position sensor  228 . 
     After the tray unit  58  is detected as in the home position from a copy operation signal or the like (or the tray home position sensor  228  detects the tray unit flag  57   a ), the stack trays  18  are set at the predetermined positions with respect to the downstream deliver roller pair  17  based on the detection signal of the measuring sensor  54 , and the stack trays  18  receive the sheets delivered from the downstream delivery roller pair  17 . 
     This apparatus also has a structure that the stack tray  18  is moved down by a prescribed amount at each stack of the sheet or sheet bundle to maintain the topmost level of the sheet on the stack tray  18  at a position of the prescribed amount from the downstream delivery roller pair  17 . 
     In this apparatus, an MPU  200  (see, FIG. 42) in the finisher unit C as described below can recognize what amount of clocks the tray unit  58  travels form the home position or namely, the traveling amount of the stack tray  18 . 
     The apparatus thus structured, can determine the positions of the stack tray  18  and the topmost surface of the sheets on the stack tray  18  and can recognize the stacked amount of the stacked sheets on the stack tray  18  (the height of the stacked sheets). 
     As described above, the apparatus recognizes that sheets are fully stacked if it is over the predetermined amount based on the processing mode and the sheet size upon detecting he position of the stack tray  18  and the stacked amount of the sheets P stacked on the stack tray  18  through he level detection in a manner. 
     However, even if the fully stacked state is detected, the sheets P, in some case, may be not settled yet due to curling or a state of the trapped rear end of the sheets P on the stack tray  18 . Therefore, the apparatus may stop the operation in judging as it is the full stacked state though in fact not fully stacked, thereby possibly reducing the productivity. 
     In this embodiment, the apparatus stops the operation upon judging that the sheets are fully stacked only when detecting that the topmost sheet on the stacked tray  18  is at a prescribed level or higher, or when detecting plural times that the stacked amount of the sheets on the stack ray  18  exceeds the prescribed amount. More specifically, the apparatus stops the operation upon moving up and down the stack tray  18  and judging that the sheets are fully stacked only when detecting plural times (three times in this embodiment) that the stacked amount of the sheets on the stack tray  18  exceeds the prescribed amount upon detecting the sheet stacked amount on the stack tray  18  at every operation (or after completion of the operation). 
     Moreover, in this embodiment, the apparatus performs detection of the fully stacked state as described above at every sheet delivery of a prescribed number (e.g., five sheets) onto the stacked tray  18 . 
     This apparatus thus can detect as to whether the sheet stacked amount exceeds the prescribed amount after solving curling or a state of the tapped rear end of the sheets P occurred on the stack tray  18 , can prevent erroneous recognition in the detection of the fully stacked sheets because the apparatus jusges that the sheets are fully stacked only when detecting successively that the sheet stacked amount exceeds the prescribed amount and stops the operation, and can provide adequate productivity. 
     It is to be noted that the apparatus has a structure for suggesting to the user that the stacked sheet (or sheet bundle) should be removed from the stack tray  18  when the apparatus detects the fully stacking of the sheets and stops its operation. 
     [System stop timing during detection of fully stacked sheets] 
     However, if image formation is stopped upon detection that the sheets are fully stacked as described above even while the sorting operation is going on, the sheet bundle in a midway of the sorting operation is stacked on the stack tray  18 , and removal of this may make complicated handling of the stacked sheets because the sorting operation ends. On the other hand, a margin to some extent may usually be set for detection of the fully stacked sheets on the stack tray  18 , and even where the sheets are detected as full, further sheets can be stacked thereon. 
     In this embodiment, as shown in FIG. 40, if the fully stacked state is detected (S 102 ) in a midway of the image formation and stacking operation (S 101 ), the apparatus judges whether a single bundle is completed (S 103 ), and if image formation of the single bundle is not yet completed, the image formation is continued as it is without stopping the formation. This structure avoids a sheet bundle in a midway of the sorting operation even where the sheet bundle is removed from the stack tray  18  and makes easier the handling. 
     Subsequently, the measuring sensor  54  detects the fully stacked state, and the apparatus stops the image recording as in he fully stacked while it is not in the sorting operation (S 104 ). 
     [Special sheets] 
     if sheets delivered and stacked on the stack tray  18  are special sheets, particularly, OHP sheets, because the light emitted from the measuring sensor  54  does not reflect irregularly so much on the OHP sheet surface but reflect mostly in a mirror fashion, errors of distances of 20 to 30 mm (experimental values) may occur in comparison with measurements to the ordinary sheets. If the ordinary control is made, the stack tray  18  is moved up since the apparatus recognizes that the stacked top surface is far (low) that the actual one where detecting the fully stacked state or controlling the stacked height, and in some case, the stacked sheet may be trapped at the delivery opening, so that failures in stacking such that sheets are damaged by collisions of the delivered sheets to the stacked sheets may occur. 
     In this embodiment, as shown in FIG. 41, the apparatus judges whether the stacked sheets containing the sheets fed from the apparatus multi-tray (manual feeding tray) are moved from the sheets on the stack tray  18  (S 111 ), and if such sheets are removed, an approximate detection flag is cleared (S 112 ), or namely, it is detected as not the approximate detection. 
     If the approximate detection flag is off (S 113 ), the normal tray control is performed (S 114 ) as presuming that no sheet is stacked or sheets having no error in direction of the measuring sensor are stacked. If the approximate detection flag is on (S 113 ), the apparatus does the approximate detection control (S 115 ). 
     The approximate detection control herein presumes errors in advance and amend then where the surface level of the delivered stacked sheets on the stack tray  18 , which is measured by the measuring sensor  54 , is not trustful due to special sheets or the like. In this embodiment, it indicates a possibility that the sheets fed from the apparatus multi-tray (manual feeding tray) are stacked on the stack tray  18 . 
     The approximate detection control also indicates that based on the errors in the sensor, it is controlled to be lower than the predetermined stacked surface level, more specifically, about 30 mm lower. 
     After the delivered sheets are stacked on the stack tray  18  (S 116 ), the apparatus judges whether the stacked sheets are the subject matter of the approximate detection (S 117 ). This judgment is made in the same way depending on whether the stacked sheets are fed from the apparatus multi-tray (manual feeding tray). 
     According to this judgment, when the sheets are of the subject matter of the approximate detection, the approximate detection flag is set (S 118 ), and in the subsequent tray control, the approximate detection control enters (S 115 ). 
