Abstract:
An image forming apparatus, includes: a mainframe; a photoreceptor; an exposing unit that exposes a surface of the photoreceptor to form an electrostatic latent image; and a developing unit that develops the electrostatic latent image by supplying a charged developer on the surface of the photoreceptor having the electrostatic latent image formed thereon. The photoreceptor is loadable in and unloadable from the mainframe separately from the developing unit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus for forming an image, such as a color laser printer. 
   2. Background Art 
   As an apparatus for forming a multicolor image, such as a four-color image, on a recording medium, such as recording paper, a tandem type apparatus has been known, which has, for each of colors, a photoreceptor, an exposing unit for exposing the photoreceptor to form an electrostatic latent image on the surface thereof, and a developing unit for developing the electrostatic latent image by attaching a charged developer on the surface of the photoreceptor having the electrostatic latent image formed thereon. A four-cycle type apparatus has also been known, which has one pair of an exposing unit and a photoreceptor, and plural developing unit for the respective colors arranged around the photoreceptor. 
   The four-cycle type apparatus is not suitable for speeding up the image formation because the exposing and developing steps of the photoreceptor are sequentially carried out with changing colors. In the tandem type apparatus, on the other hand, the exposing and developing steps of the photoreceptors can be simultaneously carried out for the colors, and a multicolor image can be formed by sequentially transferring the developers attached on the photoreceptors corresponding to the respective colors on a recording medium. Therefore, the tandem type apparatus for forming a multicolor image exhibits a color image forming speed equivalent to the case of a monochrome image, and is suitable for high-speed image formation. 
   SUMMARY OF THE INVENTION 
   In the apparatus for forming an image, the photoreceptor and the developing unit should be replaced as needed. In the tandem type apparatus for forming a multicolor image, plural pairs of a exposing unit and a photoreceptor are provided corresponding to the respective colors, and therefore, it is necessary to prevent the photoreceptor and the developing unit (which are, in some cases, integrally configured as an exchangeable process cartridge) from interfering with the exposing unit upon replacement. Under the circumstances, it has been proposed that the exposing unit is evacuated to prevent from interfering upon replacement of the process cartridge (as described, for example, in JP-A-2001-166555). 
   In the case where the exposing unit is evacuated upon replacement of the process cartridge, the relative positions of the exposing unit of the respective colors are slightly deviated to cause a possibility of color drift. However, it is difficult in the configuration of the conventional apparatus for forming a multicolor image that the process cartridge is replaced without evacuation of the exposing unit. In particular, it is significantly difficult that the photoreceptor is replaced without evacuation of the exposing unit because the photoreceptor is disposed in the back of the apparatus for forming a multicolor image as viewed from the user. 
   Further, the photoreceptor drums of the tandem type color laser printer are required to have an extremely high dimensional accuracy for preventing color drift because images of the respective colors are transferred from the four different photoreceptor drums onto the single transfer medium. Therefore, the processing of the photoreceptor drums brings about high cost. 
   As described, for example, in JP-A-10-48898, a tandem type color laser printer may have image forming units for each of colors, which integrally retains a toner cartridge, a photoreceptor drum and a developing roller, and the whole of the image forming unit is replaced in the case where the toner is emptied out. 
   However, in the case where the whole of the image forming unit is replaced, the expensive photoreceptor drum is also replaced when the toner is emptied out, so as to increase the running cost and to increase the amount of industrial waste. 
   According to an aspect of the invention, an image forming apparatus may include: a mainframe; a photoreceptor; an exposing unit that exposes a surface of the photoreceptor to form an electrostatic latent image; and a developing unit that develops the electrostatic latent image by supplying a charged developer on the surface of the photoreceptor having the electrostatic latent image formed thereon. The photoreceptor is loadable in and unloadable from the mainframe separately from the developing unit. 
   According to this aspect, the surface of the photoreceptor is exposed with the exposing unit to form an electrostatic latent image, and the latent image is developed by attaching the charged developer to the surface of the photoreceptor with the developing unit. The developers thus developing the electrostatic latent image on the surface of the photoreceptor provided is transferred to the recording medium to form an image on the recording medium. The apparatus of this aspect can be easily subjected to speeding up owing to the tandem type system. 
   The invention may provide a photoreceptor cartridge to be loaded in an image forming apparatus, wherein the image forming apparatus includes a developing unit that develops an electrostatic latent image by supplying a charged developer. The photoreceptor cartridge includes a photoreceptor having a surface on which the electrostatic latent image is formed to be developed by the developing unit. The photoreceptor cartridge is loadable in and unloadable from a mainframe of the image forming apparatus while being separated from the developing unit. 
