Abstract:
An image forming apparatus includes a main body having a sheet ejecting section, a placing tray formed on an upper portion of the main body to receive a sheet ejected from the main body, an image reading device for reading an image disposed above the sheet ejecting section, and a sheet post-processing unit disposed between the image reading device and the placing tray. The post-processing unit includes a supporting device capable of moving between a support position for supporting an upstream side of an ejected sheet in a transfer direction and a retreat position for allowing the sheet to drop onto the placing tray, a post-processing device disposed at a side adjacent to the supporting means for applying post-processing to the sheet supported on the supporting means, and a sheet shift device laterally moving the sheet into and from the post-processing device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application of patent application Ser. No. 09/927,413 filed on Aug. 13, 2001, now U.S. Pat. No. 6,685,180. 
    
    
     BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an image forming apparatus provided with a post-processing device. 
     As a conventional sheet post-processing device, a sheet post-processing device shown in FIG. 11, which is disclosed in Japanese Patent Publication (KOKAI) No. H1-313261, is known. A sheet post-processing device  100  in FIG. 11 is disposed outside an image forming apparatus main body  101 , and a placement tray  102  is disposed outside the sheet post-processing device  100 . 
     In FIG. 11, a sheet S copied in the image forming apparatus main body  101  and fed out by a paper ejection roller  105  is transferred to the sheet post-processing device  100  attached to an outside of the image forming apparatus main body  101 . The sheets S transferred to the sheet post-processing device  100  are stapled by a staple unit  103  provided in the sheet post-processing device  100 . 
     The sheets S in which stapling is finished are pushed by an abutting member  104  to be placed on the placement tray  102 . The abutting member  104  supports rear ends of the sheets S in a transferring direction to thereby align the rear ends, and when the stapling is finished, the abutting member  104  pushes the rear ends of the sheets S toward the placement tray  102 . 
     The sheet post-processing device  100  is attached to an outside of a side surface of the image forming apparatus main device  101  shown in FIG. 11, and the placement tray  102  is attached to an outside of the sheet post-processing device  100 . Therefore, when the sheet post-processing device  100  is installed in the image forming apparatus main body  101 , an installation area for the image forming apparatus main body  101  becomes large. 
     In order to solve the aforementioned problem, there is an image forming apparatus  106  shown in FIG. 12, which is disclosed in Japanese Patent Publication (KOKAI) No. 2000-86076. In this structure, a sheet post-processing device  107  is installed between an image forming apparatus  106  and an image reading device  108 . In the image forming apparatus  106  structured as described above, an installation area for the image forming apparatus  106  is reduced by a portion of the sheet post-processing device  107  assembled inside the image forming apparatus main body  106 . 
     However, a placement tray  109  on which the post-processed sheets S are placed remains to be projected outside from a side surface of the image forming apparatus  106 . Usually, a size of the placement tray  109  is extremely larger than that of the sheet post-processing device  107 , so that if the placement tray  109  remains to be projected, the installation area is not reduced. 
     Thus, as in a structure shown in FIG. 13 which is disclosed in Japanese Patent Publication (KOKAI) No. H8-277059, there has been known a structure in which a sheet post-processing device  110  is attached above a placement tray  111 . The sheet post-processing device  110  includes an internal tray  112 . The internal tray  112  is formed of two trays, that is, a first tray  112   a  and a second tray  112   b . The internal tray  112  structured as described above opens like a door from a connecting portion between the two trays toward the placement tray  111 . 
     The sheets S are stapled on the internal tray  112 , and when the stapling is finished, the internal tray  112  comes to an open condition. When the internal tray  112  is in the open condition, a set of the sheets S falls onto the placement tray  111  by its own weight to be placed on the placement tray  111 . 
     Therefore, since the internal tray  112  for supporting an entire surface of the sheet to which the stapling is applied and the placement tray for placing the stapled sheets are disposed vertically in parallel, the installation area for the post-processing device  110  can be reduced. 
     However, since the internal tray  112  is opened and closed like a door, the sheet post-processing device  110  is required to have a height sufficient for allowing the internal tray  112  to open. Thus, it is extremely difficult to assemble the sheet post-processing device  110  inside the image forming apparatus. Supposing that the sheet post-processing device  110  is assembled inside the image forming apparatus, when the sheet post-processing device  110  which is considerably high in order to open the internal tray is assembled inside the image forming apparatus, the height of the image forming apparatus is increased. 
     When the height of the image forming apparatus is increased, in case a document subjected to the image forming is set on the image forming apparatus, a position of setting the document becomes high. If the position of setting the document is high, it becomes difficult to confirm the position of setting. Therefore, there is a problem that this tall image forming apparatus is difficult to use. 
     Moreover, in order to install the tall sheet post-processing device  110  described above inside the image forming apparatus, a large space is required inside the image forming apparatus. However, in the known image forming apparatuses, since it has been tried to minimize a size thereof as small as possible, the large space described above is normally not formed in the known image forming apparatuses. Therefore, in the known image forming apparatus, especially, there has been a problem that the sheet post-processing device  110  can not be installed inside the image forming apparatus. 
     Also, as shown in FIG. 14, there has been known a structure disclosed in Japanese Patent Publication (KOKAI) No. 8-143211, in which an auxiliary guide  150  for supporting only a rear end portion of a sheet is disposed above a displacement tray  140  at the highest portion of a plurality of trays to be freely capable of projecting and retracting, such that the sheet is supported by the auxiliary guide  150  and a stapler  155  is moved forward and backward with respect to the sheet in a direction orthogonal to a sheet transferring direction to carry out the stapling process. 