     If the sheets are not of the subject matter of the approximate detection, the apparatus implements the stacked amount detection and the sheet height control (S 121 ), but if they are of the subject matter of the approximate detection, the apparatus implements only the stacked amount detection (S 119 ). 
     In the above embodiment the control is described in which all the sheets fed from the apparatus manual feeding opening are processed entirely as special sheets. Herein, a control for turning on and off the approximate detection flag in judging whether the stacked sheets are special. 
     This judgment is made by a calculation of a distance by the measuring sensor  54  at two points where the stack tray  18  is moved at the two points having the different heights at which a traveling amount is known in advance. On the other hand, the distance that the stack tray  18  is moved is measured by a traveling amount detecting means not shown. If the differential between the measured value and the difference of the distance measured b the measuring sensor is equal to or more than a prescribed amount (in general, the measure value by the sensor is larger), the apparatus judges that the stacked sheets are the subject matter of the approximate detection. 
     By this operation, even if the sheets to be conveyed are special sheets such as the OHP sheets that the measuring sensor  54  cannot measure easily, or even if the sheets are changed to special sheets in a midway, the apparatus can do the sheet processing substantially the same as the normal sheets. 
     The measurement of the OHP sheets done by the measuring sensor creates a shift of a certain amount (20 to 30 mm) in comparison with plain paper as described above, but the deviations in the measure values according the sheet number are in the same way as the plain paper. Therefore, if the delivered sheets are recognized as the OHP sheets, the apparatus can do the detection of the fully stacked state and control for stacked height in use of the measuring sensor  54  in the same manner as the normal cases by shifting in a certain amount the stack tray  18  downward. 
     [Structure of the control system for the finisher unit] 
     Referring to FIG. 42, the structure of the control system for the finisher unit C of the sheet processing apparatus B is briefly described. 
     In FIG. 42, numeral  200  represents the MPU as a control means. The MPU  200  receives input signals from the loading sensor  28 , the entry sensor  27 , the buffer sensor  26 , the delivery sensor  29 , the measuring sensor  54 , the stack sensor  53 , the drive motor rotation detecting sensor  55 , the staple cartridge sensor  13   a , the staple detection sensor  13   b , the staring staple detection sensor  13   c , the stacker motor clock sensor  227  for detecting the pulse amount of the encoder  226  provided on the output shaft of the stacked motor  209 , the tray home position sensor  228  detecting the home position of the tray unit  58  (or its stack tray  18 ), and the like. 
     Based on the above signals, the apparatus drives, through respective driers D 1  to D 11 , a first flapper solenoid  201  switching the first flapper  21 , a second flapper solenoid  202  switching the second flapper  22 , a third flapper solenoid  203  switching the third flapper  23 , the buffer roller  23 , the downstream delivery roller pair  16 , and the knurled belt  32 , and moves up and down the shutter portion  34  by the reverse rotation. The apparatus also control a paddle solenoid  206  for engagement and disengagement of the drive force from the buffer conveyance motor  204  to rotate the paddle  31 , a side guide motor  207  for moving the side guide  11  in sliding the side guide  11 , a reference guide solenoid  208  for escaping the reference guide  37  from the staple tray  12  during the sheet shift, the drive motor  40  for rocking the rocking guide  20  and driving rotatively the downstream delivery roller  17   a  in the normal and reverse directions, the stacker motor  209  for moving up and down the stack tray  18 , a stapler motor  210  for staple operation of the stapler  13  and feeding of the staples, a stapler traveling motor  211  for moving the position of the stapler  13 , and so on. 
     The respective motors control the traveling amount, speed, and so on according to the control input pulse and the input from the encoder detecting the rotation amount. 
     [Stitcher unit] 
     Respective structures of portions in the stitcher unit D in the sheet processing apparatus B is described in detail next. As described above, the stitcher unit D as shown in FIG. 3 delivers sheets in providing the folding operation after the sheets delivered from the image forming apparatus body are conveyed in the vertical path  60  composed of path guides  60   a ,  60   b  and are stapled at the center of the sheets by means of the stapler unit  61 . 
     The sheets P delivered from the image forming apparatus body A are fed to the vertical path  60  of the stitcher unit D in co-operation with the first flapper  21 , and are stacked and aligned while the lower end of the sheets is in contact with the stopper  62 . An upper roller pair  63  is provided as a conveying means at an upper portion of the vertical path  60 , and plural flappers  64  are formed on the downstream side of the pair. In this embodiment, the flappers  64  are constituted of a first flapper  64   a  and a second flapper  64   b , which allow to change selectively the conveyance route according to the size of the sheets P. 
     A movable guide  65  is provided around the flappers  64 . The guide  65  is urged toward the flapper  64  by an urging means  65   a  to constitute a part of the conveyance route of the vertical path  60 . This movable guide  65  can expose the inside of the vertical path  60  near the flappers  64  by pivotally movement by gripping a handle  65   b , thereby allowing recovery for sheets when jamming occurs. 
     Plural sheet sensors  66  are arranged at positions opposing to the flapper  64  with respect to the vertical path  60 . A first upper sensor  66   a  is placed between the upper roller pair  63  and the first flapper  64   a ; a second upper sensor  66   b  is placed at a position opposing to the first flapper  64 ; and a third upper sensor  66   c  is placed at a position opposing to the second flapper  64   b . Those sheet sensors  66  can detect existence of the passing sheet and the front end or rear end of the sheet. 
     [Lower roller pair] 
     The stapler unit  61  as describe below is arranged around the center of the vertical path  60 , and an anvil  61   d  is placed at a position opposing to the stapler unit  61  with respect to the vertical path  60 . A lower roller pair  67  is formed as a conveying means on a downstream side of the stapler unit  61 , and the pair  67  includes a drive roller  68  as a drive rotary body for transmitting the drive force from the drive source not shown, and a pickup roller  69  as a movable rotary body driven to rotate in pushing the sheet to the drive roller  68 . 
     As shown in FIG. 43, the pickup roller  69  is mounted at one end of a conveyance roller arm  69   a , and the other end of the conveyance roller arm  69   a is rotatively supported to the path guide  60   b  of the vertical path  60  through a pivotal shaft  69   b . An elastic member  69   c  is attached around the center of the conveyance roller arm  69   a , thereby urging the pickup roller  69  to the drive roller  68 . Meanwhile, a pressing releasing arm  70  driven by a solenoid not shown is formed at the conveyance roller arm  69   a , and the arm  70  is ale to separate the pickup roller  69  from the drive roller  68 . Therefore, the pickup roller  69  can change its position between the pressing position for pushing the sheet to the drive roller  68  and the separation position for separating the roller  69  from the drive roller  68 . 
