Abstract:
An electrophotographic image forming apparatus for forming an image on a recording material, the electrophotographic image forming apparatus includes a drum cartridge including an electrophotographic photosensitive member drum; a developing cartridge including a developing roller for developing an electrostatic latent image formed on the electrophotographic photosensitive drum using a developer; a supporting member movable between an inside position and a retracted position in the state that supporting member supports the drum cartridge and the developing cartridge, wherein the inside position is inside the main assembly of the apparatus, and the retracted position is retracted from the main assembly of the apparatus; wherein the supporting member supports the drum cartridge and the developing cartridge independently demountably therefrom, wherein mounting and demounting directions of the drum cartridge relative to the supporting member and mounting and demounting directions of the developing cartridge are different from each other.

Description:
This is a divisional of co-pending U.S. patent application Ser. No. 12/536,908, filed Aug. 6, 2009. 
    
    
     FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an electrophotographic image forming apparatus, which is structured so that a drum cartridge and a development cartridge are removably mountable in the main assembly of the image forming apparatus, and also, so that it forms an image on recording medium while the drum cartridge and development cartridge remain in the main assembly of the image forming apparatus. Here, an electrophotographic image forming apparatus means an apparatus which forms a color image on recording medium, with the use of an electrophotographic image formation process. As examples of an electrophotographic image forming apparatus, an electrophotographic copying machine, an electrophotographic printer (for example, color laser beam printer, color LED printer, etc.), a facsimile apparatus, a word processor, etc., can be included. 
     There have been known electrophotographic color image forming apparatuses (which hereafter will be referred to simply as image forming apparatuses) which form a color image on recording medium. In the case of a conventional electrophotographic color image forming apparatus, two or more electrophotographic photosensitive drums (which hereafter will be referred to as photosensitive drums) are disposed in parallel, and two or more development rollers are disposed so that they oppose the photosensitive drums, one for one, and also, so that each of the electrostatic latent images formed on the photosensitive drums, one for one, is developed with a developer which is different in color from the developer used for developing the other electrostatic latent images. Incidentally, disposing two or more photosensitive drums in parallel is generally referred to as a tandem arrangement. 
     There have been known structural arrangements which allow multiple development cartridges having a development roller, to be removably mounted in the main assembly of an image forming apparatus of the tandem type, in such a manner that the development rollers oppose the photosensitive drums one for one (disclosed in U.S. Patent Application 0147881/2007, for example). 
     However, the multiple photosensitive drums become different in the length of their service lives, because the frequency with which each of the developers different in color is used is different from the frequency with which the other developers are used, and also, the amount by which each developer is used is different from the amount by which the other developers are used. Thus, it is desired that an electrophotographic color image forming apparatus is structured so that each photosensitive drum can be independently replaced from the other photosensitive drums. 
     SUMMARY OF THE INVENTION 
     Thus, the primary object of the present invention is to provide an electrophotographic image forming apparatus structured so that each of the drum cartridges and development cartridges for the image forming apparatus can be independently mounted onto, or removed from, the drum cartridge supporting member and development cartridge supporting member of the apparatus, respectively, independently from the other drum cartridges and development cartridges. 
     Here, the drum cartridge mentioned above is a cartridge having an electrophotographic photosensitive drum. The development cartridge mentioned above is a cartridge having a development roller for developing an electrostatic latent image formed on the corresponding electrophotographic photosensitive drum, with the use of developer. Further, the supporting member is a member of the main assembly of the image forming apparatus, which moves between its innermost position in the main assembly of the electrophotographic image forming apparatus, and its outermost position, or its outside position, in which the supporting member is when it is outside the main assembly  1 . 
     Another object of the present invention is to provide an electrophotographic image forming apparatus which is superior to a conventional electrophotographic image forming apparatus, in terms of the operational efficiency with which the drum cartridges and development cartridges can be replaced. 
     Another object of the present invention is to provide an electrophotographic image forming apparatus structured so that the direction in which the drum cartridges are mounted onto, or removed from, the supporting member is different from the direction in which the development cartridges are mounted into, or removed from, the supporting member. 
     According to an aspect of the present invention, there is provided a An electrophotographic image forming apparatus for forming an image on a recording material, said electrophotographic image forming apparatus comprising a drum cartridge including an electrophotographic photosensitive member drum; a developing cartridge including a developing roller for developing an electrostatic latent image formed on said electrophotographic photosensitive drum using a developer; a supporting member movable between an inside position and a retracted position in the state that supporting member supports said drum cartridge and said developing cartridge, wherein the inside position is inside said main assembly of said apparatus, and the retracted position is retracted from said main assembly of said apparatus; wherein said supporting member supports said drum cartridge and said developing cartridge independently demountably therefrom, wherein mounting and demounting directions of said drum cartridge relative to said supporting member and mounting and demounting directions of said developing cartridge are different from each other. 
     As described above, according to the present invention, each of the drum cartridges and development cartridges can be removed from the supporting member, independently from the others. Further, according to the present invention, each of the drum cartridges and development cartridges can be attached to the supporting member, independently from the others. 
     Further, the present invention improved the operational efficiency with which the drum cartridges and development cartridges are replaced. 
     Further, the present invention can make the direction in which the drum cartridges are mounted onto, or removed from, their supporting member, different from the direction in which the development cartridges are mounted onto, or removed from, the supporting member. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of the image forming apparatus in the first preferred embodiment of the present invention, and shows the general structure of the apparatus. 
         FIG. 2  is a cross-sectional view of the image forming portion of the image forming apparatus in  FIG. 1 , and shows the structure of the image forming portion. 
         FIG. 3  is perspective views of one of the drum cartridges, and one of the development cartridges, respectively, in the first preferred embodiment. 
         FIG. 4  is a sectional view of the image forming apparatus in the first preferred embodiment, the supporting member of which is in its outermost position relative to the main assembly of the apparatus. 
         FIG. 5  is a perspective view of the supporting member in the first embodiment. 
         FIG. 6  is a perspective view of the supporting member, and the cartridges on the supporting member, in the first preferred embodiment, and shows the relationship between the supporting member and the cartridges. 
         FIG. 7  is a schematic drawing for describing the positional relationship between one of the drum cartridges, and the corresponding development cartridge, in the first preferred embodiment. 
         FIG. 8(   a ),  FIG. 8(   b ) and  FIG. 8(   c ) are views illustrating the mechanism for placing the development cartridges in contact with the corresponding photosensitive drums, or separating the development cartridges from the corresponding photosensitive drums, in the first preferred embodiment. 
         FIG. 9  is a sectional view of the image forming apparatus in the second preferred embodiment of the present invention, and shows the general structure of the apparatus. 
         FIG. 10  is a cross-sectional view of the image forming portion of the image forming apparatus in  FIG. 9 , and shows the structure of the image forming portion. 
         FIG. 11  is perspective views of one of the development cartridges, and one of the drum cartridges, respectively, in the second preferred embodiment. 
         FIG. 12  is a sectional view of the image forming apparatus in the second preferred embodiment, the supporting member of which is in its outermost position relative to the main assembly of the apparatus. 
         FIG. 13  is a perspective view of the supporting member in the second preferred embodiment. 
         FIG. 14  is a perspective view of the mechanism for placing the development cartridges in contact with the corresponding photosensitive drums, or separating the development cartridges from the corresponding photosensitive drum, in the second preferred embodiment. 
         FIG. 15  is a perspective view of the supporting member, and the cartridges on the supporting member, in the second preferred embodiment, and shows the positional relationship between the supporting member and the cartridges. 
         FIG. 16  is a schematic drawing for describing the positional relationship between one of the drum cartridges, and the corresponding development cartridge, in the second preferred embodiment. 
         FIG. 17(   a ),  FIG. 17(   b ) and  FIG. 17(   c ) illustrate the mechanism for placing the development cartridges in contact with the corresponding photosensitive drums, or separating the development cartridges from the corresponding photosensitive drums. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the appended drawings. It should be noted here that unless specifically noted, the materials and shapes of the structural components of any of the image forming apparatuses in the following preferred embodiments of the present invention, and the positional relationship among the structural components, are not intended to limit the present invention in scope. 
     (Embodiment 1) 
     First, referring to  FIGS. 1-8 , the electrophotographic color image forming apparatus (which hereafter will be referred to simply as image forming apparatus) in the first preferred embodiment of the present invention will be described. 
     &lt;Image Forming Apparatus&gt; 
     Referring to  FIG. 1 , the overall structure of the image forming apparatus in the first preferred embodiment of the present invention will be described.  FIG. 1  is a sectional view of the image forming apparatus in the first preferred embodiment of the present invention, and shows the general structure of the apparatus. 
     The main assembly  1  of the image forming apparatus  100  is provided with image forming portions  3 K,  3 Y,  3 M, and  3 C, which form black, yellow, magenta, and cyan images, respectively. The image forming portions are disposed in parallel. Hereafter, the suffix of each of the referential codes  3 K,  3 Y,  3 M, and  3 C, which indicates the color of the images formed by the image forming portions, may be left out; each of the image forming portions  3 K,  3 Y,  3 M, and  3 C may be referred to simply as “image forming portion  3 ”. So may be the suffix of each of the referential codes of the various components of the image forming apparatus. 
       FIG. 1  shows the state of the image forming apparatus when the tray unit  33  (supporting member), which will be described later, is in its preset innermost position, and the drum cartridges  31  and development cartridge  32  are ready for image formation. The main assembly  1  is what remains after the drum cartridges  31  and development cartridges  31  are removed from the image forming apparatus  100 . 
     The image forming portions  3 K,  3 Y,  3 M, and  3 C are provided with electrophotographic photosensitive drums  311 K,  311 Y,  311 M, and  311 C, which are for bearing black, yellow, magenta, and cyan images, respectively. 
     The main assembly  1  is also provided with a laser unit  2 , which is above these image forming portions  3 . The laser unit  2  projects beams of laser light LK, LY, LM, and LC upon the photosensitive drums  311 K,  311 Y,  311 M, and  311 C with which the image forming portions  3 K,  3 Y,  3 M, and  3 C are provided, respectively. As the beam of laser light is projected upon the photosensitive drum  311 , an electrostatic latent image is formed, which reflects the information of the image to be formed. Hereafter, the suffixial letters K, Y, M, and C, which indicate the color of the laser beam, may be left out; each of the four beams of laser light may be referred to simply as beam L of laser light. 
     Further, the main assembly  1  is provided with a transfer unit  4 , which is below the combination of the four image forming portions  3 . The transfer unit  4  transfers onto a sheet  61  of recording medium, an image formed of developer on photosensitive drum  311 . Here, recording medium is medium on which an image is formed with the use of an electrophotographic image formation process. As the concrete examples of the sheet  61  of recording medium, a sheet of paper, an OHP sheet, a piece of fabric, etc., can be listed. 
     The main assembly  1  is also provided with a recover unit  5 , which is located below the combination of the image forming portions  3 . The recover unit  5  recovers the developer t, which remained adhered to a transfer belt  41  of the unit  4  after the transfer of the image formed of developer. Further, the main assembly  1  is provided with a cassette  6 , which is below the unit  5 . The cassette  6  is where multiple sheets  61  of recording medium are stored in layers. 
     Further, the main assembly  1  is provided with a fixation unit  7  for fixing the developer image to the sheet  61  after the transfer of the developer image onto the sheet  61 . The fixation unit  7  is on the left side (in drawing) of the combination of the image forming portions  3 . Further, the main assembly  1  is provided with a discharging portion  8  for discharging the sheet  61  out of the main assembly  1  after the fixation of the developer image to the sheet  61 . The discharging portion  8  is above the unit  7 . 
     Further, the main assembly  1  is provided with a conveying portion  6 , which is on the right side (in drawing) of the unit  4 . 
     &lt;Conveying Portion&gt; 
     Next, referring to  FIG. 1 , the conveying portion  6  will be described in more detail. The conveying portion  6  is for conveying the sheet  61  to the unit  4 . It has a feed roller  63 , a pair of conveyance rollers  64 , a pair of registration rollers  65 , etc., in addition to the abovementioned cassette  61 . 
     The roller  63  feeds the sheets  61  in the cassette  62 , into the main assembly  1 , one by one, by rotating as an image forming operation continues. After being fed out into the main assembly  1  by the roller  63 , each sheet  61  is conveyed by the pair of rollers  64  to the pair of roller  65 , which is located further downstream. 
     The moment the leading edge of the sheet  61  arrives at the nip between the pair of rollers  65 , the pair of rollers  65  is stationary. Thus, if the sheet  61  happens to be askew as it arrives at the nip, it is straightened by the pair of the rollers  65  (nip), which is remaining stationary. Thereafter, the rotation of the pair of rollers  65  is started with preset timing so that the developer image can transferred onto the sheet  61 , across the preset portion of the sheet  61 . Thus, the sheet  61  is conveyed to the transfer unit  4 . 
     &lt;Image Forming Portion&gt; 
     Next, referring to  FIG. 2 , the image forming portion  3  will be described in more detail.  FIG. 2  is a sectional view of one of the image forming portions  3  of the image forming apparatus  100  in this embodiment, and shows the structure of the image forming portion  3 . 