     However, in this device, it is necessary to move the stapler  155 , which is relatively large and heavy, with respect to the sheet, and a motor for moving the stapler  155  also becomes bigger, so that the sheet post-processing device can not be made small. In addition, since the apparatus includes a plurality of trays, as in the aforementioned apparatus of FIG. 11, the sheet post-processing device is attached to the outside of the side surface of the image forming apparatus main body, so that an installation area for the image forming apparatus main body becomes large. 
     Accordingly, a first object of the invention is to provide an extremely compact sheet post-processing device. 
     A second object of the invention is to provide an image forming apparatus, in which an installation area for an entire image forming apparatus is not increased even if the sheet post-processing device is attached by selecting the installation site for the compact sheet post-processing device. 
     Further, a third object of the invention is to provide an image forming apparatus assembled with the sheet post-processing device, which can be easily assembled inside the known image forming apparatus and can be used easily without increasing an installation area of the image forming apparatus. 
     Further objects and advantages of the invention will be apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     To achieve the aforementioned objects, the present invention provides a sheet post-processing device for carrying out post-processing, such as a binding process or punching process, with respect to sheets ejected from an image forming apparatus main body. The sheet post-processing device comprises: a placing tray for placing sheets thereon; supporting means capable of moving between a support position for supporting upstream sides, in the transferring direction, of the sheets ejected above the placing tray, and a retreat position for allowing the supported sheets to drop onto the placing tray; post-processing means fixedly disposed at one end side of the supporting means and applying post-processing to the sheets supported on the supporting means; sheet shift means for shifting the sheets to a position where the sheets are released from the post-processing means; and control means for controlling the sheet shift means to shift the sheets from the post-processing means after the post-processing by the post-processing means is carried out. Also, the control means moves the supporting means to the retreat position. 
     Accordingly, since the placing tray and the supporting means are overlapped vertically, the space for the supporting means in the sheet transferring direction can be omitted, and there is no need to move the post-processing means, such as a stapler device, resulting in providing an extremely compact sheet post-processing device. 
     Also, the placing tray includes a first placing section, which supports forward ends of the sheets in the transferring direction, and a second placing section located below the supporting means. The second placing section is lower than the first placing section. Accordingly, the sheets can be extended over the supporting means and the placing tray to be processed, so that the post-processing can be surely carried out. 
     Also, the present invention provides an image forming apparatus, which comprises a placing tray formed on an upper surface of an image forming apparatus main body and placing sheets ejected from the image forming apparatus thereon; an ejecting section projected above the placing tray and disposed to be spaced away from the placing tray, in which the sheet ejecting section includes a sheet ejection port; and a sheet post-processing device including a unit formed of supporting means and post-processing means. The supporting means is provided between the placing tray and the sheet ejection port, and is capable of moving between a support position for supporting upstream sides, in the transferring direction, of the ejected sheets, and a retreat position for allowing the supported sheets to drop onto the placing tray. The post-processing means is disposed to be adjacent to one end side of the supporting means, and provided for applying post-processing to the sheets supported on the supporting means. 
     Further, the image forming apparatus described above further includes an image reading device for reading an image, which is disposed above the placing tray on the upper surface of the image forming apparatus main body and the ejecting section. The sheet post-processing device formed of the unit is disposed between the image reading device and the placing tray and located adjacent to the sheet ejection port. 
     Accordingly, there is no need to increase an installation area for the entire image forming apparatus. Even in case of the known image forming apparatus, the sheet post-processing device can be easily assembled therewith, and the image forming apparatus assembled with the sheet post-processing device, which is easy to use, can be provided without increasing the installation area for the image forming apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing an entire structure of a first embodiment; 
     FIG. 2 is an enlarged schematic view showing a main part of the first embodiment; 
     FIG. 3 is an enlarged schematic view showing a portion inside a sheet post-processing device of the first embodiment; 
     FIG. 4 is an enlarged schematic view showing a portion inside the sheet post-processing device of the first embodiment; 
     FIG.  5 ( a ) is a schematic view showing a state before sheets are aligned by alignment plates; 
     FIG.  5 ( b ) is a schematic view showing a state when the sheets are being aligned by the alignment plates; 
     FIG.  5 ( c ) is a schematic view showing a state when the sheets are aligned by the alignment plates; 
     FIG.  5 ( d ) is a schematic view showing a state when the sheets are pushed out from a processing section by the alignment plates; 
     FIG.  6 ( a ) is a schematic view showing an initial position before the sheets in the first embodiment are released; 
     FIG.  6 ( b ) is a schematic view showing a state when the sheets in the first embodiment are being released; 
     FIG.  6 ( c ) is a schematic view showing a state after the sheets in the first embodiment are released; 
     FIG. 7 is an enlarged schematic view showing an inside of a sheet post-processing device of a second embodiment of the invention; 
     FIG. 8 is a schematic view showing a third embodiment of the invention; 
     FIG. 9 is an enlarged schematic view showing an inside of a sheet post-processing device of the third embodiment; 
     FIG. 10 is a schematic view showing a fourth embodiment of the invention; 
     FIG. 11 is a schematic view showing an example of a conventional image forming apparatus with a sheet post-processing device; 
     FIG. 12 is a schematic view showing another example of a conventional image forming apparatus with a sheet post-processing device; 
     FIG. 13 is a schematic view showing an example of a conventional sheet post-processing device; and 
     FIG. 14 is a schematic view showing a part of still another example of a conventional image forming apparatus with a sheet post-processing device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 to  6 ( c ) show a first embodiment of the invention, and FIG. 1 is a view showing an entire structure of an image forming apparatus  1  which is provided with a sheet post-processing device FS, an automatic document feeder DF, and an image reading device Y. 
     The image reading device Y and the automatic document feeder DF are mounted above the image forming apparatus  1 , and a paper ejection port  2  for ejecting a sheet S on which the image is formed is provided in a space between the image forming apparatus  1  and the image reading device Y. The sheet post-processing device FS is connected to the paper ejection port  2 . 