     [Pressing of the pickup roller] 
     When the sheet P is conveyed by the lower roller pair  67 , the roller  69  is pressed as shown in FIG.  44 ( a ) on the sheet after the front end of the sheet passes by the position of the pickup roller  69 , and the sheet P is carried while nipped by the pickup roller  69  and the drive roller  68 . The subsequently fed sheet proceeds at that time toward the drive roller side of the already stacked sheets P, and is conveyed in skidding together with the already stacked sheets. 
     If the pickup roller  69  is normally in pressed contact with the drive roller  68 , the roller may exert conveyance force to the sheets P that have reach the stopper  62  and been stacked thee and may fold the sheets. In this embodiment, the pickup roller  69  comes in pressed contact with the roller  68  only when necessary, so tat the sheets are aligned well and can be stacked precisely to the vertical path  60 . 
     [Separation of the pickup roller] 
     As shown in FIG.  44 ( b ), the pickup roller  69  is separated at a position where the front end of the sheet P come close to a prescribed position from the stopper  62 . In this embodiment, the prescribed position from the stopper  62  is set for 10 mm in this embodiment, and after the pickup roller  69  is separate, the sheet P is conveyed to the stopper  62  from the inertial moment prior to this moment and the weight of the sheet itself. It is to be noted that the position of the front end of the sheet P is recognized by a conveyance distance after the front end of the sheet passes by the sheet sensor  66 . 
     If the sheet is conveyed while the pickup roller  69  is in pressed contact with the drive roller  68  until the sheet P reaches the stopper  62 , the sheet may be bent or may impair proper alignment due to occurrences of rebounding when the pickup roller  69  is separated. In this embodiment, the pickup roller  69  is separated at an early stage, thereby preventing the sheets from overly conveyed and avoiding the above problem. 
     [Drive roller when the pickup roller is separated] 
     As described above, if the pickup roller  69  is separated before the sheet P is stacked on the stopper  62 , the sheet P may proceed with great force in the vertical path  60  where the stacked number is not so large, and rebounding of the sheet may create disorder in alignment. If the stacked number is large, the friction opposing to smooth passage may increase due to narrower space in the vertical path  60 , so that the sheet may not reach the stopper  62 . 
     This embodiment is structured that the drive roller  68  keeps drive rotation even after the pickup roller  69  is separated. The sheet P conveyed at that time receives only weak conveyance force from contact force because the sheet is not in pressed contact with the drive roller  68 . Accordingly, the sheet P is surely conveyed to the stopper  62 , and can be aligned certainly because pushed. 
     [Vertical path shape] 
     The sheet P stacked upon hitting the stopper  62  is aligned in the width direction by an alignment member  71 . At that time, the sheet P is in an upright state, and if the sheet is flexible, the sheet is folded. In this embodiment, to solve such a problem, a projection  60   c  projecting in the conveyance route of the vertical path  60  is formed at the path guide  60   a,  thereby bending the stacked sheets horizontally, or namely creating rigidity in the vertical direction. Accordingly, the sheets P can be stacked without folding of the sheets. 
     On the other hand, if the sheets remain bent in the horizontal direction, it is not favorable when the stapler unit  61  makes the stapling operation. In this embodiment, as shown in FIG. 45, the vertical path  60  is bent between the upper portion and the lower portion of the stapler unit  61 , thereby making the sheets P bent around the center to the vertical direction. That is, the sheets P are stacked where the lower portion is bent in the horizontal direction and where the center portion is bent in the vertical direction. This structure can stack the sheets without folding the sheets and can make the position for executing the staple operation flat. 
     [Stopper Mechanism] 
     Referring to FIG. 47, a drive mechanism for the stopper  62  is described. Sliding members  62   a  are mounted on both ends of the stopper  62  and supported slidably along a stopper frame  72 . The stopper  62  is securely coupled to a stopper drive belt  73  wound around a drive pulley  73   a  and idler pulley  73   b.  A drive gear  73   d  is fixed to a rotary shaft  73   c  of the drive pulley  73   a  and is connected to a stopper drive motor  74 . That is, if the stopper drive motor  74  rotates, the stopper drive belt  73  rotates upon receiving the drive force and can drive the stopper  62  up and down. 
     A stopper sensor  75  is provided on a sheet stacking surface of the stopper  62  and can detect the sheet P where the front end of the sheet P hits the stopper  62 . A flag  62   b  is formed at the lower portion of the stopper  62 , and a stopper home sensor  76  detects the stopper  62  when the stopper  62  reaches the home position. 
     [Stapler unit] 
     A mechanism of the stapler unit as a sheet stapling means for rendering the staple operation of the sheet bundle is described next. 
     As shown in FIG. 48, the stapler unit  61  is mounted at two locations symmetrically in the horizontal direction with respect to the center in the sheet width direction by a support plate  77  secured to the frame at a center position in the conveyance direction of the sheet bundle aligned by the vertical path  60 . 
     In FIG. 48, the stapler unit  61  is constituted of a forming portion  61   b  serving as a staple shooting means located on an upper side and supported pivotally around a rotary shaft  61   a,  a drive unit  61   c,  and a anvil  61   d.    
     The vertical path  60  extends below the stapler unit  61  to guide the sheet bundle by the path guides  60   a,    60   b  and the anvil  61   d.  The vertical guide  60  is structured so that a guide surface  60   b   1  of the path guide  60   b  for guiding the sheet bundle and a stapling surface  61   d   1  of the anvil  61   d  for stapling the guided sheet bundle are angled with alpha to each other. The path guide  60     1   with the angle alpha on the upper surface side for forming the vertical path has a cutoff hole  60   a   1  of a size not interfering with the forming portion  61   b  when the forming portion  61   b  of the stapler unit  61  is moved pivotally. 
     A staple cartridge  61   e  is detachably attached to the forming portion  61   b,  and in the staple cartridge  61   e,  the staples  61   f  connected as a plate form are filled in a number of about 2000 to 5000. The staples  61   f  in the plate form filled in the staple cartridge  61   e  are urged downward by a spring  61   g  provided at a topmost end of the staple cartridge  61   e,  and the spring  61   g  gives the conveyance force to a staple feeding roller  61   h  disposed on a lowermost side. 