     As described above, the image forming apparatus  100  is provided with the four image forming portions  3  ( 3 K,  3 Y,  3 M, and  3 C) which form black, yellow, magenta and cyan images, respectively. The four image forming portions  3  are the same in basic structure, although they are different in the color of the developer t they use. Thus,  FIG. 2  shows the image forming portion  3 K, which forms a black image, as the image forming portion which represents the four image forming portions  3 . 
     The image forming portion  3  is provided with a charge roller  312  ( 312 K,  312 Y,  312 M, or  312 C), as a charging means (processing means), in addition to the photosensitive drum  311 . Hereafter, the suffixial letters K, Y, M, and C, which indicate the color with which the four charge rollers  312 K,  312 Y,  312 M, and  312 C are associated may be left out; each of the charge rollers may be referred to simply as a charge roller  312 . The same holds true in the case of the components of the image forming apparatus  100 , other than the abovementioned components. 
     The four image forming portions  3 K,  3 Y,  3 M, and  3 C are structured so that the development cartridges  32 K,  32 Y,  32 M, and  32 C having the development rollers  321 K,  321 Y,  321 M, and  321 C, which are developing means (processing means), are removably attachable in the image forming portions  3 K,  3 Y,  3 M, and  3 C, respectively. 
     Further, the four image forming portions  3 K,  3 Y,  3 M, and  3 C are provided with cleaning rollers  313 K,  313 Y,  313 M, and  313 C, respectively, which are cleaning means (processing means). 
     Next, the image formation process which is carried out by each image forming portion  3  will be described. 
     Each photosensitive drum  311  is cylindrical. It has a cylindrical substrate, and photosensitive layers which cover the peripheral surface of the cylindrical substrate. The photosensitive layers are formed of organic photosensitive substances. The photosensitive drum  311  is rotatably supported. It rotates in the clockwise direction ( FIG. 2 ) when the image forming apparatus  100  forms an image. 
     The charge roller  312  is a roller for charging the photosensitive drum  311 . As charge bias is applied to the charge roller  312  from a bias charging means (unshown), the charge roller  312  uniformly charges the peripheral surface of the photosensitive drum  311 . 
     After the charging of the peripheral surface of the photosensitive drum  311 , a beam L of laser light is projected by a laser unit  2  upon the charged peripheral surface of the photosensitive drum  311 , while being modulated with the information regarding the image to be formed, as described above, whereby an electrostatic latent image is effected on the peripheral surface of the photosensitive drum  311 . 
     The development cartridges  32 K,  32 Y,  32 M, and  32 C have development rollers  321 K,  321 Y,  321 M, and  321 C, respectively, which bear developer t. 
     The cartridges  32 K,  32 Y,  32 M, and  32 C have also development blades  322 K,  322 Y,  322 M, and  322 C, respectively, which regulate in thickness the layer of developer t having adhered to the development roller  321 , and also, charge the developer t. 
     Further, the cartridges  32 K,  32 Y,  32 M, and  32 C have developer storage portions  323 K,  323 Y,  323 M, and  323 C, respectively, which store the developer t. The cartridge  32 K has a developer storage portion  323 K, which stores black developer t. It forms a black developer image on the drum  311 K. The cartridge  32 Y has a storage portion  323 Y, which stores yellow developer t. It forms a yellow developer image on the drum  311 Y. The cartridge  32 M has a storage portion  323 M, in which magenta developer t is stored. It forms a magenta developer image on the drum  311 M. The cartridge  32 C has a storage portion  323 C in which cyan developer t is stored. It forms an image of cyan color, on the drum  311 C. 
     Further, the cartridges  32 K,  32 Y,  32 M, and  32 C have stirring members  324 K,  324 Y,  324 M, and  324 C, which convey the developer t to the development rollers  321 K,  321 Y,  321 M, and  321 C while stirring the developer t in the developer storage portions  323 K,  323 Y,  323 M, and  323 C, respectively. 
     During an image forming operation, each development roller  321  rotates in the counterclockwise direction ( FIG. 2 ). As the developer t borne on the peripheral surface of the development roller  321  is moved past the development blade  322  while remaining in contact with the development blade  322 , the developer t becomes charged. Thus, as the development roller  321  is rotated further, the charged developer t is adhered to the electrostatic latent image (which has just been formed on photosensitive drum  311 ) by the development bias applied to the development roller  321  by the bias applying means (unshown), in the area in which the development roller  321  opposes the photosensitive drum  311 . In other words, the electrostatic latent image is developed by the combination of the development roller  321  and the developer t on the development roller  321 . 
     After the formation of a developer image (development of electrostatic latent image) on the photosensitive drum  311  by the cartridge  32 , the developer image is transferred onto the sheet  61  (of recording medium), which is on a transfer belt  41  and being conveyed by the transfer belt  41  ( FIG. 1 ). The developer t, which is remaining on the photosensitive drum  311  after the transfer, that is, the developer t, which failed to be transferred from the photosensitive drum  311  onto the sheet  61 , is recovered by a cleaning roller  313 . 
     As the above described steps of the electrostatic image formation process are repeated by the image forming portion  3 , the intended image is completed (formed) on the sheet  61 . 
     As for the developer t recovered by the cleaning roller  313 , it is adhered to the photosensitive drum  311  with a preset timing, and then, is recovered into a recovery unit  5  by way of the belt  41 . 
     Incidentally, the method for recovering the residual developer t on the photosensitive drum  311 , that is, the portion of the developer image on the photosensitive drum  311 , which failed to be transferred onto the sheet  61 , does not need to be limited to the above described one. That is, any of the known methods, for example, the method which uses a cleaning blade, may be used as needed. 
     &lt;Transfer Unit&gt; 
     Next, referring to  FIG. 1 , the transfer unit  4  will be described. 
     The unit  4  has: the transfer belt  41  which conveys the sheet  61  by bonding the sheet  61  to the transfer belt  41 ; a belt tension roller  42 , which provides the belt with tension; and a belt driving roller  43  for driving the belt  41 . Further, the unit  4  has: a transfer roller  44  ( 44 K,  44 Y,  44 M, and  44 C) for transferring the developer image after the formation of the developer image on the photosensitive drum  311 ; and a roller  45  which is disposed in such a manner that it opposes the recovery roller  51  of the recovery unit  5  with the presence of the transfer belt  41  between the two rollers  45  and  51 . 
     The belt  41  adheres the sheet  61  to itself so that it can reliably convey the sheet  61 . As the sheet  61  is conveyed by the belt  41 , it sequentially moves through the four nips formed by the four photosensitive drums  311  and corresponding transfer rollers  44 , one for one. While the sheet  61  is moved through each nip, a preset transfer bias is applied to the transfer roller  44  by a bias applying means (unshown). Thus, the four developer images on the four photosensitive drums  311 , one for one, are sequentially transferred onto the sheet  61 , effecting thereby a color image (developer image) on the sheet  61 . 
     The roller  43  is the roller for rotating the belt  41 , and is rotated by a driving means (unshown) in the counterclockwise direction ( FIG. 1 ). The roller  45  is positioned so that it is pressed against the roller  51  with the presence of the belt  41  between the two rollers  45  and  51 . Thus, the belt  41  is kept pinched in the nip which the two rollers  45  and  51  form. The roller  42  provides the belt  41  with a preset amount of tension, by being aided by a pressure applying means (unshown). 
     &lt;Recovery Unit&gt; 
     Next, referring to  FIG. 1 , the recovery unit  5  will be described. 
     The unit  5  has: the recovery roller  51  for scraping away the residues on the belt  41 ; a scraper  52  for scraping down the residues recovered by the roller  51 ; and a recovery container  53 . 
     There are such residues as the residue from the developer t adhered to the belt  41  by the roller  313  as described above, and paper dust, on the belt  41 . The roller  51  is rotated by a driving means (unshown), and a preset bias is applied to the roller  51  by a bias applying means (unshown), so that these residues are recovered by the roller  51 . 
     The scraper  52  is made up of a piece of flexible sheet. One of its lengthwise edges is in contact with the roller  51 . With the provision of this structural arrangement, the residues recovered by the roller  51  are scraped down from the peripheral surface of the roller  51  by the scraper  52 , and are recovered into the container  53 . 
     &lt;Fixation Unit&gt; 
     Next, referring to  FIG. 1 , the fixation unit  7  will be described. 
     The fixation unit  7  has a heat roller  71  and a pressure roller  72 . The roller  71  has a heating means (unshown), which is inside the roller  71 . It is heated to a preset temperature level. The pressure roller  72  is kept pressed upon the roller  71  by a pressing means (unshown) so that a preset amount of pressure is maintained between the two rollers  71  and  72 . 
     While the sheet  61 , which is bearing the developer image transferred onto the sheet  61  by the transfer unit  4 , is conveyed through the nip portion between the rollers  71  and  72 , heat and pressure is applied to the sheet  61  and the developer image thereon, whereby the developer image becomes fixed to the sheet  61 , yielding thereby the sheet  61  bearing a fixed developer image. 
     &lt;Discharge Portion&gt; 
     Next, referring to  FIG. 1 , the discharge portion  8  will be described. 
     The discharge portion  8  has a pair of conveyance rollers  81 , a pair of discharge rollers  82 , and a delivery portion  83  (tray). 
     After the fixation of the developer image to the sheet  61 , the sheet  61  is conveyed out of the fixation unit  7 , and then, is conveyed to the discharge portion  8 . 
     The rollers  81  are for conveying the sheet  61  to the pair of rollers  82 , and are rotated, along with the rollers  82 , by a driving means (unshown). The rollers  82  are for discharging the sheet  61  out of the main assembly  1 . The delivery portion  83  is the portion into which the sheet  61  is discharged in such a manner that it will be placed on top of the preceding sheet  61 , after the fixation of the developer image to the sheet  61 , that is, after the completion of the image on the sheet  61 . 
     &lt;Unitization of Components with Limited Service Life, and Consumables (Placement of Components with Limited Service Life, and Consumables, in Cartridge)&gt; 
     Next, the unitization of components with a limited service life, and consumables, will be described. 
     As described above, each image forming portion  3  of the image forming apparatus  100  in this embodiment is provided with the photosensitive drum  311 , charge roller  312 , cleaning roller  313 , and development roller  321 . 
     Some of the various members (components), which make up the image forming portion  3 , wear out. Thus, they have to be replaced as they reach the end of their service life. Here, the end of their service life refers to when they reach the point beyond which they become unsatisfactory for the image forming apparatus to continue to form images satisfactory in quality to a user, because of their deterioration and/or wear. 
     In this embodiment, therefore, the components which are likely to wear and/or deteriorate, are placed together in a cartridge to make simpler the operation for replacing them. More concretely, each image forming portion is made up of a drum cartridge  31  ( 31 K,  31 Y,  31 M, and  31 C) and the development cartridge  32  ( 32 K,  32 Y,  32 M, and  32 C) described above. The cartridge  31 K and  32 K are paired to form an image forming portion, and so are the cartridges  31 Y and  32 Y, cartridges  31 M and  32 M, and cartridges  31 C and  32 C. 
     The cartridge  31  has the photosensitive drum  311 , charge roller  312  (charging means), and cleaning roller  313  (cleaning means), which were unitized. The development cartridge  32  has the development roller  321  (developing means) and developer storage portion  323 , which were unitized. 
     If any of the cartridges  31  and  32  reaches the end of its service life, a user can replace the cartridge(s) having reached the end of its service life, with a brand-new one to ensure that the image forming apparatus  100  continues to form high quality images. 
     &lt;Drum Cartridge&gt; 
     Next, referring to  FIGS. 2 and 3(   a ), the drum cartridge  31  will be described.  FIG. 3(   a ) is a perspective view of the drum cartridge  31  in the first preferred embodiment of the present invention. As described above, the four image forming portions  3  ( 3 K,  3 Y,  3 M, and  3 C) are basically the same in structure, although they are different in the color of the developer t. Therefore, shown in  FIG. 3(   a ) is the drum cartridge  31 K, as the cartridge which represents all the cartridges  31 , as it does in  FIG. 2 . 
     The cartridge  31  has the photosensitive drum  311 , charge roller  312 , cleaning roller  313 , and the drum cartridge frame  314  ( 314 K,  314 Y,  314 M, and  314 C) to which the preceding three components are attached to unitize them. 
     Further, the frame  314  is provided with: a drum cartridge guide  3141   a  ( 3141   a K,  3141   a Y,  3141   a M, and  3141   a C) (guiding members, by which frame  314  is guided) which guides the cartridge  31  when the cartridge  31  is mounted into, or removed from the main assembly  1 ; a drum cartridge guide  3141   b  ( 3141   b K,  3141   b Y,  3141   b M, and  3141   b C) (guiding members, by which frame  314  is guided) which guides the cartridge  31  when the cartridge  31  is mounted into, or removed from the main assembly  1  ( FIGS. 2 and 3(   a )). The guides  3141   a  and  3141   b  (guiding members by which frame  314  is guided) are parallel to the lengthwise direction of the cartridge  31  (axial line of photosensitive drum  311 ). Further, they are different in the direction in which they outwardly protrude from the frame  314  ( FIG. 2) . 