     When a document d is placed on a document table 3 of the automatic document feeder DF, the document d is transferred to a document supply path  4  by respective transfer rollers, and reaches a reading section  5 . An image of the document d which has reached the reading section  5  is read by an image reading element or sensor  6  of the image reading device Y. 
     The document d, which has passed through the reading section  5  such that the image thereof is read, is transferred from a document ejection path  7  to a document return tray  8 . Here, in case there are images on both surfaces of the document d, the document d once transferred to the document return tray  8  is reversely sent to the document supply path  4  again. Then, the document d is reversed, and the sensor  6  reads an image on the surface opposite to the surface on which the image is read in the aforementioned process. 
     The image read by the sensor  6  as described above is sent as an analog signal to an image processing section  9 . After the image processing section  9 , which has received the image signal, carries out analog processing, analog-to-digital conversion, shading correction, image compression process and the like, the processed image signal is sent to an image writing section  10 . 
     In the image writing section  10 , which has received the processed image signal, the image signal as an output light from a semiconductor laser is irradiated to a photosensitive drum of an image forming section  11 , to thereby develop the image on the drum. 
     In the image forming section  11 , the image is transferred to the sheet S. The sheets S are supplied one by one by respective rollers from a cassette paper supply section  12  to a paper supply path  13 . The image forming section  11  described above is disposed in the paper supply path  13 , and the image forming section  11  transfers the image of the document d developed on the photosensitive drum to the sheet S passing through the paper supply path  13 . 
     The sheet S on which the image is transferred as described above is sent to a further downstream side of the paper supply path  13 . In the downstream side of the image forming section  11  in the paper supply path  13 , there is provided a fixing section  14  which fixes the image transferred in the image forming section  11  to the sheet S. When the image is fixed in the fixing section  14  as described above, the sheet S is sent further downstream, and ejected from a paper ejection port  2  through a paper ejection path  15 . The paper ejection port  2  is provided with rollers  17 , and the sheet S is ejected by the rollers  17 . 
     Incidentally, in case the image is formed on both sides of the sheet S, instead of sending the sheet S from the fixing section  14  to the paper ejection path  15 , the sheet S is sent from the fixing section  14  to a duplex path  16 . In the duplex path  16 , the sheet S sent to the duplex path  16  is placed such that a surface on which an image is not formed faces the image forming section  11  side, and the sheet S is sent to the paper supply path  13  again. Then, the sheet S in which the image is formed on both sides is ejected to the paper ejection port  2  through the paper ejection path  15  as in the one-side image forming. 
     The paper ejection port  2  is continuously connected to the sheet post-processing device FS, and the image-formed sheet S is sent from the paper ejection port  2  to the sheet post-processing device FS. Then, post-processing by the staple is carried out in the sheet post-processing device FS, and when the post-processing is finished, the sheets S are stored on a placing tray  18 . 
     The placing tray  18  is formed of a first placing section  18   a  located at substantially the same height as the paper ejection port  2 , and a second placing section  18   b  which is provided at an upper stream side than the first placing section  18   a  and located at a position lower than the first placing section  18   a . The first placing section  18   a  extends to the second placing section  18   b  through a slope  18   c.    
     Next, the sheet post-processing device FS will be explained in detail with reference to FIGS.  2  through  6 ( c ). FIG. 2 is an enlarged view of the sheet post-processing device FS shown in FIG.  1 . 
     Here, in order to specify directions of the sheet S, a side of the sheet S, which is parallel to a direction of transferring the sheet S, is defined as a transferring direction, and a side of the sheet S, which is orthogonal to the transferring direction, is defined as a width direction. Also, a case of using a staple unit is explained as a post-processing unit for the sheets S. 
     The sheet post-processing device FS includes a support plate  19  for supporting a rear end of the sheet S in a transferring direction; arms  20  which push the transferred sheet S onto the support plate  19 ; paddles  22  which make the rear end of the sheet S placed on the support plate  19  to abut against a regulating plate  21  to thereby align the rear end of the sheet S; alignment plates  23   a  and  23   b  for aligning the width direction of the sheet S; a staple unit  26  which carries out post-processing with respect to the aligned sheets S; and a cover  27  for covering these members. 
     In this first embodiment, the support plate  19  has a function of a release mechanism for releasing the sheet S, which will be explained later. Also, in the sheet post-processing device FS, a controller, not shown, is provided, and first, second, third and fourth motors M 1 , M 2 , M 3  and M 4 , described later, are controlled by the controller. 
     In the structure described above, steps after the sheet S is ejected from the image forming apparatus  1  and is sent to the sheet post-processing device FS, in which the sheets S are post-processed after several sheets S are stacked, until the processed sheets S are placed on the placing tray  18 , are explained in detail. 
     As shown in FIG. 2, the cover  27  is formed of a ceiling surface  27   a  and side surfaces  27   b  covering four sides thereof, and a bottom of the cover  27  is opened. Also, the cover  27  is provided with a guide section  27   c  which is parallel to the ceiling surface  27   a.    
     An inlet  28  for the sheet S is formed on the side surface  27   b  of the cover  27 , which contacts the image forming apparatus  1 , and the sheet post-processing device FS is assembled with the image forming apparatus  1  such that the inlet  28  is continuously connected to the paper ejection port  2  of the image forming apparatus  1 . Therefore, the sheet S ejected from the paper ejection port  2  of the image forming apparatus  1  is sent to the inlet  28  of the sheet post-processing device FS by the rollers  17 . 
     At first, a first sheet S is transferred to the sheet post-processing device FS as described above. An inlet sensor  29  is disposed at the inlet  28  of the sheet post-processing device FS, and checks that the sheet S is transferred into the sheet post-processing device FS. Then, the sheet S is entirely sent by the rollers  17 , and when the inlet sensor  29  checks the rear end of the sheet S, the inlet sensor  29  sends a signal to a solenoid  30 . 