     The staples  61   f  fed by the staple feeding roller  61   h  are formed individually into a rectangular letter-U shape by rocking the forming portion  61   b  around the rotary shaft  61   a  as the center in the row direction (the counterclockwise direction in FIG.  48 ). That is, when a stapler motor  61   i  starts moving, an eccentric cam gear  61   k  rotates through a gear series  61   j.  By operation of an eccentric cam mounted to the eccentric cam gear  61   k  as a united body, the forming portion  61   b  performs the stapling operation (clinching operation) by its rocking movement in the arrow direction in FIG. 48 (toward the anvil  61   d ), thereby stapling the sheet bundle by folding the hit staple  61   f  at the anvil  61   d  located at the lower surface of the sheet bundle. 
     A flag not shown is disposed coaxially with the eccentric cam gear  61   k,  and the apparatus detects the flag with a stapler sensor not shown to monitor whether the stapler unit  61  is in a clinching state, ends the clinching operation (or it is before clinching start). 
     [Filling of the staples] 
     In the staple operation as described above, if not staple exists in the staple cartridge  61   e  as a staple filling member, the cartridge  61   e  has to be replaced. Now, filling of staples for the stapler unit  61  is described. 
     To the stapler unit  61  according to the embodiment, two staple cartridges  61   e  are to be attached for stapling two positions in the sheet width direction as shown in FIG. 49, and if all staple has gone from either staple cartridges  61   e,  the stapler unit can detect the no staple status. 
     When the no staple status is detected, the stapler unit  61  as shown in FIG. 49 is pulled in the arrow direction, and staples are newly filled in the staple cartridge  61   e.  If oily one staple cartridge  61   e  becomes the no staple status where the other staple cartridge  61   e  still has some staples, the staples in the other staple cartridge  61   e  may be used up soon even where the staples are filled in the one staple cartridge  61   e,  and it is unproductive to pull the stapler unit  61  upon stopping the operation of the apparatus to fill the staples. 
     In this embodiment, when the no staple status is detected, the control panel as a display indicates to prompt to fill staples at the same time in the two staple cartridges  61   e  by pulling the stapler unit  61  to fill the staples in the one staple cartridge  61   e  in the no staple status and by replacing the remaining staples in the other staple cartridge  61   e  with new staples even where some staples are remaining the other staple cartridge  61   e.    
     According to this display, where the staples are filled simultaneously in the two staple cartridges  61   e,  the two staple cartridges generally enter in the no staple status at the same time, so that when the one staple cartridge  61   e  holds no staple, the other staple cartridge  61   e  holds only few remaining staples even where both do not enter in the no staple status at the same time. Filling the staples in both cartridges at the same time makes this operation more productive than filling the staples individually in each staple cartridge  61   e  when each runs out the staples. 
     It is to be noted that although in this embodiment the two staple cartridges  61   e  are placed, staples in the all cartridges can be replaced at the same time even in a case that three or more staple cartridges  61   e  are installed in the stapler unit  61 . 
     The apparatus is required to detect as to whether the staple initialization is made where the stapler motor  61   i  (see, FIG. 48) is to drive to initialize the staple state after the staples are filled as described above. Although such a staple initialization can be detected by a sensor or the like, the initialization is this embodiment is detected by checking the drive current value of the stapler motor  61   i.    
     That is, where staples are set to the initialized position by drive of the stapler motor  61   i,  the stapler motor  61   i  is subject to a small load prior to the staple initialization (the empty shot state), and therefore, the current driving the stapler motor  61   i  is small as shown in FIG.  50 ( a ). On the other hand, when the staples are set to the initial position (the staple shot state), the stapler motor  61   i  is subject to a large load, and therefore, the current driving the motor becomes larger than that in the empty shot state as shown in FIG.  50 ( b ). Accordingly, when the value of the current driving the stapler motor  61   i  is detected, the apparatus detects that it is before the staple initialization if the detected current value is smaller than the prescribed value and that the staple initialization is done if the detected current value is larger than the prescribed value. 
     Such detection of the staple initialization in use of the drive current of the stapler motor  61   i  eliminates necessity for installing a special sensor for the staple initialization and can reduce the number of parts and costs. 
     [Post-processing of the staple operation] 
     After the sheet bundle P, which is conveyed and aligned at the vertical path  60  as described above, is stapled at the center in the sheet conveyance direction by drive of the stapler unit  61 , the stopper  62  is moved down to convey the sheet bundle to the folding position. As shown in FIG. 51, at that time, if the staple  61   f  after the staple operation is fitted in a groove  61   d   2  on the anvil  61   d,  the staple  61   f  is trapped at the groove  61   d   2  even if the stopper  62  moves downward, thereby possibly causing conveyance failures of the sheet bundle P. 
     To solve such a problem, in this embodiment, as shown in FIG. 51, the pickup roller  69  of the lower roller pair  67  is structured to be rocked once in the arrow direction in FIG. 51 before the stapled sheet bundle P is conveyed downward. This rocking movement of the pickup roller  69  as the position changing means pushes the sheet bundle P toward the path guide  60   a,  and therefore, the staple  61   f  fitted in the groove  61   d   2  on the anvil  61   d  is surely taken out of the groove  61   d   2 . Accordingly, the stopper  62  does not move down while the staple  61   f  is fitted in the groove  61   d   2 , and the apparatus can surely prevent failures in sheet conveyance from occurring. 
     Although in this embodiment, exemplified is an example that the pickup roller  69  is rocked to shift the position of the sheets to render the staple  61   f  escape from the groove  61   d   2 , a special member as shifting means for shifting the sheets so as to take the staple  61   f  out of the groove  61   d   2  may be provided instead of the pickup roller  69  and be executed so. 
     It is to be noted that when the sheet bundle is conveyed down by moving the stopper  62  downward, the pickup roller  69  is spaced from the drive roller  68 , and the drive roller  68  is driven to rotate in a direction to move the sheet P down. 
     This structure does not create any frictional load during moving down of the sheet when the sheet bundle falls by its weight even where the sheet is in contact with the drive roller  68  formed of a material having a high frictional co-efficient. The sheet surely follows the dissenting stopper  62   b,  thereby guaranteeing the accuracy in the folding position. 