     The frame  314  is also provided with: a drum cartridge positioning front hole  3143   a  ( 3143   a K,  3143   a Y,  3143   a M, and  3143   a C) (positioning hole by which frame  314  is positioned) which precisely positions the cartridge  31  relative to the unit  33 ; and a drum cartridge positioning front hole  3143   b  ( 3143   b K,  3143   b Y,  3143   b M, and  3143   b C) (positioning hole, by which frame  314  is positioned) which precisely positions the cartridge  31  relative to the unit  33  ( FIG. 3(   a )). The positioning holes  3143   a  and  3143   b  are portions of protrusions, one for one, protruding from the trailing end of the frame  314  (in terms of the direction indicated by arrow mark X, that is, the direction in which the cartridge  31  is inserted into the unit  33 ), in the direction intersectional (perpendicular) to the cartridge insertion direction X. 
     Further, the frame  314  is provided with a drum cartridge positioning front hole (groove)  3142  ( 3142 K,  3142 Y,  3142 M, and  3142 C) (positioning members, by frame  314  (cartridge  31 ) is guided) which precisely positions the cartridge  31  relative to the unit  33 . The positioning hole  3142  is at the leading end of the frame  314  in terms of the direction X in which the cartridge  31  is inserted into the unit  33 , and its axial line coincides with that of the photosensitive drum  311 . That is, the leading end of the cartridge  31 , in terms of the cartridge insertion direction X, is precisely positioned relative to the unit  33  by the hole  3142  of the frame  314 , which is at the leading end of the cartridge  31 , whereas the trailing end of the cartridge  31  is precisely positioned relative to the unit  33  by the holes  3143   a  and  3143   b  of the frame  314 , which is at the trailing end of the frame  314  in terms of the cartridge insertion direction X. As described above, the axial line of the hole  3142  coincides with the axial line of the photosensitive drum  311 . Therefore, as the cartridge  31  is precisely positioned relative to the unit  33 , the photosensitive drum  311  is also precisely positioned relative to the unit  33 . Incidentally, the axial lines of the  3142 ,  3143   a , and  3143   b  are parallel to the cartridge insertion direction X. Also as described above, the photosensitive drum  311 , charge roller  312 , and cleaning roller  313  are integral parts of the cartridge  31 , and therefore, removably mounted in the main assembly  1 . 
     The cartridge  31  integrally holds the charge roller  312  (charging means) and cleaning roller  313  (cleaning means), which are processing means, and photosensitive drum  311 , and is removably mountable in the main assembly  1 . Thus, the cartridge  31  may be referred to as a process cartridge, because, a process cartridge is a cartridge in which at least one of the charging means and cleaning means, which are processing means, and the photosensitive drum, are integrally disposed so that they can be removably mounted in the main assembly  1  of the image forming apparatus  100 . Incidentally, this embodiment is not intended to limit the present invention in terms of the structure of the cartridge  31 . For example, what is required of the cartridge  31  is that it has at least the photosensitive drum  311 , and is removably mountable in the main assembly  1 . It is possible that the cartridge  31  has only the photosensitive drum  311 , that is, it does not have any of the aforementioned processing means. In such a case, the charge roller  312  (charging means) and cleaning roller  313  (cleaning means) are to be attached to the main assembly  1 . 
     &lt;Development Cartridge&gt; 
     Next, referring to  FIGS. 2 and 3(   b ), the development cartridge  32  will be described in more detail.  FIG. 3(   b ) is a perspective view of the cartridge  32  in the first preferred embodiment of the present invention. 
     The cartridge  32  has the development roller  321 , development blade  322 , stirring member  324 , and a development cartridge frame  325  ( 325 K,  325 Y,  325 M, and  325 C) to which the preceding components are attached to be unitized. The development cartridge frame  325  has a development storage portion  323 . That is, the cartridge  32  is an integration of the development roller  321 , development blade  322 , stirring member  324 , and developer storage portion  323 , and is removably mountable in the main assembly  1 . 
     The frame  325  is provided with a pair of developer cartridge positioning shafts  3251   a  ( 3251   a K,  3251   a Y,  3251   a M, and  3251   a C) and  3251   b  ( 3251   b K,  3251   b Y,  3251   b M, and  3251   b C), which are positioning portions for precisely positioning the cartridge  32  relative to the unit  33 . More specifically, the positioning shaft  3251   a  projects from one of the lengthwise ends of the cartridge  32  (direction parallel to axial line of development roller  321 ), and the positioning shaft  3251   b  projects from the other lengthwise end of the cartridge  32 . The axial lines of the shafts  3251   a  and  3251   b  coincide with the axial line of the development roller  321 . Thus, the cartridge  32  is precisely positioned relative to the unit  33  in such a manner that the development roller  321  is precisely positioned relative to the unit  33 . 
     Further, the frame  325  is provided with a pair of separation bosses  3252   a  ( 3252   a K,  3252   a Y,  3252   a M, and  3252   a C) and  3252   b  ( 3252   b K,  3252   b Y,  3252   b M, and  3252   b C), which project from the lengthwise ends of the frame  325 , one for one, in the direction parallel to the lengthwise direction of the frame  325 . Each of the separation bosses  3252   a  and  3252   b  is one of the members which make up the means (mechanism) for placing the development roller  321  in contact with the photosensitive drum  311 , or separating the development roller  321  from the photosensitive drum  311 . This means for placing the development roller  321  in contact with, or separating from, the photosensitive drum  311 , will be described later in more detail. 
     &lt;Tray Unit System&gt; 
     Next, referring to  FIGS. 1 and 4 , the tray unit system will be described.  FIG. 4  is a schematic sectional view of the tray unit (supporting member)  33  in the first preferred embodiment of the present invention, when the tray unit  33  is in its outermost position relative to the main assembly  1 . 
     The image forming apparatus  100  in this embodiment is provided with the unit  33 , which is a supporting member for supporting the image forming portions  3 . The image forming apparatus  100  is structured so that when its main assembly  1  is on a horizontal surface, the unit  33  is horizontally movable relative to the main assembly  1 . Further, the unit  33  is supported by the main assembly  1  so that it is movable between its innermost position in the main assembly  1  and its outermost position relative to the main assembly  1 . If a user wants to move the unit  33  out of the main assembly  1 , the user is to open the main assembly cover  11 , and then, horizontally pull the unit  33  outward in a straight line, as shown in  FIG. 4 . Incidentally, the main assembly cover  1  is capable of taking the closed position and open position, in which it keeps the opening  1   a  of the main assembly  1  closed or open, respectively. The unit  33  is movable, while supporting the cartridges  31  and  32 , between the innermost position IP ( FIG. 1 ) in the main assembly  1 , and the outermost position OP (FIG.  4 ) relative to the main assembly  1 . Thus, when the unit  33  is moved between the innermost position IP and outermost position OP, it moves through the opening la. That is, the opening  1   a  is the opening which allows the unit  33  to move between the inward and outward side of the main assembly  1 . By the way,  FIG. 1  is a drawing for showing the state of the unit  33  when the unit  33  is in the innermost position IP, and  FIG. 4  is a drawing for showing the state of the unit  33  when the unit  33  is in the outermost position OP. The outermost position OP is the unit position which allows a user to mount the cartridges  31  and  32  into the unit  33 , or remove the cartridges  31  and  32  from the unit  32 . The innermost position IP is the unit position which allows the unit  33  to keep the cartridges  31  and  32  in the image forming portions in the main assembly  1 . The image forming position is the cartridge position in which the cartridges  31  and  32  contribute to the image formation process. That is, the image forming position is the position in which the cartridges  31  and  32  carry out the image formation process. In this embodiment, when the unit  33  is in its innermost position IP (image forming position), the photosensitive drum  311 , which the cartridge  31  has, is in contact with the belt  41 . 
     The unit  33  is a member (unit) for supporting multiple cartridges  31  and  32 . The unit  33  and cartridge  31  and  32  are structured so that the cartridges  31  and  32  can be individually and removably mountable in the unit  33 . If a user wants to replace the cartridge  31  or cartridge  32 , the user is to move the unit  33  out of the main assembly  1  before the user replace the cartridge(s). That is, the user is to pull the unit  33  out of the main assembly  1  (from innermost position IP,  FIG. 1 ), all the way to the outermost position OP ( FIG. 4 ), and then, the user is to replace the cartridge(s)  31  and  32  which needs to be replaced, with brand-new cartridge(s)  31  and  32 , while keeping the unit  33  in the outermost position OP. After the completion of the cartridge replacement operation, the user is to move the unit  33  back into the innermost position IP in the main assembly  1  by horizontally moving the unit  33  in a straight line. In other words, in this embodiment, the cartridges  31  and  32  are removably mountable in the main assembly  1 . That is, the cartridges  31  and  32  are removably positioned in their image forming portions described above. Referring to  FIG. 4 , the main assembly  1  is provided with a pair of inner walls  103  and a pair of tray guiding rails  101  and  102 . The inner walls  103  are on the immediately inward side of the corresponding outer walls of the main assembly  1 , and face each other across the internal space of the main assembly  1 . One of the inner walls  103  is at one of the widthwise ends of the unit  33 , and the other inner wall  103  is at the other widthwise end of the unit  33 . The guide rail  101  is a part of one of the mutually facing inner walls  103 , and is in the form of a groove. The guide rail  102  is a part of the other inner wall  103 , and is also in the form of a groove. The guide rails  101  and  102  are positioned so that they squarely oppose each other as do the pair of inner walls  103 . Further, the rails  101  and  102  have tray positioning portions  101   a  and  102   a , respectively, which are the rail positioning portions of the unit  33 . Here, the widthwise direction of the unit  33  is the direction intersectional (perpendicular) to the directions Z 1  and Z 2  ( FIG. 5 ) in which the unit  33  is moved relative to the main assembly  1 . 
     Next, referring to  FIG. 5 , the unit  33  has a pair of handholds  332  and  333 , which have tray guiding bosses  332   a  and  333   a , respectively, which are used to guide the unit  33  when the unit  33  is mounted into the main assembly  1 . The guide boss  332   a  is guided by the rail  102  when the unit  33  is moved between the innermost position IP and outermost position OP. The guide boss  333   a  is guided by the rail  101  when the unit  33  is moved between the innermost position IP and outermost position OP. 
     Further, the cover  11  (which can be opened or closed) has a tray contacting portion  11   a  for keeping the unit  33  in the innermost position IP. As the cover  11  is closed when the unit  33  is in the innermost position IP, the tray contacting portion  11   a  keeps the unit  33  in the innermost position IP by coming into contact with the guide boss  332   a.    
     As a user moves the unit  33  from the outermost position OP to the innermost position IP, the guide bosses  333   a  and  332   a  move while remaining engaged with the guide rails  101  and  102 , respectively, whereby the unit  33  is regulated in its movement. Therefore, the unit  33  remains stable in its movement. It is after the arrival of the bosses  333   a  and  333   b  at the tray positioning portions (of main assembly  1 )  101   a  and  102   a , respectively, when the cover  11  is to be closed. As the cover  11  is closed to completely cover the opening  1   a , the unit  33  is precisely positioned in the innermost position IP by the cover  11 . In this embodiment, the boss  333   a  is precisely positioned by the tray positioning portion  101   a , whereby the unit  33  is precisely positioned relative to the main assembly  1 , as will be described later. Also in this embodiment, the cartridges  31  and  32  are precisely positioned relative to the unit  33 , as will be described later. Thus, as the unit  33  is precisely positioned relative to the main assembly  1 , the cartridges  31  and  32  are also precisely positioned relative to the main assembly  1 . In other words, when the unit  33  is in the innermost position IP, the cartridges  31  and  32  are in their image forming positions described above. As for the position of the bosses  333   a  and  332   a , they are at the widthwise ends of the unit  33 , one for one. Further, in terms of the lengthwise direction of the unit  33 , in which the unit  33  is moved into the main assembly  1 , the boss  333   a  is at the downstream end of the unit  33 , and the boss  332   a  is at the upstream end of the unit  33 . Thus, the unit  33  is precisely positioned relative to the main assembly  1  by its lengthwise ends and widthwise ends. Therefore, it is ensured that the unit  33  is precisely positioned relative to the main assembly  1 . Here, the abovementioned lengthwise direction of the unit  33  is the direction parallel to the direction Z 1 , that is, the direction in which the unit  33  is pushed into the main assembly  1 . The abovementioned widthwise direction of the unit  33  is the direction perpendicular to the direction Z 1 , that is, the direction in which the unit  33  is pushed into the main assembly  1 . Further, the cover  11  can be opened or closed to expose or cover the opening  1   a , respectively. The cover  11  is rotatably movable about the axial line of the shaft  11   b.    
     The employment of the above described tray unit system makes it possible to perform the cartridge replacement operation outside the main assembly  1 , that is, in a wide-open space, making it easier to perform the cartridge replacement operation. In addition, it makes it unnecessary for a user to remove the cartridges  31  and  32  one by one from within the main assembly  1  when it is necessary to remove the sheets  61  having jammed up in the main assembly  1 . More concretely, as the user releases a stopper (unshown), the user can pulled (remove) the combination of the unit  33  and cartridges  31  and  32 , out of the main assembly  1  by grasping the handholds  332  and  333 . Thus, the employment of the above described tray unit system can improve the image forming apparatus in terms of the efficiency with which the jammed sheets  61  can be removed. 