     The solenoid  30  is disposed at the ceiling surface  27   a  of the cover, and as shown in FIG. 3, the solenoid  30  is connected to a rotating lever  32  through a solenoid shaft  31 . One side of the rotating lever  32  is fixed to a shaft  33 , and the arms  20  are fixed to the shaft  33 . In other words, the rotating lever  32  and the arms  20  are connected through the shaft  33 . 
     Also, as described above, the solenoid shaft  31  and the rotating lever  32  are connected to each other, and a connecting position thereof is a position close to a side opposite to a side to which the shaft  31  is fixed. One end of a spring  34  is fixed to the connection portion between the solenoid shaft  31  and the rotating lever  32 , and the other end of the spring  34  is fixed to the side surface  27   b  of the cover in the image forming apparatus  1  side. 
     In the structure described above, when a signal notifying that the rear end of the sheet S passes through the inlet is sent from the inlet sensor  29  to the solenoid  30 , the solenoid  30  is turned on, and the solenoid shaft  31  is extended. When the solenoid shaft  31  is extended, the rotating lever  32  is pushed down toward a downside in FIG. 3 while extending the spring  34 . When the rotating lever  32  is pushed down, the shaft  33  connected thereto is rotated in an arrow direction in FIG.  3 . When the shaft  33  is rotated, accordingly, the arms  20  are rotated in the arrow direction in FIG.  3 . 
     By the rotation of the arms  20  as described above, the rear end portion of the sheet S transferred to the sheet post-processing device FS is dropped downwardly in FIG.  3 . The rear end portion of the sheet S dropped downwardly is placed on the support plate  19 . 
     The support plate  19  is located below the arms  20  and at a position down from the inlet  28 , and the support plate  19  has a size for supporting only the rear end portion of the sheet S. Also, the support plate  19  is formed freely rotatably at a shaft  35  supported at the side surface  27   b  of the cover. A detailed method of rotating the support plate will be explained later. 
     As described above, by dropping the sheet S by the arms  20 , the rear end of the sheet S is surely placed on the support plate  19 . Since the support plate  19  has the size for supporting only the rear end of the sheet S as described above, if the arms  20  do not flaps off the sheet S, the sheet S transferred by the rollers  17  might pass through the support plate  19 . 
     As described above, only the rear end of the sheet S is placed on the support plate  19 , and a forward end portion of the sheet S passes through an inside of the sheet post-processing device FS to be supported by the first placing section  18   a  of the placing tray  18 . Namely, the sheet S is supported such that the sheet S extends over the support plate  19  and the first placing section  18   a.    
     When the arm  20  flaps off the rear end of the sheet S onto the support plate  19 , the extended spring  34  is returned to an initial position. When the spring  34  is returned to the initial position, the shaft  33  is rotated in a direction reverse to the arrow direction in FIG. 3, to thereby return the arms  20  to the initial position. 
     When the sheet S is placed on the support plate  19 , the paddles  22  shown in FIG. 2 are rotated to push back the sheet S in a direction reverse to the forwarding direction. By pushing back the sheet S, the rear end of the sheet S is allowed to abut against the regulating plate  21 , to thereby align the rear end of the sheet S. 
     Also, as shown in FIG. 4, the paddles  22  are connected to a shaft  39 , and the paddles  22  are rotated by the rotation of the shaft  39 . The shaft  39  is connected to the second motor M 2 , and the motor M 2  is disposed at the side surface  27   b  of the cover. 
     When the rear end of the first sheet S is aligned as described above, the second sheet S is transferred from the image forming apparatus  1 . The transferred second sheet S is disposed on the first sheet S placed on the support plate  19 , and a rear end of the second sheet S is aligned by the arms  20 . 
     As described above, a predetermined number of sheets S is placed on the support plate  19 . When the predetermined number of the sheets S is placed on the support plate  19  and the rear ends of the sheets S are aligned, subsequently, the alignment plates  23   a  and  23   b , which are provided at both sides in the width direction of the sheets S, align the width directions of the sheets S. 
     Namely, the alignment plates  23   a  and  23   b  are respectively formed of alignment sections  24   a  and  24   b  perpendicularly colliding with the side surfaces in the width direction of the sheets S, and rack-formed sections  25   a  and  25   b  which are orthogonal to the alignment sections  24   a  and  24   b  and disposed at upper portions of the alignment sections  24   a  and  24   b . Racks  40   a  and  40   b  are formed on side surfaces of the rack-formed sections  25   a  and  25   b , and pinions  41   a  and  41   b  to be engaged with the racks  40   a  and  40   b  are provided. The pinion  41   a  is rotated by the third motor M 3 , and the pinion  41   b  is rotated by the fourth motor M 4 . The third motor M 3  and the fourth motor M 4  are fixed to the ceiling surface  27   a  of the cover. 
     Also, slide holes  42  are respectively formed in the alignment sections  24   a  and  24   b , and the guide  27   c  is inserted into the slide holes  42 . 
     In this structure, after the paddles  22  align the rear ends of the sheets S, the fourth motor M 4  is rotated. When the fourth motor M 4  is rotated, the alignment plate  23   b  allows the sheets S to abut against the alignment plate  23   a , to thereby align the width direction of the sheets S. 
     Movements of the alignment plate  23   b  at this time are shown in FIGS.  5 ( a ) to  5 ( d ). FIG.  5 ( a ) shows an initial state before the alignment plate  23   b  is moved. From this state, when the fourth motor M 4  is rotated so that the pinion  41   b  is rotated, the alignment plate  23   b  is moved in a direction toward the alignment plate  23   a , that is, in a leftward direction in FIG.  5 ( a ). When the alignment plate  23   b  is moved, the alignment plate  23   b  abuts against the sheets S as shown in FIG.  5 ( b ). The alignment plate  23   b  which abuts against the sheets S is further moved toward the left in the figure, and pushes the sheets S against the alignment plate  23   a . By pushing the sheets S against the alignment plate  23   a , the width directions of the sheets S are aligned as shown in FIG.  5 ( c ). 