     [Fine adjustments of the staple position and the folding position] 
     To perform the staple operation and the folding operation as described below accurately with respect to the center in the conveyance direction of the sheet bundle, the stopper  62  is required to move so that the center of the sheet bundle is positioned precisely at the staple position as well as the folding position. However, the length of the sheets may be not constant due to deviations when the sheets are cut or extensions or contractions due to humidity, and in some case, the staple position and the folding position may be out of the center of the sheet bundle P. In such a case, the position of the stopper  62  as a supporting means for supporting the lower end of the sheets is required to be finely adjusted. 
     In a conventional apparatus, the stopper is structured to be screwed in a long hole bored in a frame supporting the stopper, and the position of the stopper is finely adjusted in the range of the long hole, thereby finely adjusting the staple position and the folding position of the sheet bundle as described above. 
     However, such an apparatus is required to loosen the screw to finely adjust the position as described above and to fasten the screw after a fine adjustment is made, so that it requires laborious work for adjustment as well as makes tough to do a fine adjustment precisely. Such adjustment work is also done only by service persons. 
     In this embodiment, to solve such a problem, the shift amount of the stopper  62  is finely adjustable by a drive mechanism (see FIG. 47) as described above according to designations from an input means such as a control panel formed on the image forming apparatus body A or the sheet processing apparatus B. 
     More specifically, the apparatus is structured so that the shift amount (moving up amount) when the stopper  62  is moved from the home position to a lower end stopping position (stapling stopping position) as a stopping position for the staple operation according to the sheet size is finely adjustable according to the designation from a control panel (not shown) formed on the image forming apparatus body A. For example, the shift amount from the home position to the lower end position of the sheet of the respective sizes is normally constant for every sheet size, but the shift amount is increased or decreased by an amount according to the designation from the control panel as an input means to finely adjust the shift amount when the stopper  62  is moved up, thereby changing the stopping position by the portion of the fine adjustment. 
     Alternatively, the apparatus is structured so that the shift amount (moving down amount) when the stopper  62  is moved, after the sheets are stapled, from the lower end stopping position (stapling stopping position) according to the sheet size to a lower end stopping position (folding stopping position) during the folding operation is finely adjustable according to a dip switch (not shown) or a control panel on an electrical circuit substrate formed on the sheet processing apparatus B. The shift amount of the stopper  62  from the lower end stopping position during the staple operation to a lower end stopping position during the folding operation is constant (e.g., 70 mm in this embodiment) regardless the sheet size, but the shift amount is increased or decreased by an amount according to the dip switch on the electrical circuit substrate to finely adjust the shift amount when the stopper  62  is moved down, thereby changing the stopping position by the portion of the fine adjustment. 
     With such a constitution, the stopper for receiving the lower end of the sheet is finely adjustable precisely and easily. 
     The sheet may extend longitudinally due to roller pressure, temperature, humidity, and the like during the conveyance of the sheet. Therefore, where the sheet length is detected by a sheet length detecting means during conveyance of the sheet, the stopping position of the stopper  62  can be automatically adjusted according to the detected results. 
     For example, as shown in a flowchart of FIG. 52, when the sheet is conveyed to the vertical path  60 , any one of the upper sensors  66   a  to  66   c  detects this according to the sheet size. When this sensor detects the front end of the sheet (S 131 ), the pulse counter, not shown, for conveyance motor is turned on (S 132 ) to count the pulse number up to a time from passing of the rear end of the sheet at the sensor position until the sensor is turned off (S 133 , S 134 ). From this operation, the length of the sheet fed in the vertical path  60  can be detected precisely. The shift amount of the stopper  62  from the home position to the staple stopping position and the shift amount from the staple stopping position to the folding stopping position are finely adjusted upon automatic calculation by the control means according to the length of the conveyed sheet (slightly extended sheet) (S 135 ), and the stopper  62  is moved according to the shift amount after the fine adjustment. 
     Where the stopper shift amount is automatically adjusted finely according to the sheet length detected during the conveyance, the user does not have to set for fine adjustment, and the sheet can be stopped at the staple position and the folding position precisely and easily. 
     [Folding of the sheet bundle] 
     Thus, the staple position is conveyed until reaching the position of the folding roller  78  disposed below the stapler unit  61  by the down movement of the stopper  62 , and the sheets are hit by a striking plate  79   a  at the staple position and folded in folio on conveyed through nipping the sheets by the folding roller  8  while the sheets are in folio. Referring to FIG. 53 to FIG. 57, the sheet folding structure is described next. 
     As shown in FIG. 53, the folding roller  8  is constituted of a stable roller  78   a  pivotally movable around a secured rotary shaft  78   c,  and a movable roller  78   b  attached pivotally to a support arm  80  which can be pivotally moved around a pendulum  80   a  with respect to the apparatus frame. The folding roller  78  is constituted in which a spring  81  engaged at one end of the support arm  80  renders both rollers  78   a,    78   b  in pressed contact with each other. This structure allows the pitch between the folding rollers  78  to be corrected according to the thickness of the sheet bundle P to be nipped. 
     The structure for driving the folding roller  8  is as shown in FIG.  54  and FIG. 55 with a motor pulley  83  secured to an output shaft  82   a  of a folding motor  82 . The drive force of the motor pulley  83  is transmitted to a pulley of an idler gear pulley  85  via a timing belt  84 . The idler gear pulley  85  is formed coaxially with the pulley and the gear portion. 
     Respective folding gears  86 ,  87  are secured to the shaft of the folding roller  78  described above, and both gears  86 ,  87  are in mesh with each other. One end of the folding gear  86  is in mesh with the gear portion of the idler gear pulley  85 . The folding gear  86  is also meshing an idler gear  88 . 
     The rotary force of the folding motor  82  is transmitted to the idler gear pulley  85  from the motor pulley  83  through the timing belt  84 . The rotation of the idler gear pulley  85  is transmitted to the folding gear  87  from the folding gear  86 , thereby driving the folding roller  87 . The rotation force is also transmitted at the same time to the idler gear  88  in meshing with the folding gear  86 . The idler gear  88  also transmits the rotation force to the delivery roller as described below. 