   Preferably, the photoreceptor includes a plurality of photoreceptors for a plurality of colors, which are integrally retained. According to this configuration, the photoreceptor cartridge integrally retains the photoreceptors for each of colors, and is detachable from the mainframe as being separately from the developing unit. Due to this, the photoreceptors can be detached without interference with the exposing unit of the image forming apparatus as being separated from the developing unit. Therefore, the photoreceptor can be easily replaced without movement of the exposing unit, and thus, occurrence of color drift in the image forming apparatus can be fairly prevented. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be more readily described with reference to the accompanying drawings: 
       FIG. 1  is a schematic cross sectional view showing a laser color printer according to an embodiment of the invention. 
       FIGS. 2A and 2B  are a side view and a perspective view, respectively, showing a configuration of a supporting member of the printer. 
       FIG. 3  is an elevational view showing a configuration of a photoreceptor cartridge of the printer. 
       FIGS. 4A and 4B  are perspective views showing a detailed configuration of the supporting member. 
       FIG. 5  is an elevational view showing the photoreceptor cartridge before mounting. 
       FIG. 6  is an elevational view showing a modified embodiment of the photoreceptor cartridge. 
       FIG. 7  is a cross sectional view showing another embodiment of a color laser printer as the apparatus for forming an image. 
       FIG. 8  is a cross sectional view showing the color laser printer shown in  FIG. 7  in a state where a front cover thereof is opened. 
       FIG. 9  is a cross sectional view showing the color laser printer shown in  FIG. 7  in a state where a rear cover thereof is opened. 
       FIG. 10  is an enlarged cross sectional view showing a process part of the laser color printer shown in  FIG. 7  (in a state where drum cartridges corresponding to a yellow developing cartridge and a magenta developing cartridge are mounted). 
       FIG. 11  is a cross sectional view showing another embodiment of a color laser printer as the apparatus for forming an image. 
       FIG. 12  is a cross sectional view showing another embodiment of a color laser printer as the apparatus for forming an image. 
       FIG. 13  is a cross sectional view showing another embodiment of the color laser printer shown in  FIG. 12  in a state where an openable cover thereof is opened. 
       FIG. 14  is an enlarged cross sectional view showing a process part of the laser color printer shown in  FIG. 12  (in a state where only a drum cartridge corresponding to a black developing cartridge is mounted). 
       FIG. 15  is a schematic side view showing a positional relationship of photoreceptor drums, developing rollers and transfer rollers. 
       FIG. 16  is a cross sectional view showing another embodiment of a color laser printer as the apparatus for forming an image. 
       FIG. 17  is a cross sectional view showing the color laser printer shown in  FIG. 16  in a state where an openable cover thereof is opened. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The First Preferred Embodiment 
   Embodiments of the invention will be described with reference to the drawings.  FIG. 1  is a schematic cross sectional view showing a color laser printer  1  as an apparatus for forming a multicolor image, to which the invention is applied. The color laser printer  1  shown in  FIG. 1  has a visual image forming part  4 , an intermediate transfer material in a belt form  5 , a fixing part  8 , a paper feeding part  9  and a paper delivery tray  10   b.    
   The visual image forming part  4  has, for each of visual image forming steps with respective toners of magenta (M), cyan (C), yellow (Y) and black (Bk), developing devices  51 M,  51 C,  51 Y and  51 Bk as a developing unit, photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk as a photoreceptor, cleaning rollers  70 M,  70 C,  70 Y and  70 Bk as a cleaning unit, charging devices  71 M,  71 C,  71 Y and  71 Bk as a charging unit, and exposing unit  72 M,  72 C,  72 Y and  72 Bk. 
   These configurational elements will be described in more detail below. The developing devices  51 M,  51 C,  51 Y and  51 Bk have developing rollers  52 M,  52 C,  52 Y and  52 Bk, respectively. The developing rollers  52 M,  52 C,  52 Y and  52 Bk each is configured in the form of a cylinder with electroconductive silicone rubber as a base material having formed on the surface thereof a coated layer of a resin or rubber material containing fluorine. The developing rollers  52 M,  52 C,  52 Y and  52 Bk may not be configured with electroconductive silicone rubber but may be configured with electroconductive urethane rubber. The ten-point average roughness (Rz) of the surface thereof is from 3 to 5 μm, which is smaller than 9 μm, the average particle diameter of the toner. 