     Also, at this time, while the width direction of the sheets S is aligned, the rear end portions of the sheets S are inserted into the processing section  26   a  of the staple unit  26  as shown in FIG.  5 ( c ). 
     Although not shown in the figures, the staple unit  26  includes staples and staple driving means for driving the staples, and the staple is driven to the sheets S at the processing section  26   a . Therefore, as described above, the staple driving means, not shown, drives the staple to the sheets S guided to the processing section  26   a  by the alignment plates  23   a  and  23   b , to thereby carry out the post-processing. 
     At this time, the position where the staple is driven is the rear ends of the sheets S, and the rear ends of the sheets are supported by the support plate  19 . Since the rear ends to be stapled are supported by the support plate  19 , a stability in stapling can be maintained as compared with a case of supporting the other portions of the sheets. Namely, without displacement of the sheet S, the staple can be surely provided to the sheets S. 
     When the sheets S are post-processed as described above, the fourth motor M 4  is rotated reversely to the rotation at the time of aligning the sheets S, such that the alignment plate  23   b  is moved toward a right side as shown in FIG.  5 ( d ). Concurrently with the movement of the alignment plate  23   b  toward the right side, the third motor M 3  is rotated in the same direction as that of the fourth motor M 4 , to thereby move the alignment plate  23   a  toward the right side. By moving the alignment plates  23   a  and  23   b  toward the right side in FIG.  5 ( d ), the sheets S are moved to the right side, so that the post-processed sheets S are disengaged from the processing section  26   a  of the staple unit  26 . When the post-processed sheets S are completely disengaged from the processing section  26   a , the first motor M 2  shown in FIG. 4 is rotated in the direction of an arrow A. 
     The first motor M 1  is provided with a pinion gear  38 , and it is structured that a gear  37  of a fan-shaped member  36  is engaged with the pinion gear  38 . Namely, the gear  37  is formed at an arc portion of the fan-shaped member  36 , and the gear  37  is engaged with the pinion gear  38 . When the first motor M 1  is rotated in the direction of the arrow A in FIG. 4, the pinion gear  38  is also rotated in the direction of the arrow A. Then, by engaging the pinion gear  38 , the fan-shaped member  36  is rotated in a direction of an arrow B in FIG.  4 . When the fan-shaped member  36  is rotated, the shaft  35  and the support plate  19  are integrally rotated. Incidentally, the first motor M 1  is fixed to the side surface  27   b  of the cover. 
     When the support plate  19  is rotated as described above, the sheets S supported by the support plate  19  are dropped onto the placing tray  18 . Namely, the support plate  19  is rotated from the initial state in which the rear ends of the sheets S are supported by the support plate  19  as shown in FIG.  6 ( a ), and the sheets S placed on the support plate  19  are dropped down as shown in FIG.  6 ( b ). The dropped sheets S are placed on the placing tray  18  as shown in FIG.  6 ( c ). At this time, the rear end portions of the post-processed sheets S are placed on the second placing section  18   b  of the placing tray  18 , and the forward end portions of the sheets S are placed on the first placing section  18   a  as shown in FIG.  6 ( c ). 
     When the support plate  19  is rotated such that the sheets S are placed on the placing tray  18  as described above, the first motor M 1  is rotated reversely to the direction of the arrow A in FIG.  4 . In accordance therewith, the fan-shaped member  36  is rotated reversely to the direction of the arrow B, and the support plate  19  is returned to the initial position. 
     As described above, while the support plate  19  supports the transferred sheets S, the support plate  19  has a releasing function for dropping and releasing the post-processed sheets S. 
     According to the first embodiment described above, by rotating the support plate  19 , the post-processed sheets S can be dropped right under the support plate  19 . Furthermore, since the placing tray  18  is provided under the support plate  19 , by merely rotating the support plate  19 , the sheets S can be placed on the placing tray  18 . Therefore, it is not necessary to provide the placing tray  18  outside the image forming apparatus  1 , so that a floor space for installation can be reduced. 
     Also, since the support plate  19  supports only the rear end portions of the sheets S transferred from the image forming apparatus  1  such that the forward end portions of the sheets are supported by the placing tray  18 , a size of the support plate  19  in the transferring direction can be reduced. By reducing the size of the support plate  19 , a space for rotating the same can be small. Namely, the entire sheet post-processing device FS can be made compact. Therefore, the compact sheet post-processing device FS can be easily assembled with the image forming apparatus, and there is no such a problem that the image forming apparatus  1  becomes tall. 
     Further, in the known image forming apparatus, even if the image reading device Y and the automatic document feeder DF are disposed above the image forming apparatus  1 , the sheet post-processing device FS can be installed in a space between the image forming apparatus  1  and the image reading device Y. 
     Incidentally, although the staple unit is adopted as the sheet post-processing unit in this embodiment, it is needless to say that other post-processing unit, such as a punching, can be used. Also, although the first motor M 1  is used for rotating the support plate  19  in the embodiment, other driving device, such as a solenoid, can be used instead. Further, though the solenoid is used for rotating the arm  20 , other driving device can be used instead. 
     Incidentally, the image forming apparatus  1  has a post-processing execution mode, in which stapling or punching is carried out to each set of a predetermined number of sheets by using the sheet post-processing device to provide a required number of post-processed sets of the sheets, and a straight ejection mode, in which the sheets are directly stacked and placed onto the placing tray  18  without carrying out the post-processing described above. The device of the embodiment can be easily adapted to both of these modes. 