     In FIG. 54, the numeral  79  is a projecting unit as a projecting means, and is constituted of a projecting plate  79   a,  holders  79   b,    79   d,  axes  79   c,    79   e,  and so on. The projecting plate  79   a  is supported by the holders  79   b,    79   d,  and the axes  79   c,    79   e  are secured to the holder  79   b.  A roller not shown is rotatively mounted on outer peripheral surfaces of the axes  79   c,    79   e,  and the roller slides in a groove  89  formed in the housing frame. 
     The numeral  90  is a projecting motor, and a motor pulley  90   a  is secured to the output shaft of the motor. The numeral  91  is an idler pulley, in which a pulley portion and a gear portion are formed coaxially. A timing belt  92  is wound around the pulley portion of the idler gear pulley  91  and the motor pulley  90   a.  The gear portion of the idler gear pulley  91  is in mesh with a gear  126  having an axis  93  as a part. As shown in FIG. 55, flags  95   a,    95   b  are fixed to a rotary shaft  94   a  of a gear  94 . The flags  95   a,    95   b  have cutoffs as a part. A projecting home sensor  96   a  and a projecting position sensor  96   b  are proved at positions where the cutoffs of the flags  95   a,    95   b  are detected. The projecting home sensor  96   a  is arranged to detect the projecting plate  79   a  at a position deeper than the sheet conveyance surface constituted by the path guides  60   a,    60   b,  and the projecting position sensor  96   b  is arranged to detect the projecting plate  79   a  at a position where the projecting plate  79  is inserted. 
     As shown in FIG. 55, a rotational plate  97  having a shaft  97   a  in substantially the same way as the gear  94  is secured to the other end of the gear  94  on the rotary shaft  94   a,  and is structured to rotate in synchrony with the gear  94 . 
     The rotary force of the projecting motor  90  is transmitted to the idler gear pulley  91  from the motor pulley  90   a  through the timing belt  92 . The gear  94  rotates because the idler gear pulley  91 , thereby moving the axis  93  circularly. One end of a link  98  is fitted to the axis  93 , and the other end of the link  98  is fitted to the axis  79   c  of the projecting unit  79 . Thus, the circle movement of the axis  93  is transmitted to the axis  79   c  of the projecting unit  79  through the link  98 , and the axis  79   c  moves linearly along the groove  89  on the frame fitting to the axis  79   c  together through a roller, not shown. 
     The other end of the projecting unit is, in substantially the same manner, converting the circle movement of the rotational plate  97  and the axis  97  on the rotational plate  97  to a linear movement of the projecting unit through the link. The projecting unit  79  thus moves slidably in always parallel to the folding roller  78  in the arrow direction of FIG. 54 upon receiving drive at the opposing ends. 
     Moreover, as shown in FIGS. 55,  56 , stopper shafts  97   b,    97   c  are formed on a surface opposite to the axis  97   a  of the rotational plate  97 ; a stopper member  99  is provided pivotally around the a shaft  99   a  as a center on the housing frame and is urged toward the rotary shaft  94   a  by a spring  100 . As described above, the projecting unit  79  moves in sliding by rotary movement of the rotational plate  97 ; the rotational plate  97  rotates in the arrow direction of FIG.  56 ( b ); the projecting unit  79  projects therein; when the projection position sensor  96   b  reaches the detected position (upper left position in FIG.  54 ), the stopper shaft  97   c  and stopper member  99  are fitted to each other, and the rotational plate  97  rotates no more. Therefore, the projecting unit  79  is immobilized at the projecting position. When the projecting unit  79  is returned to the home position, the projecting motor  90  is rotated in a direction opposing to the direction at a time for projecting the unit, thereby disengaging the stopper member  99  and the stopper shaft  97   c  from each other. When the projecting unit returns to the home position, the stopper shaft  97   b  and the stopper member  99  are engaged with each other at a position (upper right position in FIG. 54) detected by the home sensor  96   a,  and therefore the rotational plate  97  rotates no more. 
     As described above, the apparatus is structured so that the projecting unit  79  moves horizontally from the normal and reverse rotations of the projecting motor  90 . 
     [Distance between nips of the folding roller and center adjustment of the projecting plate] 
     As described above, in the folding roller  78 , the movable roller  78   b  moves up according to the thickness of the sheet bundle P. That is, the distance between the nips of the folding roller  78  may change. To the contrary, if the projecting position of the projecting plate  79   a  is always constant, the projecting plate  79   a  does not always strikes the center of the space between the nips of the folding roller  78 , so that the sheet bundle P may be not be folded at a stapling position. 
     In this embodiment, the apparatus is so structured that a cam member allow the projecting plate  79   a  to strike always the center of the space between the nips even if the space between the nips of the folding roller  78  is deviated. This structure is described in reference to FIG. 57 in detail. 
     As shown in FIG. 57, the pushing unit  79  is structured to be rotatable around a sliding roller  79   g  as a center (the outer diameter of the sliding roller  79   f  is smaller than the width of the groove  89 ), and a cam member  101  to attached to both shafts  78   c,    78   d  of the folding roller  78 . The cam member  101  has a cam groove  101   a  capable of engaging with the movable roller shaft  78   d  and a guide portion  101   b  of the projecting unit  79 , and the projecting unit  79  is slidable on the guide portion  101   b  and is urged downward by a spring  102 . 
     The cam member  101  is mounted pivotably around a shaft  78   c  as a center of the stable roller  78   a.  When the movable roller  78   b  moves up upon rotation of the movable roller  78   b  around the center  80   a  (see, FIG.  53 ), the shaft  78   d  of the movable roller  78   b  pushes up the cam groove  101   a.  By this pressing, the cam member  101  rotates in the counterclockwise direction in FIG. 57, and as shown in FIG.  57 ( b ), the guide portion  101   b  pushes up the projecting unit  79 . It is to be noted that the am groove  101   a  and the guide portion  101  are shaped to always project the projecting plate  79   a  to the center of the space between the nips of the folding roller  78 . 
     By providing the cam member  101  thus structured, the projecting center of the projecting plate  79  is adjusted to strike always the center of the space between the nips, thereby surely folding the sheet bundle P at the staple position. The center adjustment of the projecting plate  79   a  can be done by replacement of the cam member  101  as described above, so that the structure cannot be complicated. 
     It is to be noted that although the sheet bundle P is nipped by the folding roller  78  upon folding the sheet bundle P by projection of the projecting plate  79   a,  wrinkles may occur on an inner sheet due to frictional force between the folded sheet and the projecting plate  79   a  if the projecting plate  79   a  is located at a position where the projecting unit  79  is projected even after the folding roller  78  nips the sheet bundle P. 