     In the case of an image forming apparatus structured so that the unit  33  is to be horizontally pulled out of the main assembly  1  as in the case of the image forming apparatus in this embodiment, the operation for replacing the cartridges  31  and  32  can be performed without retracting the laser unit  2 . Further, even if the image forming apparatus is structured so that the original reading apparatus (unshown) is in the top portion of the main assembly  1 , the operation for replacing the cartridges  31  and  32  can be performed without retracting the original reading apparatus. In other words, not only does the employment of the above described tray unit system improve an image forming apparatus in terms of the cartridge replacement efficiency, but also, makes it unnecessary to provide an image forming apparatus with a mechanism or structural arrangement dedicated to the retraction of the unit  2  and original reading apparatus. That is, the employment of the tray unit system is advantageous also from the standpoint of structural simplification. 
     &lt;Tray Unit&gt; 
     Next, referring to  FIGS. 5 and 6(   a ), the tray unit  33  will be described in more detail.  FIG. 5  is a perspective view of the tray unit (supporting member) in the first preferred embodiment of the present invention.  FIG. 6(   a ) is a perspective view of the tray unit in the first preferred embodiment of the present invention, when the tray unit is holding the cartridges. 
     The unit  33  has a tray frame  331 , and the pair of handholds  332  and  333 . The handholds  332  and  333  are where a user is to place his or her hand(s) to grasp the unit  33  when moving the unit  33  relative to the main assembly  1 . The handhold  332  is to be grasped by a user to push the unit  33  into the main assembly  1  (direction of arrow mark Z 1 ), or pull the unit  33  out of the main assembly  1  (direction of arrow mark Z 2 ). In terms of the direction (indicated by arrow mark Z 1 ) in which the unit  33  is pushed into the main assembly  1 , the handhold  332  is at the upstream end of the unit  33 . The handhold  333  is to be grasped by a user when the user removes the unit  33  from the main assembly  1 . For example, when it is necessary to remove jammed recording sheet(s), a user is to release the stopper (unshown), and remove the unit  33  from the main assembly  1  by grasping the handholds  332  and  333 , so that the jammed recording sheets can be removed. 
     Further, the unit  33  is provided with a pair of separation bars  334   a  and  334   b  ( FIG. 5 ), which are two of the components that make up the means for placing each of the development rollers  321  in contact with the corresponding photosensitive drum  311 , or separating each of the development rollers  321  from the corresponding photosensitive drum  311  ( FIG. 5 ). 
     The unit  33  is provided with drum positioning shafts (drum positioning members on supporting member side)  335  ( 335 K,  335 Y,  335 M, and  335 C), which correspond in position to the holes  3142 , with which the cartridges  31  are provided one for one. Each positioning shaft  335  is projecting inward of the unit  33  in the direction intersectional (perpendicular) to the direction in which the unit  33  is moved relative to the main assembly  1 . The number of the shafts  335  matches the number of the cartridges  31  supportable by the unit  33  so that there will be one shaft  335  per cartridge  31  ( FIG. 5 ). 
     Further, the unit  33  is provided with drum positioning bosses (drum positioning portion on supporting member side)  336   a  ( 336   a K,  336   a Y,  336   a M, and  336   a C), which correspond in position to the positioning holes  3143   a , with which the cartridges  31  supported by the unit  33  are provided, one for one. The boss  336   a  is projecting outward from one of the widthwise end walls of the unit  33 , in the direction intersectional (perpendicular) to the direction in which the unit  33  is moved relative to the main assembly  1 . The number of bosses  336  matches the number of the cartridges  31  so that there will be one boss  336   a  per cartridge  31 . Incidentally, the widthwise direction of the unit  33  is the directions (indicated by arrow marks Z 1  and Z 2 ), which is perpendicular to the direction in which the unit  33  is moved relative to the main assembly  1 . 
     Further, the unit  33  is provided with drum positioning bosses (drum positioning portion on supporting member side)  336   b  ( 336   b K,  336   b Y,  336   b M, and  336   b C), which correspond in position to the positioning holes  3143   b , with which the cartridges  31  supported by the unit  33  are provided, one for one. The bosses  336   b  are projecting outward from one of the widthwise end walls of the unit  33 , in the direction intersectional (perpendicular) to the direction in which the unit  33  is moved relative to the main assembly  1 . Each boss  336   b  is a part of the unit  33 , as is each boss  336   a  ( FIG. 5 ). 
     Further, the unit  33  is provided with guide rails (guide rails of unit  33 )  337   a  ( 337   a K,  337   a Y,  337   a M, and  337   a C), which engage with the cartridge guides  3141   a , one for one, with which the cartridges  31  are provided. More concretely, there are four guide rails  337   a , which extend in the direction perpendicular to the directions Z 1  and Z 2  ( FIG. 5 ) in which the unit  33  is moved relative to the main assembly  1 , that is, the widthwise direction of the unit  33 , being disposed with equal intervals. Thus, as each cartridge  31  is inserted into the unit  33 , it moves while the guide  3141   a  remains engaged with the guide rail  337   a ; the cartridge  31  is guided to the deepest end of the unit  33  ( FIG. 5 ) by the guides  337   a  and  3141   a . Incidentally, the direction indicated by the arrow mark Z 1  is the direction in which the unit  33  is pushed into the main assembly  1 , and the direction indicated by the arrow mark Z 2  is the direction in which the unit  33  is pulled out of the main assembly  1  ( FIG. 5 ). 
     Further, the unit  33  is provided with guide rails (guides which belong to unit  33 )  337   b  ( 337   b K,  337   b Y,  337   b M, and  337   b C), which engage with the cartridge guides  3141   b , one for one, with which the cartridges  31  are provided. Thus, as each cartridge  31  is inserted into the unit  33 , it moves while the guide  3141   b  remains engaged with the guide rail  337   b ; the cartridge  31  is guided to the deepest end of the unit  33  by the guides  337   b  and  3141   b  ( FIG. 5 ). As described above, the unit  33  and cartridges  31  are structured so the pair of guides  3141   a  and  3141   b , with which each cartridge  31  is provided, engage with one of the pairs of rails  337   a  and  337   b . Thus, it is ensured that each cartridge  31  reliably advances into the unit  33 . 
     Further, the unit  33  is provided with openings (drum cartridge entrance-exit)  338  ( 338 K,  338 Y,  338 M, and  338 C), through which the cartridges  31  are mounted into, or removed from, the unit  33 , one for one. More concretely, the left lateral wall of the unit  33 , as seen from the upstream side in terms of the unit insertion direction, is provided with four openings  338 , which are positioned with equal intervals. The aforementioned pair of boss  336   a  and  336   b  are positioned in a manner of opposing each other across the corresponding opening  338 . 
     Further, the unit  33  is provided with four pairs of development cartridge guiding grooves (development cartridge guiding grooves which belong to supporting member)  339   a  ( 339   a K,  339   a Y,  339   a M, and  339   a C) and  339   b  ( 339   b K,  339   b Y,  339   b M, and  339   b C), which guide the cartridges  32 , one for one. In terms of the widthwise direction of the unit  33 , the guiding grooves  339   a  are at one end of the unit  33 , being positioned with equal intervals, and the guiding groove  339   b  are at the other end of the unit  33 , being positioned with equal intervals (in terms of directions Z 1  and Z 3 ), so that the guiding grooves  339   a  and  339   b  squarely oppose each other, one for one, across the internal space of the unit  33 . 
     Each of the aforementioned pairs of development cartridge positioning shaft  3251   a  and  3252   a  engages into the corresponding guide groove  339   a , and each of the aforementioned pairs of cartridge positioning shaft  3251   b  and  3252   b  engages into the corresponding guiding groove  339   b . That is, when the cartridge  32  is mounted into the unit  33 , the shaft  3251   a  comes into contact with the walls of the guiding groove  339   a , being thereby guided downward toward the bottom of the unit  33 , and the shaft  3251   b  comes into contact with the walls of the guiding groove  339   b , being thereby guided downward toward the bottom of the unit  33 . 
     The unit  33  can be moved relative to the main assembly  1  while all the cartridges  31  and  32  are supported by the unit  33  ( FIG. 6(   a )). Thus, as the unit  33  is moved in the direction indicated by the arrow mark Z 1 , it moves into the main assembly  1 , whereas as the unit  33  is moved in the direction indicated by the arrow mark Z 2  while the unit  33  is in the main assembly  1 , it comes out of the main assembly  1 . 
     When a user wants to perform the operation for replacing the cartridge(s)  31  and/or  32 , the user is to pull the unit  33  out of the main assembly  1  before the user starts the operation; the operation is to be performed when the unit  33  is in the outermost position OP. 
     &lt;Mounting of Drum Cartridge into Tray Unit, and Removal of Drum Cartridge from Tray Unit&gt; 
     Next, the method for mounting or removing the cartridge  31  will be described with reference to the appended drawings, in particular,  FIGS. 5 and 6(   b ).  FIG. 6(   b ) is a perspective view of the combination of the tray unit  33  and the four cartridges  31  and four cartridge  32 , in the first preferred embodiment of the present invention, and is for describing the operation for mounting the cartridges  31  and  32  into the tray unit  33 , or removing the cartridges  31  and  32  from the tray unit  33 . 
     The image forming apparatus  100  is structured so that each cartridge  31  is independently mounted into, or removed from, the unit  33 , from the cartridges  32 . 
     The method for mounting the cartridge  31  into the unit  33  is as follows: First, the cartridge  31  is to be inserted into the unit  33  in the direction parallel to the axial line of the drum  311  (lengthwise direction of cartridge  31 ) through the opening  338 , while positioning the cartridge  31  so that the cartridge guides  3141   a  and  3141   b  engage with the guide rails  337   a  and  337   b , respectively, of the unit  33 . That is, the cartridge  31  is to be inserted into the rearmost end of the unit  33  while the guides  3141   a  and  3141   b  are guided by the rails  337   a  and  337   b , respectively. Since the guides  3141   a  and  314   b  remain engaged with the rails  337   a  and  337   b , the cartridge remains roughly horizontal while it is mounted into the unit  33 . Thus, the cartridge  31  is removably supported by the unit  33 . 
     As the cartridge  31  is inserted to almost the deepest end of the unit  33 , the positioning shaft  335  of the unit  33  engages into the positioning hole  3142 , with which the leading end of the cartridge  31  (in terms of cartridge insertion direction X) is provided. Then, lastly, the positioning bosses  336   a  and  336   b  of the unit  33  engage into the positioning holes  3143   a  and  3143   b , respectively, with which the trailing end of the cartridge  31  (in terms of cartridge insertion direction X) is provided. As a result, the cartridge  31  becomes precisely positioned relative to the unit  33 . 
     When the unit  33  is moved from its outermost position OP to its innermost position IP while the unit  33  is holding the cartridges  31 , the drum cartridge frame  314  comes into contact with the inner walls  103 , whereby the cartridge  31  is precisely positioned relative to the main assembly  1  in terms of its lengthwise direction. 
     If a user wants to remove the cartridge  31  from the unit  33 , the user has only to pull the cartridge  31  in the direction parallel to the axial line of the photosensitive drum  311  so that it will come out through the opening  338 . 
     Incidentally,  FIG. 6(   b ) shows the combination of the unit  33  and cartridges  31  and  32 , when the cartridge  31 M is halfway out of, or halfway into, the unit  33 . 
     &lt;Mounting of Development Cartridge into Tray Unit, and Removal of Development Cartridge from Tray Unit&gt; 
     Next, referring to the appended drawings, in particular,  FIGS. 3(   b ),  5 , and  6 ( b ), the method for mounting or removing the cartridge  32  will be described. 
     As described above, the unit  33  is provided with the four pairs of cartridge guiding grooves  339   a  and  339   b  ( FIG. 5 ). Further, the unit  33  and cartridge  32  are structured so that the direction in which each cartridges  32  is allowed to move, and the attitudinal changes which might occur to the cartridge  32 , when the cartridge  32  is mounted into, or removed from, the unit  33 , are regulated by the angle and shape of the guiding grooves  339   a  and  339   b . Thus, even in the case where the cartridges  32  are mounted into, or removed from, the unit  33  when the cartridges  31  are already in the unit  33 , it does not occur that the cartridges  31  interfere with the mounting or removal of the cartridges  32 . 
     While the unit  33  is in its outermost position, the cartridges  31  are kept separated from the corresponding cartridges  32  by the resiliency of the tension springs  3341 , as will be described later. Therefore, it is possible to prevent the problem that when the cartridges  31  and  32  are mounted into, or removed from, the unit  33 , the photosensitive drums  311  and development rollers  321  become damaged by coming in contact with each other. 
     As described above, each cartridge  32  is provided with a pair of development cartridge positioning shafts  3251   a  and  3251   b.    