     Namely, when an instruction of carrying out the post-processing with respect to the ejection sheet is issued, as explained above, a predetermined number of the sheets is supported by the support plate  19 . This state of the support plate  19  constitutes a support state, which is shown in FIG.  6 ( a ). Then, after the predetermined number of the sheets is supported by the support plate  19  and the post-processing is carried out, the support plate  19  becomes a release state in which the sheets S are dropped and released on the placing tray  18  as shown in FIG.  6 ( b ). 
     On the other hand, in case an instruction of carrying out the straight ejection mode is issued, as shown in FIG.  6 ( b ), the support plate  19  is held at the position for allowing the sheets S to be dropped and released without supporting the sheets S. Namely, the support plate  19  is in the release state shown in FIG.  6 ( b ) from the beginning without taking the support state shown in FIG.  6 ( a ). 
     Incidentally, the initial position or state of the support plate  19  before setting of the respective modes can be either the support state or the release state. When the support plate  19  is in the support state as the initial state, after setting of carrying out the straight ejection mode, the support plate  19  can be moved to the position of FIG.  6 ( b ) as the release state. On the contrary, when the initial state of the support plate  19  is set at the position of FIG.  6 ( b ) as the release state, after setting the post-processing execution mode, the support plate  19  can be moved to the support state in which the sheets are supported. By structuring the device as described above, the device can be easily adapted to any of the post-processing execution mode or the straight ejection mode. 
     FIG. 7 shows a second embodiment of the invention, wherein means for releasing the post-processed sheets and the alignment plates are integrally formed. The constituents other than this character are the same as in the first embodiment, so that the same references as those in the first embodiment are used to designate the constituents, to thereby omit the detailed explanations therefor. 
     In the second embodiment, the alignment sections  44   a  and  44   b  and rack-formed sections  45   a  and  45   b  are respectively formed in the alignment plates  43   a  and  43   b , and the alignment plates  43   a  and  43   b  are further provided with support sections  46   a  and  46   b . The rack-formed sections  45   a  and  45   b  are disposed respectively at upper portions of the alignment sections  44   a  and  44   b , and the support sections  46   a  and  46   b  are disposed respectively at lower portions of the alignment sections  44   a  and  44   b  such that the alignment plates  43   a  and  43   b  have U-shaped forms. 
     In addition, racks  47   a  and  47   b  are formed in the rack-formed sections  45   a  and  45   b , such that racks  47   a  and  47   b  engage the pinions  41   a  and  41   b . The pinion  41   a  is rotated by the third motor M 3 , and the pinion  41   b  is rotated by the fourth motor M 4 . 
     In the second embodiment structured as described above, the first sheet S is ejected from the paper ejection port  2  of the image forming apparatus  1 , and the sheet S is sent from the inlet  28  of the sheet post-processing device FS into the sheet post-processing device FS. The sheet S sent into the sheet post-processing device FS is dropped off by the arms  20 , so that the rear end of the sheet S is placed on the support sections  46   a  and  46   b . At this time, the forward end of the sheet S is placed on the first placing section  18   a  of the placing tray  18 . When the sheet S is placed on the support sections  46   a  and  46   b  as described above, the paddles  22  push the rear end of the sheet S against the regulating plate  21 , to thereby align the rear end of the sheet S. 
     Then, the second sheet S and the third sheet S are successively transferred in order from the image forming apparatus  1 , and as in the first sheet S, these sheets are respectively placed on the support sections  46   a  and  46   b  to thereby align the rear ends thereof. 
     When a predetermined number of sheets S is placed on the support sections  46   a  and  46   b  as described above, the fourth motor M 4  is rotated, and the alignment plate  43   b  is moved in a direction toward the alignment plate  43   a . In accordance with the movement of the alignment plate  43   b , the sheets S supported by the support section  46   b  of the alignment plate  43   b  are moved. As described above, the alignment plate  43   b  and the sheets S are moved in the direction toward the alignment plate  43   a , and the sheets S are aligned by the alignment section  44   a  of the alignment plate  43   a  and the alignment section  44   b  of the alignment plate  43   b.    
     When the sheets S are aligned by allowing the sheets S to abut against the alignment plate  43   a , the sheets S are inserted into the processing section  26   a  of the staple unit  26 . When the sheets S are inserted into the processing section  26  as described above, the sheets S are stapled by the staple unit  26 . When the sheets S are post-processed by stapling, the third motor M 3  and the fourth motor M 4  are rotated, so that the alignment plates  43   a  and  43   b  are simultaneously moved in a direction toward a right lower side in FIG.  7 . Thus, the post-processed sheets S are disengaged from the processing section  26   a.    
     When the post-processed sheets S are completely disengaged from the processing section  26   a , only the fourth motor M 4  is rotated, and the alignment plate  43   b  is further moved in the direction toward the right lower side in FIG.  7 . By moving only the alignment plate  43   b  as described above, the sheets S supported by the support sections  46   a  and  46   b  are disengaged from the support sections  46   a  and  46   b . The sheets S disengaged from the support sections  46   a  and  46   b  are placed on the placing tray  18  disposed below the sheet post-processing device FS. 
     As described above, while the support sections  46   a  and  46   b  support the transferred sheets S, the support sections  46   a  and  46   b  have a releasing mechanism for releasing the post-processed sheets S. 
     According to the second embodiment described above, by merely increasing a distance between the alignment plate  43   a  and the alignment plate  43   b , the sheets S in which the post-processing is completed can be released onto the placing tray  18 . Namely, since there is no need to drop the sheets S by rotating the support sections  46   a  and  46   b , a space for rotating the support sections  46   a  and  46   b  is not required. Therefore, the sheet post-processing device FS can be made much smaller. 
     This compact sheet post-processing device FS can be assembled with the known image forming apparatus which is not provided with a large space for installing the sheet post-processing device therein. 