     In this embodiment, as shown in FIG. 58, the apparatus is structured that after the sheet bundle P is nipped by the folding roller  78 , the projecting plate  79   a  is returned to the home position. If the projecting plate  79   a  is returned too early, the projecting plate  79   a  returns before the folding roller  78  nips the sheet bundle P. Therefore, in this embodiment, the projecting plate  79   a  is pulled back at a time that the folding roller  78  rotates in a prescribed amount to fold the sheet bundle P. 
     This operation prevent wrinkles from occurring where the folded sheet bundle P proceeds without suffering from friction to the projecting plate  79   a,  because the projecting plate  79   a  escapes to the home position at a time when the sheet bundle P is conveyed upon nipped by the folding roller  78 . 
     [Folding Operation] 
     Folding operation for sheet bundle P is executed by projecting the center of the sheet bundle P already subjecting to the staple operation by the projecting plate  79   a,  and by conveying the sheet bundle P upon nipping the sheet bundle P with the folding roller  78  at the center of the sheet bundle P which is struck by the projecting plate  79   a.    
     Images are recorded on respectively upstream half and downstream half, with respect to the sheet center, in the conveyance direction of respective sheets constituting the sheet bundle. Similarly, such image formation thus described is made ion double sides of the sheet, and such image formation on the sheet bundle subjecting to the staple and folding operations is made according to the page order. 
     When the sheet bundle is pulled by the folding roller as described above, if an image is formed at the center of the sheet bundle, the image (toners) makes lower the sheet frictional coefficient, thereby causing a failure in pulling of the sheet bundle by the folding roller and possibly creating wrinkles and tears. 
     Referring to FIG. 59, a mechanism of occurrences of tears and wrinkles of the sheet during the folding operation is briefly described. It is to be noted that a sheet bundle made of the two sheets P 11 , P 12  is exemplified herein. 
     Where the frictional force between the folding roller  503  and the first sheet P 11  is denoted as F 1 , where the frictional force between the first sheet P 11  and the second sheet P 12  is denoted as F 2 , and where the binding force of the staples fastening the sheet bundle is denoted as F 3 , ordinary folding operation is generally made while the forces satisfy the formula F 1 =F 2 +F 3 . However, to satisfy the above formula when the frictional force F 2  between the sheets P 11 , P 12  is made smaller, the binding force F 3  of the staple is required larger. At that time, even where the sheet strength is durable enough against the binding force F 3  of the staple, the second sheet P 12  may be pulled by the staple, thereby inflicting scars as shown in FIG.  59 ( a ), and creating warps as shown in FIG.  59 ( b ). If the sheet bundle passes by the folding roller  503  in this state, wrinkles as shown in FIG.  59 ( c ) may occur. Moreover, if the sheet is broken down doe to the binding force F 3  of the staple, tears as shown in FIG.  59 ( d ) may occur. 
     In this embodiment, as shown in FIG. 60, a margin (folding margin t) of a certain size is provided at a center of the sheet (sheet bundle) P as a folded area). More specifically, image formation to the sheet P subjecting to a processing in the stitcher unit D is done during image recording in the image forming apparatus body A by avoiding in advance the folding margin t at the sheet center (in this embodiment, it is set as t=about 5 to 8 mm, margin width 2t=about 10 to 16 mm). 
     It is to be noted that in the drawing, Ps denotes the center (center in the conveyance direction) on the sheet P and that Pg denotes the image recording areas ion the sheet P. 
     With this structure, the margin (folding margin t) formed on the sheet P in advance makes the frictional force between the sheet P and the folding roller  78  larger where the sheet bundle is pulled by the folding roller  78 , so that wrinkles and tears as described above are suppressed, and so that the sheet bundle P is pulled in surely. 
     If the width of the margin is narrower than about 10 mm, adequate frictional force may not be obtained when the sheet bundle is pulled, and if the margin width exceeds about 16 mm, the image forming area may become smaller. Therefore, it is desirable to set the margin width between about 10 mm and 16 mm. 
     It is to be noted that although in this embodiment image formation is made in providing the margin of the prescribed width as described above for the sheet subjecting to the folding operation, the setting of the margin can be reset or cancelled where a larger image formation area is required. Where the sheet number of the sheets to be fold is few, the setting of the margin of the prescribed width can be cancelled to form images with an ordinary margin width (e.g., about 4 mm) because tears may not be created even where the margin of the folded portion is narrowed. With this operation, the wide image formation area can be obtained. 
     In a case where the sheet bundle subjecting to the staple and folding operations as described above has a sheet forming a front cover (during a front cover mode), if images are formed on a back side of the sheet forming the front cover (hereinafter, referred to as “front cover sheet”), silicon oil or the like may adhere when transferred toner images are fixed by a fixing means on the back side of the front cover sheet, thereby making lower the frictional coefficient between the back side of the front cover sheet and the sheet in contact with the front cover sheet. It is to be noted that though a fine quality paper (thick paper, cardboard paper, as well) can be used frequently, if silicon oil adheres on the fine quality paper, the frictional coefficient is further lowered in comparison with other sheets (plain papers). If such a front cover sheet is nipped by the folding roller to fold the sheet and is pulled in, the sheet may be skipped between the sheets because the frictional coefficient is so small between the back side of the front cover sheet and the sheet in contact with the front cover sheet, so that only the front cover sheet may be pulled. 
     In this embodiment, during the front cover mode, the apparatus does not make recording on the back side of the front cover sheet. More specifically, during the image recording period in the image forming apparatus body A, the front cover sheet is delivered without passing through the re-feeding path  7  (see, FIG. 1) after the image is recorded on the surface of the front cover sheet and is sent to the stitcher unit D. 
     It is to be noted that since in this embodiment the front cover sheets (fine quality sheets or the like) are to be fed from a multi-tray  8  (see, FIG.  1 ), the sheet is controlled to be delivered without passing through the re-feeding path  7  if the last sheet fed to the stitcher unit D is fed from the multi-tray  8 . 
     This structure prevents the back side of the front cover sheet from contacting with the fixing means (namely, a fixing roller provided on a side of the image surface), and therefore, the silicon oil will not adhere on the back side of the front cover sheet, thereby preventing the frictional coefficient from lowering, and preventing the front cover sheet from being pulled solely. 