     Thus, a user who wants to mount a cartridge  32  into the unit  33  is to insert the cartridge  32  as follows: First, the cartridge  32  is to be positioned so that the positioning shafts  3251   a  and  3251   b  align with the guiding grooves  339   a  and  339   b , respectively, and then, to move the cartridge  32  downward so that the shafts  3251   a  and  3251   b  follow the grooves  339   a  and  339   b , respectively. Also as described above, in this embodiment, the unit  33  is to be horizontally pulled out in straight line (direction indicated by arrow mark Z 2 ) from the main assembly  1  while the main assembly  1  remains on a horizontal surface. Then, each cartridge  32  is to be mounted into (supported by) the unit  33  by being moved vertically downward while the unit  33  is in its outermost position. Thus, if the user wants to take any of the cartridges  32  out of the unit  33 , the user has only to carry out in reverse the above described operation for mounting the cartridge  32  into the unit  33 . That is, all that the user has to do is to pull the unit  33  out of the main assembly  1  into its outermost position, and move the cartridge vertically upward ( FIG. 6(   b )). 
     Also as described above, when each cartridge  32  is mounted into the unit  33 , it is moved vertically moved downward into the unit  33 , whereas when the cartridge  32  is removed from the unit  33 , it is moved vertically upward. However, the direction in which the cartridge  32  is moved when it is mounted into, or removed from, the unit  33  is not perfectly vertical; it is slightly angled relative to the vertical direction, as will be evident from  FIG. 5 . 
     Incidentally,  FIG. 6(   b ) shows the combination of the unit  33  and the cartridges  31  and  32 , when the cartridge  32 Y is being mounted into, or removed from, the unit  33 . 
     &lt;Direction in Which Cartridge is Mounted into Unit  33 , and Direction in Which Cartridge is Removed from Unit  33 &gt; 
     As described above, in this embodiment, the image forming apparatus  100  is structured so that each of the cartridges  31  and  32  can be independently mounted into, or removed from, the unit  33 , from the other cartridges  31  and  32 . Further, the image forming apparatus  100  is structured so that each cartridge  31  is paired with the corresponding cartridge  32 , and multiple (four) pairs of cartridge  31  and  32  are aligned in parallel in the directions (indicated by arrow marks Z 1  and Z 2 ) in which the unit  33  is movable; each pair of cartridges  31  and  32  are supported by the unit  33  so that their lengthwise directions are intersectional (perpendicular) to the abovementioned moving directions Z 1  and Z 2  of the unit  33  ( FIGS. 6(   a ) and  6 ( b )). Incidentally, the lengthwise direction of each cartridge  31  is the direction parallel to the lengthwise direction (axial line) of the drum  311  in the cartridge  31 . Further, the lengthwise direction of each cartridge  32  is the direction parallel to the lengthwise direction (axial line) of the development roller  321  in the cartridge  32 . 
     Further, the image forming apparatus  100  (unit  33 ) is structured so that the direction in which each cartridge  31  is mounted into, or removed from, the unit  33 , are different from the direction in which each cartridge  32  is mounted into, or removed from, the unit  33 . That is, the image forming apparatus  100  (unit  33 ) is structured so that the direction in which each cartridge  31  is mounted into, or removed from the unit  33 , is perpendicular to the direction in which each cartridge  32  is mounted into, or removed from the unit  33 . Further, in the image forming apparatus  100  (unit  33 ), each cartridge  31 , and each cartridge  32 , are structured so that each cartridge  31  and  32  can be independently mounted into, or removed from, the unit  33 , from the other cartridges  31  and  32 . More concretely, the image forming apparatus  100  (unit  33 ) is structured so that each cartridge  31  is to be horizontally mounted or dismounted in the direction parallel to the axial line of the photosensitive drum  311 , whereas each cartridge  32  is to be mounted or dismounted in the direction which is roughly vertical and is perpendicular to the axial line of the development roller  321 . It should be noted here that while any of the cartridges  31  and  32  remains properly situated (supported) in the unit  33 , the axial lines of the photosensitive drum  311  and development roller  321  in the cartridge are parallel to each other. 
     Further, as described above, the image forming apparatus  100  is structured so that while the main assembly  1  is on a horizontal surface, the unit  33  is horizontally movable, and also, so that the direction in which each cartridge  31  is mounted into, or dismounted from, the unit  33  is horizontal, and is perpendicular to the direction in which the unit  33  is movable. 
     Also as described above, while the main assembly  1  remains horizontal, the moving direction of the unit  33  (indicated by arrow marks Z 1  and Z 2 ) is perpendicular to the direction in which each cartridge  31  is mounted into, or dismounted from, the unit  33 . The moving direction of the unit  33  is roughly perpendicular to the direction in which each cartridge  32  is mounted into, or dismounted from, the unit  33 . Here, the moving direction of the unit  33  means the direction in which the unit  33  is moved between its innermost position in the main assembly  1  and the outermost position from the main assembly  1 . 
     In order to minimize in size the main assembly  1  of the image forming apparatus  100 , the image forming apparatus  100  in this embodiment is structured so that while the main assembly  1  remains on a horizontal surface, each cartridge  32  and the corresponding cartridge  31  partially overlap each other in terms of the vertical direction. 
     Next, referring to  FIG. 7 , the abovementioned setup will be described.  FIG. 7  shows the cartridge  32 C and  32 K as the examples of the cartridges  32 , and the cartridge  31 K. The cartridge  31 K is extending beyond the area sandwiched by two lines L 1  and L 2 , that is, the maximum gap between the cartridges  32 C and  32 K, by its hatched portions a 1  and a 2  in the drawing. In other words, the image forming apparatus  100  is structured so that the cartridge  31 K fits into the space between the bottom portions of the adjacent two cartridges  32 K and  32 C. 
     That is, while the two cartridges  31  and  32  remain properly mounted (supported) in the unit  33 , a part (hatched portion in  FIG. 7 ) of the cartridge  31  remains under the adjacent two cartridges  32 , reducing in size the space for supporting (mounting) the cartridges  31  and  32 . Thus, this setup can reduce in size the main assembly  1 . 
     Even though the image forming apparatus  100  (unit  33 ) in this embodiment is structured so that while the cartridges  31  and  32  remain properly supported in the unit  33 , a part of each cartridge  31  is positioned below the corresponding cartridge  32 , the cartridges  31  and  32  are not affected in terms of the efficiency with which they can be mounted or dismounted, because the image forming apparatus  100  (unit  33 ) in this embodiment is also structured so that the direction in which each cartridge  31  is mounted or dismounted is perpendicular to the direction in which each cartridge  32  is mounted or dismounted. The employment of this structural arrangement makes it possible for each of the cartridges  31  and  32  to be independently mounted into, or dismounted from, the unit  33 , from the other cartridges. In other words, it is possible to exchange only the cartridge which needs to be replaced. 
     &lt;Mechanism for Placing Development Roller in Contact With, or Separating Development Roller from Photosensitive Drum&gt; 
     Next, referring to  FIGS. 1 ,  5  and  8 , the means for placing a development roller  32  in contact with, or separating the development roller  32  from, a photosensitive drum  311  will be described. Hereafter, this means may be referred to simply as a development roller moving means (mechanism). The means for moving a cartridge  32  is in the form of a mechanism for placing a development roller  321  in contact with, or separating from, a photosensitive drum  311 .  FIGS. 8(   a ),  8 ( b ), and  8 ( c ) are drawings for describing the working of the development roller moving means in the first preferred embodiment of the present invention, and are side views of the unit  33 , as seen from the side where the openings  338  are present. 
     In order to form an image (when development roller  321  is in its development position), the drum  311  and development roller  321  must be in contact with each other. On the other hand, when mounting the cartridge  31  into the unit  33 , or dismounting the cartridge  31  from the unit  33 , it is desired that the drum  311  and development roller  321  are not in contact with each other, in order to prevent the drum  311  and/or development roller  321  from being damaged when the cartridge  31  is mounted or dismounted. 
     Moreover, in a case where a black-and-white image is formed, the cartridge  32 Y,  32 M, and  32 C are not used. Therefore, from the standpoint of the prevention of the unnecessary wear of the cartridges  32 Y,  32 M, and  32 C, it is desired that these development cartridges  32  are kept separated from the corresponding photosensitive drums  311 . 
     Thus, the image forming apparatus  100  and each of the cartridges  32  are provided with the mechanism for placing and keeping the development roller  321  in contact with, or separating and keeping separated the development roller  321  from, the photosensitive drum  311 , while keeping the cartridges  31  and  32  mounted in the unit  33 . 
     More concretely, the main assembly  1  is provided with a pair of separation bar driving gears  34   a  and  34   b  ( FIGS. 1 ,  4 , and  8 ), whereas the unit  33  is provided with a pair of separation bars  334   a  and  334   b , which are moved by the pair of gears  34   a  and  34   b , respectively. The separation bar  334   a  is at one of the widthwise ends of the unit  33 , and the separation bar  334   b  is at the other widthwise end ( FIG. 5 ). The gears  34   a  are  34   b  are positioned so that as the unit  33  is pushed into the main assembly  1 , the separation bars  334   a  and  334   b  come into contact with the gears  34   a  and  34   b , respectively. The gears  34   a  and  34   b  are rotated by the driving force from a motor M ( FIG. 8 ), which is controlled (rotated or stopped) by a controlling means C ( FIG. 8 ). The gears  34   a  and  34   b  and separation bars  334   a  and  334   b  are the primary structural components of the development roller moving means. As described before, in this embodiment, the development roller  321  is one of the integral parts of the cartridge  32 . Therefore, the roller  321  is placed in contact with, or separated from, the drum  311  by the movement of the cartridge  32 . 
     Shown in  FIG. 8  is only the side of the combination of the unit  33  and cartridges  31  and  32 , where the gear  34   b  and separation bar  334   b  are present. Since the structures and functions of the gear  34   a  and separation bar  334   a  are the same as those of the gear  34   b  and separation bar  334   b , only the structures and functions of the gear  34   b  and  334   b  will be described. 
       FIG. 8(   a ) shows the combination of the unit  33  and cartridges  31  and  32 , when all the development rollers  321  of all the cartridges  32  are remaining separated from the corresponding photosensitive drums  311 .  FIG. 8(   b ) shows the combination of the unit  33  and cartridges  31  and  32 , when only the development roller  321 K, that is, the development roller of the cartridge  32 K, is in contact with the corresponding photosensitive drum  311 , that is, the drum  311 K.  FIG. 8(   c ) shows the combination of the unit  33  and cartridges  31  and  32 , when all development rollers  321  of all cartridges  32  are in contact with the corresponding drums  311 , one for one. 
     The separation bar  334   b  has a rod portion  334   b   1 , which extends along the top edge of the lateral plate of the unit  33 . The rod portion  334   b   1  is provided with a rack portion  334   b   2 , which is at one end of the rod portion  334   b   1 . The rack portion  334   b   2  meshes with the teeth of the gear  34   b  as the unit is moved into its innermost position IP. That is, the unit  33  is provided with the rack portion  334   b   2 , which remains meshed with the teeth of the gear  34   b  when the unit  33  remains properly stored in the main assembly  1 . 
     Further, the rod portion  334   b   1  is provided with four separation seat areas  334   b   3  ( 334   b   3 K,  334   b   3 Y,  334   b   3 M, and  334   b   3 C) and four slant surfaces  334   b   4  ( 334   b   4 K,  334   b   4 Y,  334   b   4 M, and  334   b   4 C), which correspond in position to four cartridges  32 , one for one. 
     Further, the rod portion  334   b   1  is provided with four contact engagement portions  334   b   5  ( 334   b   5 K,  334   b   5 Y,  334   b   5 M, and  334   b   5 C), which correspond in position to the four cartridges  32 , one for one. 
     Further, referring to  FIG. 8 , the separation bar  334   b  is kept pulled leftward by the resiliency of a tension spring (elastic member)  3341  attached to the unit  33  by one of its lengthwise ends. Thus, when the rack portion  334   b   2  is not in mesh with the teeth of the gear  34   b  (for example, when the unit  33  is in its outermost position OP), the state of the combination of the unit  33  and cartridges  31  and  32  is as shown in  FIG. 8(   a ). 
     Referring again to  FIG. 8 , the image forming apparatus  100  is structured so that the gear  34   b  is rotatable in the clockwise and counterclockwise directions by the driving force from the motor M with which the main assembly  1  is provided, by a preset angle; the driving force (rotational force) from the motor M is transmitted to the gears  34   a  and  34   b  by a known driving force transmitting means. As the unit  33  is moved back into its innermost position IP in the main assembly  1 , the rack portion  334   b   2  is engaged with the teeth of the gear  34   b , as described above. Then, the gear  34   b  is rotated while its rotation is controlled by the controlling means C. That is, the controlling means C, with which the main assembly  1  is provided, controls the rotation of the motor M based on the information regarding the development roller separation and the information regarding the development, so that the separation bar  334   b  is moved in the leftward or rightward in  FIG. 8 . 