     Incidentally, although the sheets S are pushed from one direction, that is, from the alignment plate  43   b  so as not to move the alignment plate  43   a  in case of aligning the sheets S by the alignment plates  43   a  and  43   b  in the second embodiment, it can be arranged such that both the alignment plates  43   a  and  43   b  are moved to align the sheets S. Namely, in case of aligning the sheets S, the alignment plate  43   a  is moved in a direction toward the alignment plate  43   b , and the alignment plate  43   b  is moved in a direction toward the alignment plate  43   a , such that the alignment plates may be moved from both directions. In this case, after the sheets S are aligned, while the condition of aligning the sheets S is maintained, the alignment plates  43   a  and  43   b  are moved in the direction toward the staple unit  26 , so that the sheets S are inserted into the processing section  26   a.    
     Also, when the post-processed sheets S are released from the alignment plates  43   a  and  43   b , only the alignment plate  43   b  is moved away from the alignment plate  43   a  and the alignment plate  43   a  does not move. However, it can be structured that the alignment plate  43   a  is also moved. Namely, both the alignment plate  43   a  and alignment plate  43   b  can be moved away from each other, so as to release the post-processed sheets S. 
     FIGS. 8 and 9 show a third embodiment, wherein the support plate for supporting the sheets constitutes the means for releasing the post-processed sheets, and the support plate is extended and contracted. The structures other than this are the same as in the first embodiment. The constituents which are the same as those in the first embodiment are designated by the same references, and detailed explanations therefor are omitted herewith. 
     In the third embodiment, the sheet post-processing device FS is provided with a support plate  48  for supporting the sheets S transferred from the image forming apparatus  1 . The support plate  48  is formed of a base  48   a  and an expanding and contracting section  48   b , and the base  48   a  is rotated by the rotation of the shaft  35 . 
     The expanding and contracting section  48   b  has a cylindrical shape including a hollow inside, and a hollow portion thereof is provided with a spring  49 . Also, the base  48   a  is inserted into the cylindrical expanding and contracting section  48   b , so that the expanding and contracting section  48   b  is movable along the base  48   a  through the spring  49 . Further, a projection  50  is formed in the expanding and contracting section  48   b , and when the projection  50  is moved, the expanding and contracting section  48   b  is accordingly moved along the base  48   a  while contracting the spring  49 . The support plate  48  shown in FIGS. 8 and 9 is in the initial state, in which the expanding and contracting section  48   b  is extended to the maximum. 
     Also, in the third embodiment, below the shaft  35 , a cam  51  is formed at the side surface  27   b  of the cover  27  of the sheet post-processing device FS at the image forming apparatus  1  side. Thus, when the support plate  48  is rotated, the projection  50  formed at the expanding and contracting section  48   b  is moved along an outline curve  51   a  formed at the cam  51 . 
     When the support plate  48  is rotated from the initial state, the projection  50  is moved along the outline curve  51   a  in accordance with the rotation, such that the projection  50  is moved to get closer to the shaft  35 . When the projection  50  is moved to get closer to the shaft  35 , the expanding and contracting section  48   b  is also moved to get closer to the shaft  35 . Namely, while the expanding and contracting section  48   b  contracts the spring  49 , the expanding and contracting section  48   b  moves such that an entire length of the support plate  48  is shortened. Then, when the support plate  48  is rotated for approximately 90 degrees from the initial state, the support plate  48  has the shortest length. 
     Also, when the support plate  48  is in the initial state, in order to prevent the projection  50  from contacting the alignment plate  23   b , a portion of the alignment plate  23   b , which is located at a position corresponding to the projection  50 , is notched to form a notched portion  52 . 
     Further, in this embodiment, a first placing section  53   a  of a placing tray  53  is formed of a member which is separated from a second placing section  53   b , and by contracting a spring  54 , the first placing section  53   a  is lowered. The first placing section  53   a  is disposed to be rotatable around a shaft  55 . 
     In this structure, when the sheet S is transferred to the sheet post-processing device FS from the image forming apparatus  1 , the sheet S is placed onto the support plate  48  by the arms  20 . When a predetermined number of the sheets S is placed on the support plate  48 , the rear ends of the sheets S in the transferring direction are aligned by the paddles  22 , and the width direction of the sheets S is aligned by the alignment plates  23   a  and  23   b . Then, the aligned sheets S are stapled by the staple unit  26 . When the sheets S are stapled, the first motor M 1  is rotated to rotate the shaft  35 , resulting in rotating the support plate  48 . 
     When the support plate  48  is rotated and moved in an arrow direction in FIG. 9, the projection  50  is located at a distal end portion of the outline curve  51   a  of the cam  51 . When the support plate  48  is further rotated, the projection  50  is moved along the outline curve  51   a . When the projection  50  is moved along the outline curve  51   a  as described above, a distance between the projection  50  and the shaft  35  is shortened. Namely, the spring  49  is contracted, so that the entire length of the support plate  48  is shortened. 
     By rotating the support plate  48  in the arrow direction in FIG. 9 as described above, the sheets S placed on the support plate  48  are dropped onto the placing tray  53 . At this time, the rear ends of the sheets S are placed on the second placing section  53   b , and the forward ends of the sheets S are placed on the first placing section  53   a . After the support plate  48  allows the sheets S to drop onto the placing tray  53 , the support plate  48  is rotated reversely to the arrow direction in FIG. 9 to be returned to the initial state. At this time, since the projection  50  is moved away from the shaft  35 , the spring  49  is extended, so that the entire length of the support plate  48  is elongated. 
     According to the third embodiment, since the support plate  48  can be kept elongated in the initial state before the support plate  48  is rotated, the support plate  48  can securely support the sheets S. Also, since the sheets S can be placed on the placing tray  53  by merely rotating the support plate  48  from the initial state, it is not necessary to specially provide the placing tray outside the side surface of the image forming apparatus  1 , so that the installation area can be reduced. 