     [Detection of tears of the sheet bundle] 
     A structure detecting tears by a detecting means such as a sensor may be generally conceivable in the case where the tears occur in the sheet bundle during folding processing as described above, but if such a detecting means is provided separately to detect the tears, the number of parts and costs are increased. Because the subsequent sheet may be already conveyed when the tears are detected, paper jamming of a more serious degree may occur where the proceeding sheet is remaining. 
     In this embodiment, as shown in FIG. 61, using an intermediate sensor  103  formed near the folding roller  78  (upstream side), the stopper sensor  75  formed at the stopper  62 , and a delivery sensor  105  formed near a delivery roller  104 , when the respective sensors detect the existence of the sheet at the same time, the apparatus is structured to detect that a sheet (or sheet bundle) is torn and stays at the stitcher nit D. 
     It is to be noted that although in this embodiment the occurrence of tears in sheets is detected when all of the intermediate sensor  103 , the stopper sensor  75 , and the delivery sensor  105  detect the existence of the sheet at the same time, the invented structure is not limited to this, and for example, the apparatus may detect the occurrence of tears in sheets when both of the stopper sensor  75  and the delivery sensor  105  detect the sheet existence. 
     With this structure, a detecting means for detecting as to whether tears occur in the sheet (or sheet bundle) is not necessarily installed separately, it is possible to prevent the increase of the number of the parts and the costs. Tears of the sheets can be detected early at the folding timing of the sheets, so that jamming of the sheets can be prevented by stopping the conveyance of the sheets early. 
     [Delivery operation] 
     As described above, the sheet bundle subjecting to the folding operation is delivered on the stack tray  106  by the delivery roller  104  and stacked on the tray. A sheet bundle pressing member  107  for pressing the sheet bundle P is rotatively formed above the delivery roller  104  as shown in FIG. 63 around the rotary shaft  107   a  as a center. When the sheet bundle P subjecting to the folding operation is delivered on the stack tray  106  by the delivery roller  104  through the delivery opening, the pressing member  107  can press the end of the sheet bundle P, thereby stacking the sheet bundle P even where the bundle P is inadequately folded on the tray without unfolding the sheet bundle on the stack tray  106 . 
     This pressing member  107  is restricted so that a roller  107   b  formed at a tip of the member does not move below a prescribed height h (height not contacting with the stacking tray  16 ), but can freely move in a direction that the roller  107   b  is raised. Accordingly, any delivery failure, otherwise occurring by pressing the front end of a flexible sheet bundle P with the pressing member  107 , would not occur, and therefore, the sheets can be surely stacked on the stack tray  106 . 
     Where the sheet bundle P thus subjecting to the folding operation is delivered on the stack tray  106  by the delivery roller  104 , since the sheets expand more on the sheet bundle folded side (the downstream side in the delivery direction) in comparison with the sheet bundle open side, if the bundles are stacked as they are, only the sheet bundle side becomes higher, thereby possibly rendering the stacking property of the sheet bundles unstable. To solve such a problem, as shown in FIG. 62, a device has been proposed having a structure to deliver and stack the sheet bundles P bundle by bundle in a positionally shifting manner so that the sheet bundles P delivered by the delivery roller  505  are shiftingly stacked on the stack tray  504 . 
     However, the above structure requires a large stack tray for delivering many sheet bundles, and the apparatus may become larger, so does the installation space. 
     With this embodiment, as shown in FIG. 63, a portion near the delivery roller  104  of the stack tray  106  is made higher than the stacking surface  106   b  (projection  106   a ), the sheets are delivered so that the folded side of the sheet bundle P comes over the stacking surface  106   b  on the front end side (downstream side in the delivery direction) of the stack tray  106  and the open side comes over the projection  106   a.  By this structure, the folded side goes lower than the open side, and the height differential from the open side can be made smaller even where the folded side expands greatly more than the open side. 
     More specifically, when the sheet bundle P is thick (that is, in the case that the folded side particularly expands), the sheet bundle delivery speed by the delivery roller  104  is made slow, and the sheet bundles are delivered so that the open side of the sheet bundles P comes over the projection  106   a.  When the sheet bundle P is thin (that is, in the case that the folded side relatively does not expand so much), the sheet bundle delivery speed by the delivery roller  104  is made faster, and the sheet bundles are delivered so that the open side of the sheet bundles P does not come over the projection  106   a.    
     With this operation, the sheet bundles P delivered on the stack tray  106  can avoid a situation that only one side (the folded side) is raised, thereby making the stacking property of the sheet bundle stable. The sheet bundles P are not necessarily stacked in a shifting manner, a large stack tray may not be needed. 
     Although in this embodiment the projection is formed, the invention is not limited to this structure, and can be structured with a slant portion, the stack tray  106  itself modified in a shape having a slope, or a projection extendable from the bottom of the stack tray  106  or detachable. 
     [Description of the control system] 
     Referring to FIG. 64, the structural elements of the control system for drive controlling the respective members in the stitcher unit as described above is briefly described. 
     In FIG. 64, an MPU  200  as a control means inputs respective sensors such as first to third upper sensors  66   a  to  66   c  detecting the existence, the front end, and the rear end of the sheet conveyed by the stitcher unit, an alignment member home sensor  220  for detecting the home position of the alignment member  71 , the stopper home sensor  76  for detecting the home position of the stopper  62 , the stopper sensor  75  formed at the stopper  62  for detecting the sheet, the delivery sensor  105  formed near the delivery roller  104 , the projecting position sensor  96   b  for detecting the projecting position of the projecting plate  79   a,  the intermediate sensor  103  formed near the folded roller  78 , and so on. 
     Based on the signals from the respective sensors and the image forming apparatus body A, the MPU  200  controls,, through respective drivers D 20  to D 28 , a first flapper solenoid  201  for driving the first flapper  21  to feed the sheets to the vertical path of the stitcher unit, a switching upper solenoid  221  and a switching lower solenoid  222  for switching the first and second flappers  64   a,    64   b  on the route of the vertical path, a conveyance motor  223  for conveying sheets by driving the upper and lower roller pairs  63 ,  67 , the stapler motor  61     1 ,  a squeezing motor  224  for operating the alignment member  71 , the stopper drive motor  74  for moving the stopper  62 , the folding motor  82  for driving the folding roller  78 , the projecting motor  90  for driving the projecting plate  79   a,  and so on, and renders operations as described above.