     When the separation bar  334   b  is in the position shown in  FIG. 8(   a ), the positioning shaft  3251   b  of the cartridge  32  is in the guide groove  339   b , with which the unit  33  is provided. However, the separation boss  3252   b  of the cartridge  32  is on the top of the separation seat area  334   b   3 . Thus, the cartridge  32  is prevented from moving toward the drum  311 . That is, the cartridge  32  (more concretely, development roller  321 ) remains separated from the photosensitive drum  311  ( FIG. 8(   a )). 
     As the gear  34   b  is rotated by the driving force from the motor M by a preset angle in the clockwise direction ( FIG. 8 ), the separation bar  334   b  moves rightward ( FIG. 8(   b )). As the separation bar  334   b  moves rightward, only the separation boss  3252   b  of the cartridge  32 K slides down the slanted surface  334   b   4 K, and engages with the engaging portion  334   b   5 K ( FIG. 8(   b )). 
     The above described movement of the separation boss  3252   b K allows the cartridge  32 K to move toward the photosensitive drum  311 K in such a manner that the positioning shaft  3251   b K of the cartridge  32 K follows the guiding groove  339   b K. As a result, the cartridge  32 K (more specifically, development roller  321 K) comes into contact with the photosensitive drum  311 K ( FIG. 8(   b )). 
     During this movement of the cartridge  32 K, the other cartridges  32 Y,  32 M, and  32 C remain in their positions, in which their separation bosses  3252   b  remain on the separation seat area  334   b   3 . Therefore, their development cartridges  32  (more specifically, development rollers  321 ) remain separated from the corresponding photosensitive drums  311  ( FIG. 8(   b )). 
     As the gear  34   b  is rotated further in the clockwise direction ( FIG. 8(   c )) by the driving force from the motor M, the separation bar  334   b  moves further rightward ( FIG. 8(   c )). This further rightward movement of the separation bar  334   b  causes the separation bosses  3252   b  of the other cartridges  32 Y,  32 M, and  32 C to slide down the slanted surfaces  334   b   4 , and engage with the engaging portions  334   b   5 Y,  334   b   5 M, and  334   b   5 C, respectively. ( FIG. 8(   c )). 
     The above described movement of the separation bosses  3252   b Y,  3252   b M, and  3252   b C allows the other cartridges  32 Y,  32 M, and  32 C to move toward the drums  311  in such a manner that their positioning shafts  3251   b  follow the corresponding guiding grooves  339   b , one for one. As a result, these cartridges  32 Y,  32 M, and  32 Y (more specifically, development rollers  321 Y,  321 M, and  321 C) also come into contact with the photosensitive drums  311 Y,  311 M, and  311 C, respectively. 
     As for the cartridge  32 K, its engaging portion  334   b   5 K and separation boss  3252   b K remain engaged with each other. Therefore, the development roller  321 K remains in contact with the photosensitive drum  311 K ( FIG. 8(   c )). 
     On the other hand, as the gear  34   b , which is in the state shown in  FIG. 8(   c ), is rotated in the counterclockwise direction ( FIG. 8)  by the driving force from the motor M, the separation bar  334   b  moves leftward, moving to the position shown in  FIG. 8(   b ), and then, to the position shown in  FIG. 8(   a ). 
     As described above, when it is necessary to move the development roller  321 , which is in its development position (in which it is in contact with photosensitive drum  311 ), away from the development position, the controlling means C rotates the gear  34   b  in the counterclockwise direction to move the separation bar  334  leftward so that the engaging portion  334   b   5  separates from the separation boss  3252   b . This separation of the engaging portion  334   b   5  from the separation boss  3252   b  allows the development roller  321  to move from the abovementioned development position (in which it is in contact with photosensitive drum  311 ) ( FIGS. 8(   c )→ 8 ( b )→ and  8 ( a )). Further, when it is necessary to move the development roller  321 , which is not in its development position (in which it is in contact with photosensitive drum  311 ), into the development position, the controlling means C moves the separation bar  334  rightward by rotating the gear  34   b  in the clockwise direction. This rightward movement of the separation bar  334  causes the engaging portion  334   b   5  to engage with the separation boss  3252   b . As a result, the development roller  321  is moved into the abovementioned development position ( FIG. 8(   a )→ 8 ( b )→ 8 ( c )). 
     As described above, the development roller  321  can be placed in contact with, or separated from, the photosensitive drum  311 , by controlling the rotation of the gear  34   b  by the controlling means C. When a user wants to form a color image, all that the user has to do is to place all the development rollers  321  in contact with the photosensitive drums  311 , one for one (as shown in  FIG. 8(   c )). 
     On the other hand, if a user wants to form only a black-and-white image, all that the user has to do is for the user to place the image forming apparatus  100  in the state shown in  FIG. 8(   b ). Placing the image forming apparatus  100  in the state shown in  FIG. 8(   b ) can prevent the photosensitive drums  311  and development rollers  321  other than those for forming a black-and-white image, from being unnecessarily worn. 
     Also as described above, in a case where the teeth of the gear  34   b  are not in mesh with the teeth of the separation bar  334   b , the separation bar  334   b  is moved into the state shown in  FIG. 8(   a ), because it is under the resiliency of the tension spring (elastic member)  3341  which continuously pulls the separation bar  334   b  in the leftward direction in  FIG. 8 . 
     That is, while the unit  33  is out of the main assembly  1 , the teeth of the gear  34   b  are not in mesh with the teeth of the rack portion of the separation bar  334   b . Therefore, all the development rollers  321  remain separated from the corresponding photosensitive drums  311 . Therefore, it does not occur that when the cartridge  31  and/or cartridge  32  is mounted or dismounted, the development roller  321  and/or photosensitive drum  311  is damaged by the contact between them. 
     &lt;Advantages of Image Forming Apparatus in This Embodiment&gt; 
     As described above, the image forming apparatus  100  in this preferred embodiment is structured so that the direction in which the cartridge  31  is mounted into, or dismounted from, the unit  33 , is perpendicular to the direction in which the cartridge  32  is mounted into, or dismounted from, the unit  33 . Therefore, each cartridge  31  and each cartridge  32  can be independently mounted into, or dismounted from, the unit  33 , from the other cartridges. 
     Further, as described above, in order to minimize in size the image forming apparatus  100  in this embodiment, the image forming apparatus  100  is structured so that a part of each cartridge  31  is below the adjacent cartridge  32 . In spite of this structural arrangement, the image forming apparatus  100  is not inferior to any of the conventional image forming apparatuses in terms of the ease and efficiency with which the cartridges  31  and  32  can be mounting or dismounted. 
     (Embodiment 2) 
     Next, referring to  FIGS. 9-17 , the image forming apparatus  200  in the second preferred embodiment of the present invention will be described. Any of the components, members, portions, etc., of the image forming apparatus  200  shown in  FIGS. 9-17 , which is the same in basic structure and function as that of the image forming apparatus  100  in the first preferred embodiment, is given the same numerical code or the like, and will not be described. Also in this embodiment, the referential letters “K”, “Y”, “M” and “C” are added, as color reference, to the referential codes. However, these referential letters may be left out at discretion. 
     First, referring to  FIG. 9 , the image forming apparatus  200  in the second preferred embodiment of the present invention will be described regarding its overall structure.  FIG. 9  is a schematic sectional view of the image forming apparatus  200 , and shows the overall structure of the apparatus  200 . 
     The image forming apparatus  200  is different from the image forming apparatus  100  in the first preferred embodiment, in that the laser unit  2  is below the combination of the sequentially positioned image forming portions  3 . Beams of laser lights LK, LY, LM, and LC are projected by the laser unit  2  upon the photosensitive drums  311 , with which the image forming portions  3  are provided, one for one. As a result, an electrostatic latent image, which reflects the information regarding the image to be formed, is formed on each of the photosensitive drums  311 . 
     The image forming apparatus  200  in this embodiment is also different from the image forming apparatus  100  in the first embodiment in that the transfer unit  4  for transferring the development images formed on the photosensitive drums  311 , onto the sheet  61 , is above the combinations of the image forming portions  3 . 
     Further, the image forming apparatus  200  in this embodiment is different from the image forming apparatus  100  in the first embodiment, in that the recovery unit  5  for recovering the developer t which remained adhered to the transfer belt  41  of the transfer unit  4  after the development image transfer, is above the transfer unit  4 . 
     Further, the image forming apparatus  200  in this embodiment is different from the image forming apparatus  100  in the first embodiment, in that the fixation unit for fixing the unfixed development image on the sheet  61  after the transfer of the unfixed image onto the sheet  61 , is located diagonally upward on the right side of the transfer unit  4  ( FIG. 9 ). Further, the discharging portion  8  for discharging the sheet  61  out of the main assembly  1  after the fixation of the developer image to the sheet  61  is above the fixation unit  7 . 
     The conveying portion  6 , image forming portion  3 , recovery unit  5 , fixation unit  7 , and discharging portion  8  of the image forming apparatus  200  in this embodiment are roughly the same in basic structure and function as those of the image forming apparatus  100  in the first embodiment, even though there are slight differences in their positioning and structure. Therefore, their detail descriptions will be left out. 
     &lt;Transfer Unit&gt; 
     Next, referring to  FIG. 9 , the transfer unit  4  will be described. 
     The transfer unit  4  in this preferred embodiment is different in structure from the above described transfer unit  4  in the first preferred embodiment. That is, the image forming apparatus  200  in this preferred embodiment is structured so that the four monochromatic developer images, different in color, formed on the multiple photosensitive drums  311 , one for one, are sequentially transferred in layers onto the transfer belt  41 , by the transfer unit  4 , yielding thereby a full-color image on the transfer belt  41 , and then, the four monochromatic toner images, of which the single full-color image is made up, are transferred all at once onto the sheet  61 . 
     Further, the transfer unit  4  has: a roller  45  positioned in a manner to oppose the recovery roller  51  of the recover unit  5 , with the presence of the transfer belt  41  between the roller  45  and recovery roller  51 ; and a secondary transfer roller  46  for transferring all at once the four developer images on the transfer belt  41 , onto the sheet  61 . 
     As a preset transfer bias is applied to the transfer roller  46  by a bias applying means (unshown), the transfer roller  46  transfers all at once the four developer images on the transfer belt  41 , onto the sheet  61 . 
     In the case of the first preferred embodiment, the direct transfer system was employed, which transfers the developer image on each photosensitive drum  311  directly onto the sheet  61 . In the case of this preferred embodiment, however, the indirect transfer system was employed, which transfers the monochromatic developer images, different in color, onto the transferred belt  41 , and then, transfers all at once the four developer images on the transfer belt  41 , onto the sheet  61  by the secondary transfer roller  46 . 
     As will be understood from  FIG. 9 , the employment of the indirect transfer system can reduce in length the conveyance passage for the sheet  61 . Thus, it has a merit in that it can reduce the length of time necessary for image formation, by the amount proportional to the amount by which the sheet conveyance passage is reduced. 
     &lt;Unitization of Components with Limited Service Life, and Consumables&gt; 
     Also in this preferred embodiment, each image forming portion  3  is made up of the drum cartridge  31  and development cartridge  32 . Next, referring to  FIGS. 10 and 11 , the image forming portion  3  in this embodiment will be described.  FIG. 10  is a cross-sectional view of one of the image forming portions in the second preferred embodiment, and shows the general structure of the image forming portion.  FIG. 11(   a ) is a perspective view of one of the development cartridges  32  in the second preferred embodiment.  FIG. 11(   b ) is a perspective view of one of the drum cartridges  31  in the second preferred embodiment. 
     The frame  325  of the cartridge  32  is provided with a pair of cartridge guides  3253   a  and  3253   b , that is, the portions by which the cartridge  32  is guided when the cartridge  32  is mounted or dismounted. Further, the cartridge  32  is provided with a cartridge positioning rear boss (by which cartridge is positioned)  3254 , and a pair of cartridge positioning front holes (by which cartridge is precisely positioned)  3255   a  and  3255   b  ( FIG. 11(   a )). 
     Further, the cartridge  31  is provided with a pair of cartridge supporting shafts (by which cartridge is guided and positioned)  3144   a  and  3144   b  ( FIG. 11(   b )). 
     &lt;Tray Unit&gt; 
     Next, referring to the appended drawings, in particular,  FIGS. 12 and 13 , the tray unit (supporting member)  35  will be described.  FIG. 12  is a schematic sectional view of the image forming apparatus in the second embodiment, when the supporting member of the image forming apparatus is at its outermost position relative to the main assembly of the apparatus.  FIG. 13  is a perspective view of the tray unit in the second preferred embodiment of the present invention. Incidentally, the unit  35  is the same as the unit  33  in the first preferred embodiment, except for the structural portions which will be described next. 
     The tray unit (supporting member)  35  in this embodiment has a tray frame  351 , and a pair of handholds  352  and  353 , which are to be grasped by a user when the unit  35  is moved by the user relative to the main assembly  1 . The handhold  352  is provided with a pair of tray guiding bosses  352   a  and  352   b  (by which tray unit is precisely positioned), which are at the lengthwise ends of the handhold  352 , one for one. The handhold  353  is provided with a pair of tray guiding bosses  353   a  and  353   b  (by which tray unit is precisely positioned), which are at the lengthwise ends of the handhold  353 , one for one ( FIG. 13 ). 