     Also, in the support plate  48 , as the support plate  48  is rotated, the length thereof is shortened. Thus, a space required for rotating the support plate  48  can be small. Accordingly, the sheet post-processing device FS can be made much more compact. 
     Further, as described above, since the support plate  48  is shortened in accordance with the rotation thereof, even if a large number of sheets S is placed on the placing tray  53 , the rotated support plate  48  does not contact the sheets S. Therefore, much more sheets S can be placed on the placing tray  53 . 
     In addition, since the spring  54  is disposed under the first placing section  53   a  of the placing tray  53 , when the sheets S are placed on the first placing tray  53   a , the spring  54  is contracted due to the weight of the placed sheets S. When the spring  54  is contracted, the first placing section  53   a  is rotated around the shaft  55 . As described above, in accordance with an amount of the sheets S placed on the first placing section  53   a , the first placing section  53   a  contracts the spring, so that the position of the first placing section  53   a  can be lowered. 
     By lowering the position of the first placing section  53   a  in accordance with the amount of the sheets S, even if the amount of placing the sheets S is increased, the rotation of the paddles  22  or the like is not prevented. Therefore, much more sheets S can be placed on the placing tray  53 . 
     FIG. 10 shows a fourth embodiment, wherein the placing tray is integrally formed with the cover of the sheet post-processing device. Structures other than that are the same as those in the first embodiment. The constituents which are the same as those in the first embodiment are designated by the same references as in the first embodiment, so that the detailed explanations thereof are omitted herewith. 
     In the fourth embodiment, a placing tray section  57  is formed at a cover  56  of the sheet post-processing device FS. The placing tray section  57  includes a first placing section  57   a  and a second placing section  57   b . The first placing section  57   a  is located at a position higher than that of the second placing section  57   b , and the first placing section  57   a  and the second placing section  57   b  are connected through an inclined section  57   c . It is desirable that the position of the first placing section  57   a  is at substantially the same height as that of the paper ejection port  2 . 
     Also, an end portion of the second placing section  57   b , which is opposite to the inclined section  57   c , is connected to a side surface  56   a  of the cover  56 . The second placing section  57   b  is located below the support plate  19 , and positioned to have a distance from the support plate  19  such that the support plate  19  does not collide with the second placing section  57   b  even if the support plate  19  is rotated. 
     If the placing tray does not have a raised portion, such as the first placing section  57   a , the sheet S ejected from the paper ejection port  2  is liable to be dropped from the support plate  19  due to its own weight. Namely, since the sheet is not placed on the support plate  19 , the post-processing by the staple unit  26  is not carried out with respect to the sheet. Also, even if the sheet is placed on the support plate  19 , the forward end portion of the sheet S in the transferring direction becomes heavy, so that the sheet S is not aligned neatly. 
     However, in the fourth embodiment of the invention, since the first placing section  57   a  and the second placing section  57   b  are provided in the sheet post-processing section FS, even in a image forming apparatus which does not have a first placing section, the beautiful post-processing of the sheet is made. 
     Further, according to the fourth embodiment, even in the image forming apparatus in which there is no slope in the placing tray, without providing a placing tray separately, the compact sheet post-processing device FS can be assembled therewith. Therefore, the entire image forming apparatus can be made compact. 
     As described above, according to the present invention, in the sheet post-processing device, supporting means for supporting the rear ends of the sheets to be post-processed is moved to a position of releasing the sheets in the dropping direction, and the sheet post-processing device includes means for shifting the sheets from the post-processing means. Thus, the sheet post-processing device can be made compact. 
     Further, according to the present invention, since the means for aligning the sheets supported on the supporting means is provided, post-processing in the state that the side edges of the sheets are aligned can be carried out. Also, since the shift means for moving the sheets may be also used as the aligning means, the structure can be simplified. 
     Further, since the supporting means is formed of freely rotatable supporting means, by merely rotating the supporting means, the sheets can be placed on the placing tray. Also, since the supporting means supports only the rear ends of the sheets, the size of the supporting means can be small, and it is not necessary to have a large space for rotating the supporting means. Therefore, the sheet post-processing device can be made much more compact. 
     In addition, since the supporting means may be capable of expanding and contracting freely, in accordance with the rotation of the supporting means, the supporting means can be contracted. Therefore, a, space for rotating the supporting means can be further reduced, and the entire sheet post-processing device can be made smaller. Even if the sheet post-processing device is assembled with the image forming apparatus, a height of the image forming apparatus as a whole is not increased. Since the height of the entire image forming apparatus is not increased, the image forming apparatus is used easily. 
     Also, an upper surface of the apparatus may constitute the placing tray, and the sheet post-processing device made into a unit may be provided between the placing tray and the ejection port of the image forming apparatus of a type including the sheet projection port projecting further above the placing tray. Thus, the sheet post-processing device can be assembled with the image forming apparatus without increasing an area for installing the image forming apparatus and a height thereof. 
     Also, the placing tray for placing the sheets is formed of a first placing section for supporting the forward end side in the transferring direction of the sheets supported by the supporting means, and a second placing section for supporting the rear end portions in the transferring direction of the sheets when the sheets are dropped, and the second placing section is set at a position lower than that of the first placing section. Therefore, before the sheets are dropped, the sheets can be securely supported by the supporting means and the first placing section, and when the sheets are going to drop, the sheets can be surely released from the supporting means. 
     Further, according to the present invention, the compact sheet post-processing device is provided between the image forming apparatus and the image reading device. Accordingly, it is not necessary to provide the sheet post-processing device outside the image forming apparatus, so that the image forming apparatus can be made smaller, and the installation area thereof can be reduced. 
     While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.