     As for the main assembly  200   a , it is provided with a pair of inner walls  203 , as is the main assembly  1  in the first preferred embodiment is provided with the pair of inner walls  201 . The pair of inner walls  203  oppose each other across the space in which the unit  35  is when the unit  35  is in its innermost position IP. The inner walls  203  are provided with a pair of tray guiding rails  201  and  202 , respectively, which guide the tray unit  35  when the unit  35  is moved from its innermost position IP to its outermost position OP ( FIG. 12 ), or from the outermost position OP to the innermost position IP. Further, the guide rails  201  and  202  are provided with a pair of tray positioning portions  201   a  and  202   a  (of unit  35 ). Further, a cover  11  is provided with a tray contacting portion  11   a  for keeping the unit  35  precisely positioned in the innermost position IP ( FIG. 12 ). 
     If a user wants to move the unit  35  from its outermost position OP to its innermost position, the user is to move the unit  35  by grasping the handhold  352 . As the unit  35  is moved toward the innermost position IP, the guiding bosses  353   a  and  352   a  of the unit  35  engage with the guide rails  201  and  202 , respectively, of the main assembly  200   a , whereby the unit  35  is regulated in movement. Therefore, it is ensured that the unit  35  is precisely moved. As the user moves the unit  35  to the innermost position IP, the pair of guide bosses  352   a  and  353   a  reach the pair of tray positioning portions  202   a  and  203   a , respectively. Then, the closing of the cover  11  causes the tray contacting portion  11   a  to come into contact with the guide bosses  352   a , whereby the unit  35  is precisely positioned in the innermost position IP ( FIG. 9 ). 
     Further, the unit  35  is provided with a separation bar  354 , which is one of the structural components of the means for placing each development roller  321  in contact with, or separating each development roller  321  from, the corresponding photosensitive drum  311 . 
     Further, the unit  35  is provided with a pair of drum cartridge guides  355   a  and  355   b , which are a pair of grooves for guiding the cartridge  31  when the cartridge  31  is mounted into, or dismounted from, the unit  35 . 
     Further, the unit  35  is provided with four development cartridge accommodating openings  356 , through which the cartridges  32  are inserted into, or pulled out of, the unit  35 . The unit  35  is also provided with four pairs of guide rails  357   a  and  357   b , which guide the cartridges  32  when the cartridges  32  are mounted into the unit  35 . 
     Further, the unit  35  is provided with a means (mechanism)  358  for placing each cartridge  32  (development roller  321 ) in contact with, or separating each cartridge  32 , from the corresponding photosensitive drum  311 . 
     At this time, referring to  FIG. 14 , the means  358  for placing each cartridge  32  in contact with, or separating each cartridge  32  from, the photosensitive drum  311 , will be described. Hereafter, the means  358  may be referred to as a cartridge moving means  358 .  FIG. 14  is a perspective view of the cartridge moving means mechanism (cartridge moving means) in the second preferred embodiment of the present invention. The four portions of the cartridge moving means  358  are the same in structure. Therefore, the cartridge moving means portion for the cartridges  31 C and  32 C will be described as an example of the four portions. 
     The essential components of the cartridge moving means  358  are an oscillatory rear cam  3581 , an oscillatory front cam  3582 , and a shaft  3583 . The shaft  3583  connects the oscillatory rear cam  3581  to the oscillatory front cam  3582 , and its axial line coincides with the rotational axes of the oscillatory rear and front cams  3581  and  3582 . 
     The cam  3581  is provided with a cartridge positioning rear hole  35811 , which engages with the cartridge positioning rear boss  3254  of the cartridge  32 . The cam  3582  is provided with a pair of cartridge positioning front bosses  35821   a  and  35821   b , which engage with a pair of cartridge positioning front holes  3255   a  and  3255   b  of the cartridge  32 . Further, the cam  3582  is provided with a contact portion  35822 , which engages with a separation bar pin  3541  ( FIG. 13 ) with which the separation bar  354  is provided. This structural arrangement is the same for all of the other pairs of cartridges  31  and  32 . 
     &lt;Mounting of Cartridges into Tray Unit, and Dismounting of Cartridges from Tray Unit&gt; 
     Next, referring to  FIGS. 11(   a ) and  11 ( b ), and  FIGS. 13-16 , the method for mounting each cartridge  32  and each cartridge  31  into the unit  35 , and the method for dismounting each cartridge  32  and each cartridge  31  from the unit  35 , will be described. 
     When a user wants to insert a cartridge  32  into the unit  35  (supports cartridge  32  by unit  35 ), the user is to insert the cartridge  32  into the unit  35  through the opening  356 , so that the lengthwise end portion of the cartridge  32 , which has the boss (cartridge positioning portion)  2354 , enters the unit  35  first. It is important that the cartridge  32  is inserted into the unit  35  in the direction parallel to the axial line (lengthwise direction) of the development roller  321  while the cartridge  32  is held in such a manner that the pair of guides (by which cartridge is guided)  3253   a  and  3253   b  align with the guide rails (cartridge guiding portions)  357   a  and  357   b , respectively. 
     As the cartridge  32  is inserted far enough into the deepest end of the unit  35  for the leading end of the cartridge  32  reach the rear end of the unit  35 , the boss  3254  fits into the cartridge positioning rear hole (cartridge positing portion of unit  35 )  35811 , with which the cam  3581  is provided. During this movement of the cartridge  32 , the bosses  35821   a  and  35821   b , with which the cam  3582  is provided, fit into the holes  3255   a  and  3255   b  of the cartridge  32 , respectively, which ends the mounting of the cartridge  32  into the unit  35 ; that is, the cartridge  32  is fully supported by the unit  35  ( FIG. 15(   b )). 
     Then, the unit  35  is to be moved from its outermost position OP to its innermost position IP while the cartridges  32  remain mounted in the unit  35 . As the unit  35  is moved into its innermost position IP, the development cartridge frame (cartridge positioning portion of development cartridge  32 )  325  comes into contact with the inner walls (cartridge positioning portion of main assembly  200   a )  203 , whereby the cartridge  32  is precisely positioned relative to the main assembly  200   a  in terms of its lengthwise direction. 
     If a user wants to take any of the cartridges  32  out of the unit  35 , all that the user has to do is to pull the cartridge  32  in the direction perpendicular to the axial line of the development roller  321 , through the opening  356 , after moving the unit  35  into its outermost position OP. 
     Incidentally,  FIG. 15(   a ) shows the cartridge  32 M, as an example of cartridge  32  which is being mounted into, or removed from, the unit  35 . 
     On the other hand, if a user wants to mount any of the cartridges  31  into the unit  35  (support cartridge with unit  35 ), the first step for the user to take is to align the supporting shafts (portions by which drum cartridge  31  is guided and positioned)  3144   a  and  3144   b , with which the lengthwise ends of the cartridge  31  are provided, one for one, with the drum cartridge guides (drum cartridge guiding portions)  355   a  and  355   b , respectively. Then, the user is to mount the cartridge  31  into the unit  35  so that the supporting shafts  3144   a  and  3144   b  follow the pair of guides  355   a  and  355   b , respectively. The mounting of the cartridge  31  into the unit  35  makes the cartridge  31  fully supported by the unit  35  while remaining precisely positioned relative to the unit  35  ( FIG. 15(   b )). 
     That is, the user is to move the cartridge  31  vertically downward toward the unit  35  from above the unit  35  so that the cartridge  31  snugly falls into the unit  35 . As the cartridge  31  snugly falls into the unit  35 , it is fully supported by the unit  35  while being precisely positioned relative to the unit  35  ( FIG. 15(   b )). If the user wants to take any of the cartridges  31  out of the unit  35 , all that is necessary for the user to do is to move the cartridge  31  vertically upward, after moving the unit  35  into its outermost position OP. 
     Incidentally,  FIG. 15(   a ) depicts the cartridge  31 Y, as an example of cartridge  31 , which is being mounted into, or removed from, the unit  35 . 
     &lt;Direction in Which Cartridge is Mounted into Tray Unit, and Direction in Which Cartridge is Dismounted from Tray Unit&gt; 
     As described above, in this embodiment, the image forming apparatus  200  is structured so that each of the cartridges  31  and  32  can be independently mounted into, or removed from, the unit  35 , from the other cartridges. Further, the image forming apparatus  200  is structured so that each cartridge  31  is paired with the corresponding cartridge  32 , and multiple (four) pairs of cartridge  31  and  32  are aligned in parallel in the directions (indicated by arrow marks Z 1  and Z 2 ) in which the unit  35  is movable; each pair of cartridges  31  and  32  are supported by the unit  35  so that their lengthwise directions are intersectional (perpendicular) to the abovementioned moving directions Z 1  and Z 2  of the unit  35 . Each cartridge  31  and each cartridge  32  are supported by the unit  35  so that the lengthwise direction of each cartridge  31  and the lengthwise direction of each cartridge  32  are intersectional (perpendicular) to the directions Z 1  and Z 2 , in which the unit  35  is moved ( FIGS. 15(   a ) and  15 ( b )). 
     Further, the image forming apparatus  200  (unit  35 ) is structured so that the direction in which each cartridge  31  is mounted into, or removed from, the unit  35 , is different from the direction in which each cartridge  32  is mounted into, or removed from, the unit  35 . That is, the image forming apparatus  200  (unit  35 ) is structured so that the direction in which each cartridge  31  is mounted into, or removed from, the unit  35  is, perpendicular to the direction in which each cartridge  32  is mounted into, or removed from the unit  35 . Further, the image forming apparatus  200  (unit  35 ), each cartridge  31 , and each cartridge  32  are structured so that each cartridge  31  and  32  can be independently mounted into, or removed from, the unit  35 , from the other cartridges. More concretely, the image forming apparatus  200  (unit  35 ) is structured so that each cartridge  32  is to be mounted or dismounted in the direction which is roughly vertical and is perpendicular to the axial line of the development roller  321 , whereas, each cartridge  31  is to be horizontally mounted or dismounted in the direction parallel to the axial line of the photosensitive drum  311 . It should be noted here that while any pair of cartridges  31  and  32  remains properly situated (supported) in the unit  35 , the axial lines of the photosensitive drum  311  and development roller  321  in the cartridge are parallel to each other. 
     Further, as described above, the image forming apparatus  200  is structured so that while the main assembly  200   a  is on a horizontal surface, the unit  35  is horizontally movable, and also, so that the direction in which each cartridge  32  is mounted into, or dismounted from, the unit  35 , is horizontal, and is perpendicular to the direction in which the unit  35  is movable. 
     With the provision of the above described structural arrangement, while the main assembly  200   a  remains positioned on a horizontal surface, the moving direction of the unit  35  is perpendicular to the direction in which each cartridge  31  is mounted into, or dismounted from, the unit  35 . The moving direction of the unit  35  is roughly perpendicular to the direction in which each cartridge  32  is mounted into, or dismounted from, the unit  35 . 
     In order to minimize in size the main assembly  200   a  of the image forming apparatus  200 , the image forming apparatus  200  in this embodiment is structured so that while the main assembly  200   a  remains on a horizontal surface, each cartridge  32  and the corresponding cartridge  31  partially overlap each other in terms of the vertical direction. 
     Next, referring to  FIG. 16 , the abovementioned setup will be described.  FIG. 16  shows the cartridge  31 C and  31 K. The cartridge  32 K is extending beyond the area sandwiched by two lines L 3  and L 4 , that is, the maximum gap between the cartridges  31 C and  31 K, by its hatched portions a 3  and a 4  in the drawing. In other words, the image forming apparatus  200  is structured so that the cartridge  32 K fits into the space between the bottom portions of the adjacent two cartridges  31 K and  31 C. 
     That is, while the two cartridges  31  and  32  remain properly mounted (supported) in the unit  35 , a part of the cartridge  32  remains under the cartridge  31 , reducing in size the space for supporting (mounting) the cartridges  31  and  32 . Thus, this setup can reduce in size the main assembly  200   a.    
     Even though the image forming apparatus  200  (unit  35 ) in this embodiment is structured so that while the cartridges  31  and  32  remain properly supported in the unit  35 , a part of each cartridge  32  is positioned below the corresponding cartridge  31 , the cartridges  31  and  32  are not affected in terms of the efficiency with which they can be mounted or dismounted, because the image forming apparatus  200  (unit  35 ) in this embodiment is also structured so that the direction in which each cartridge  31  is mounted or dismounted is perpendicular to the direction in which each cartridge  32  is mounted or dismounted. The employment of this structural arrangement makes it possible for each of the cartridges  31  and  32  to be independently mounted into, or dismounted from, the unit  35 , from the other cartridges. In other words, it is possible to exchange only the cartridge which needs to be replaced. 
     &lt;Mechanism for Placing Development Roller in Contact with Photosensitive Drum, and Separating Development Roller from Photosensitive Drum&gt; 
     Next, referring to  FIGS. 9 ,  14 , and  17 , the means (mechanism) (which hereafter will be referred to as development roller moving means (mechanism)) for placing the development roller in contact with, or separating from, the photosensitive drum, will be described.  FIGS. 17(   a ),  17 ( b ), and  17 ( c ) are drawings for describing the working of the development roller moving means in the preferred embodiment of the present invention, and are side views of the unit  35 , as seen from the side where the openings  356  are present. 
     Referring to  FIG. 14 , a pair of cams  3581  and  3582  are connected to each other with an oscillatory shaft  3583 , and are attached to the unit  35  so that they can be oscillatory moved. The cartridge  32  is supported by the pair of cams  3581  and  3582 , as described above. Thus, the cartridge  32  is supported so that it can be oscillatory rotated about the axial line of the oscillatory shaft  3583 . Further, the cam  3581  is provided with a torsional coil spring (elastic member)  35812 , which is disposed within the cam  3581 , as shown in  FIG. 14 . 
     On the other hand, the main assembly  200   a  is provided with a separation gear  34 , as shown in  FIG. 9 . Further, the image forming apparatus  200  is structured so that as the unit  35  is moved into its innermost position IP in the main assembly  200   a , the rack portion  3542 , with which one of the lengthwise ends of the separation bar  354  is provided, meshes with the separation gear  34 . 
     Also in this embodiment, the separation gear  34  is rotatable by a driving means M, only by a preset angle, as was the separation gear in the first preferred embodiment. The rotation of the gear  34  is controllable by a controlling means C, making it possible to move the separation bar leftward or rightward in  FIG. 17 . 
       FIG. 17  shows the various positional relationships among the cartridge moving means  358 , cartridge  32 , and photosensitive drum  311 .  FIG. 17(   a ) shows the combination of the unit  35  and cartridges  31  and  32 , when all the development rollers  321  of all the cartridges  32  are remaining separated from the corresponding photosensitive drums  311 .  FIG. 17(   b ) shows the combination of the unit  35  and cartridges  31  and  32 , when only the cartridge  32 K is in its development position, in which it remains in contact with the corresponding photosensitive drum  311 , that is, the drum  311 K.  FIG. 17(   c ) shows the combination of the unit  35  and cartridges  31  and  32 , when all the cartridge  32  are in contact with the corresponding photosensitive drums  311 , that is, all the development rollers  321  are in their development positions in which they remain in contact with the corresponding drums  311 , one for one. 
     Referring to  FIG. 17(   a ), right after the complete insertion of the unit  35  into the main assembly  200   a , all the cartridges (development roller  321 ) remain separated from the corresponding photosensitive drums  311 , because the cam  3581  is kept pressured in the counterclockwise direction by the resiliency of the spring (elastic member)  35812 . 
     Then, as the gear  34  is rotated in the counterclockwise direction ( FIG. 17 ), the separation bar  354  is moved leftward. As the separation bar  354  is moved leftward, first, the separation bar pin  3541 K, which corresponds to the cartridge  32 K, comes into contact with the contact portion  358  of the cam  3582 K. Then, as the separation bar is moved further leftward, the separation bar pin  3541 K moves upward, causing thereby the cam  3582 K and oscillatory shaft  3583 K to rotate clockwise about the axial line of the oscillatory shaft  3583 K. Thus, the cam  3581 K attached to the oscillatory shaft  3583 K also rotates clockwise. As a result, the development cartridge  32 K rotates clockwise about the axial line of the oscillatory shaft  3583 K. 
     Thus, the cartridge  32 K (more specifically, development roller  321 K) comes into contact with the photosensitive drum  311 K. That is, the development roller  321 K moves into its development position, as shown in  FIG. 17(   b ). 
     During this movement of the cartridge  32 K, the pins  3541  other than the pin  3541  which corresponds to development cartridge  32 K are not in contact with the corresponding cams  3582 . Therefore, the development rollers  321  of these cartridges  32  remain separated from the corresponding photosensitive drums  311 . 
     As the gear  34  is rotated further, the bar  354  is moved further leftward, causing the pins  3541  which correspond to the cartridges  32 Y,  32 M, and  32 C to come into contact with the contact portions  35822 , causing thereby the cams  3582  to rotate clockwise ( FIG. 17 ). As a result, the cartridges  32 Y,  32 M, and  32 Y are rotationally moved in the clockwise direction about the axial lines of the oscillatory shafts  3583 . 
     Consequently, the cartridges  32 Y,  32 M, and  32 C also come into contact with the corresponding photosensitive drums  311 , one for one, as shown in  FIG. 17(   c ). That is, the development rollers  32 , which these cartridges  32  have, come into contact with the photosensitive drums  311 , one for one. Since the pin  3541 K of the cartridge  32 K remains in contact with the contact portion  35822 K, the cartridge  32 K remains in contact with the photosensitive drum  311 K. That is, the development roller  321 K, which the cartridge  32 K has, remains in contact with the photosensitive drum  311 K. 
     Further, as the gear  34  is rotated clockwise ( FIG. 17 ) while the combination of the unit  35  and cartridges  32  and  31  are in the state shown in  FIG. 17(   c ), the bar  354  is moved rightward by the rotation of the gear  34 . Thus, the state of the combination changes to the state shown in  FIG. 17(   b ), and then, to the state shown in  FIG. 17(   a ). 
     As described above, the development roller  321 , which each cartridge  32  has, is placed in contact with, or separated from, the corresponding photosensitive drum  311  by controlling the rotation of the gear  34  with the use of the controlling means C. If a user wants to form a color image, all that is necessary for the user to do is to put the combination of the unit  35  and cartridges  32  and  31  in the state shown in  FIG. 17(   c ). 
     On the other hand, if a user wants to form only black-and-white images, the user has only to put the combination in the state shown in  FIG. 17(   b ). Placing only the cartridge  32 K (development roller  321 K) in contact with the photosensitive drum  311  to prevent the photosensitive drums  311 , which the cartridges  31 Y,  31 M, and  31 C have, and the development rollers  321 , which the cartridges  32 Y,  32 M, and  32 C have, from being unnecessarily worn. 
     Also as described above, in a case where the teeth of the gear  34  are not in mesh with the rack portion of the bar  334 , the bar  334  is kept in its rightmost position by the resiliency of the spring  35812 , as shown in  FIG. 17(   a ). 
     That is, while the unit  33  is completely out of the main assembly  200   a , the teeth of the gear  34  are not in mesh with the rack portion of the bar  334 . Therefore, all the development rollers  321  remain separated from the corresponding photosensitive drums  311 . Therefore, it does not occur that when the cartridge  31  and/or cartridge  32  is mounted or dismounted, the development roller  321  and/or photosensitive drum  311  is damaged by the contact between them. 
     In each of the above described preferred embodiments, the development position is the position of the development roller  321 , in which the development roller is in contact with the corresponding photosensitive drum  311 , whereas the abovementioned state of separation is the state in which a development roller  321  is not in contact with a photosensitive drum  311 . However, these definitions are not intended to limit the present invention in scope. For example, the development position may be such a development roller position that makes smallest the distance between the development roller and photosensitive drum. In such a case, the state of separation means the state in which the distance between the development roller and photosensitive drum is significantly larger than the distance between the development roller and photosensitive drum when the development roller is in its development position. In other words, the present invention is satisfactorily applicable to a non-contact development system, as well as a contact development system. 
     &lt;Advantages of Image Forming Apparatus in This Embodiment&gt; 
     As described above, the image forming apparatus in this preferred embodiment is structured so that the direction in which the cartridge  31  is mounted into, or dismounted from, the unit  33 , is perpendicular to the direction in which the cartridge  32  is mounted into, or dismounted from, the unit  33 . Therefore, each cartridge  31  and each cartridge  32  can be independently mounted into, or dismounted from, the unit  35 , from the other cartridges. 
     Further, as described above, in this embodiment, in order to minimize in size the main assembly  200   a , the image forming apparatus  200  is structured so that while the cartridges  31  and  32  remain supported by (mounted in) the unit  35 , a part of each cartridge  32  is below the adjacent cartridge  31 . In spite of this structural arrangement, the image forming apparatus  200  is not inferior to any of the conventional image forming apparatuses in terms of the ease with which cartridges  31  and  32  can be mounted or dismounted. 
     (Miscellanies) 
     In each of the above described preferred embodiments of the present invention, the cartridges  31  and  32  are removably mountable in the main assembly  1  or  200   a . Further, each of the tray units  33  and  35  is precisely positioned in its innermost position IP in the main assembly  1  or  200   a  while supporting the cartridges  31  and  32 , whereby the cartridges  31  and  32  are precisely positioned in their image forming positions, in which they contribute to image formation. On the other hand, if a user wants to take any of the cartridges  31  and  32  out of the main assembly  1  or  200   a , the user is to move the tray unit  33  or  35 , which is in its innermost position IP in the main assembly  1  or  200   a  and is supporting the cartridges  31  and  32 , out of the main assembly  1  or  200   a . In other words, if it is necessary for any of the cartridges  31  and  32  in the main assembly  1  or  200   a , to be replaced, the operation for the removal of the cartridge to be replaced from the tray unit  33  or  35 , and the operation for mounting a replacement cartridge into the tray unit  33  or  35 , are to be carried out after the tray unit  33  or  35  is pulled out of the main assembly  1  or  200   a.    
     Strictly speaking, the “outermost position” in each of the above-described preferred embodiments does not need to be literally outermost position; it may be any position (of the unit  33  or  35 ) outside the main assembly  1  or  200   a . It does not necessary mean the tray position that exposes the entirety of the tray  33  or  35 . All that it means is a tray position in which the tray units  33  or  35  is out of the main assembly  1  or  200   a , respectively, far enough for the cartridge(s)  31  and/ 32  in the tray units  33  or  35  to be replaced. 
     For example, referring to  FIG. 5 , the cartridge  32 C supported by the most upstream portion of the unit  33 , in terms of the direction (indicated by arrow mark Z 2 ) in which the unit  33  is pulled out, may be on the inward side of the main assembly  1  relative to the opening  1   a , for the following reason. That is, as long as the image forming apparatus  100  is structured so that as the cover  11  is opened, the portion of the main assembly  1 , which is above the opening  1   a , is exposed, a user can easily replace the cartridge  32 C, because, when the tray unit  33  is in the above described position, the cartridge  32   c  is outward of its image forming position in the image forming apparatus  100 , that is, it is on the front side of the image forming position, relative to the main assembly  1 . As described above, the “outermost position” does not need to be such a position that when the unit  33  or  35  is in the outermost position, the entirety of the unit  33  or  35  is out of the main assembly  1  or  200   a . Needless to say, it is preferable that the image forming apparatus is structured so that when the unit  33  is in its outermost position, even the cartridges  31 C and  32 C, which are supported by the most upstream portion of the unit  33 , in terms of the direction (indicated by arrow mark Z 2 ) in which the unit  33  is pulled out, are on the outward side of the opening  1   a.    
     Further, in the above described preferred embodiments, the unit  33  or  35  is moved in a straight line and in parallel to the surface on which the main assembly  1  or  200   a  was placed. However, the preferred embodiments are not intended to limit the present invention in scope. For example, an image forming apparatus may be structured so that the unit  33  or  35  moves in a diagonally upward or downward in a straight line relative to the surface on which the main assembly is placed. Further, in the above described preferred embodiments, the image forming apparatus was structured so that the cartridges  31  and  32  are supported by the units  33  or  35  in such a manner that the lengthwise direction of the cartridge  31  and that of the cartridge  32  are intersectional (perpendicular) to the moving directions (Z 1  and Z 2 ) of the unit  33  and  35 . However, these embodiments are not intended to limit the present invention in scope. For example, the structural arrangement that makes the cartridges  31  and  32  supported by the unit  33  or  35  in such a manner that the abovementioned lengthwise direction of the cartridges  31  and  32  is parallel to the moving direction of the unit  33  and  35  may be employed. Further, it is not mandatory that the image forming apparatus is structured so that the unit  33  or  35  is linearly moved. For example, the image forming apparatus may be structured so that the unit  33  or  35  is placed in the bottom portion of the main assembly  1  (or  200   a ) and is rotatable about the rotational axis of the unit  33  or  35 . In the case of such a structural arrangement, the outermost position OP is the position outside the main assembly  1  ( 200   a ), into which the unit  33  (or  35 ) is moved out by being rotated about the abovementioned rotational axis. The innermost position IP is the position in the main assembly  1  (or  200   a ), in which the entirety of the unit  33  (or  35 ) is out of the main assembly  1  (or  200   a ). Also in the case of the image forming apparatus having the above described rotationally movable unit  33  (or  35 ), the image forming apparatus may be structured so that even when the unit  33  (or  35 ) is in its innermost position, a part of the unit  33  (or  35 ) is outside the main assembly  1  (or  200   a ). 
     As described above, each of the above described preferred embodiments of the present invention made it possible for each of the drum cartridges  31  and each of the development cartridges  32  to be removably and independently supportable by the tray unit (supporting member)  33  or  35 , from the other cartridges  31  and  32 . Further, each of the above described preferred embodiments made it possible to improve an electrophotographic image forming apparatus in the operational efficiency with which each of the drum cartridges  31  and each of the development cartridges  32  are replaceable. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Applications Nos. 250447/2008 and 104387/2009 filed Sep. 29, 2008 and Apr. 22, 2009, respectively, which are hereby incorporated by reference.