   The developing devices  51 M,  51 C,  51 Y and  51 Bk have feeding rollers  53 M,  53 C,  53 Y and  53 Bk, respectively. The feeding rollers  53 M,  53 C,  53 Y and  53 Bk are electroconductive sponge rollers, which are in contact with the developing rollers  52 M,  52 C,  52 Y and  52 Bk, respectively, under pressure with an elastic force of sponge. The feeding rollers  53 M,  53 C,  53 Y and  53 Bk may be configured with a foamed body of a suitable material, such as electroconductive silicone rubber, EPDM and urethane rubber. 
   The developing devices  51 C,  51 Y,  51 Y and  51 Bk have squeezing blades  54 M,  54 C,  54 Y and  54 Bk, respectively. The squeezing blades  54 M,  54 C,  54 Y and  54 Bk are formed in a plate form having a stainless steel base end fixed to casings  55 M,  55 C,  55 Y and  55 Bk of the developing devices, and a tip end edge formed with insulating silicone rubber, insulating fluorine rubber or resin, or the like. The tip end edges of the squeezing blades  54 M,  54 C,  54 Y and  54 Bk are in contact with the developing rollers  52 M,  52 C,  52 Y and  52 Bk, respectively, under pressure from underneath of the developing rollers  52 M,  52 C,  52 Y and  52 Bk. 
   The toner housed in the casings  55 M,  55 C,  55 Y and  55 Bk of the developing devices is a positively charged non-magnetic one-component developer, which has toner mother particles having an average particle diameter of 9 μm formed by adding a known coloring agent, such as carbon black, and a charge controlling agent, such as a quaternary ammonium salt, or a charge controlling resin to a styrene-acrylic resin formed into a spherical form by suspension polymerization. The toner contains silica as an external additive on the surface of the toner mother particles. The silica as the external additive has been subjected to a known hydrophobic treatment with a silane coupling agent, a silicone oil or the like, and has an average particle diameter of 10 nm. The addition amount of the silica is 0.6% by weight based on the amount of the toner mother particles. Magenta, cyan, yellow and black toners are housed in the casings  55 M,  55 C,  55 Y and  55 Bk of the developing devices, respectively. 
   The toner is excellent in flowability because the toner is a suspension polymerization toner having such a shape that is extremely close to sphere, and contains hydrophobic silica having an average particle diameter of 10 nm in an amount of 0.6% by weight. Therefore, a sufficient charge amount can be obtained through frictional charge. Furthermore, the toner does not have sharp corners, which are found in pulverized toners. Therefore, the toner is difficult to receive mechanical forces, is excellent in followability to an electric field, and exhibits good transfer efficiency. 
   The photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk each is formed, for example, with an aluminum base material having formed thereon a positively charging photosensitive layer. The thickness of the photosensitive layer is 20 μm or more, and the aluminum base material is used as a grounding layer. In this embodiment, a slight difference is provided between the velocities of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk and an intermediate transfer material  5 . 
   The cleaning rollers  70 M,  70 C,  70 Y and  70 Bk each is a roller formed with an elastic material, such as electroconductive sponge, and are in frictionally contact with the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, respectively, from underneath of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk. The cleaning rollers  70 M,  70 C,  70 Y and  70 Bk are applied with a negative voltage, which is contrary to the toners, with an electric power source not shown in the figure, and remove the remaining toners on the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk through actions of the frictional force to the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk and an electric field formed by the applied voltage. This embodiment employs a so-called cleaner-less developing system, in which the remaining toners thus removed with the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk in the prescribed process step after completing the developing step are returned to the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, and then recovered by the developing rollers  52 M,  52 C,  52 Y and  52 Bk into the developing devices  51 M,  51 C,  51 Y and  51 Bk of the corresponding colors. 
   The charging devices  71 M,  71 C,  71 Y and  71 Bk are scorotron charging devices and are disposed to face the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk in non-contact therewith from underneath of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk on the downstream of the rotation direction of the photoreceptor drums  3 M,  3 C,  3 Y, and  3 Bk with respect to the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk. 
   The exposing unit  72 M,  72 C,  72 Y and  72 Bk each is configured with a known laser scanner unit. The exposing unit  72 M,  72 C,  72 Y and  72 Bk are disposed to overlap in the vertical direction the developing devices  51 M,  51 C,  51 Y and  51 Bk of the visual image forming part  4 , and to overlap in the horizontal direction the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk and the charging devices  71 M,  71 C,  71 Y and  71 Bk, and expose the surfaces of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk with laser light on the downstream of the rotation direction of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk with respect to the charging devices  71 M,  71 C,  71 Y and  71 Bk. The surfaces of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk are exposed with laser light corresponding to image data by the exposing unit  72 M,  72 C,  72 Y and  72 Bk to form electrostatic latent images of each of colors on the surfaces of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk. 
   The toners are positively charged, and then the toners are fed to the developing rollers  52 M,  52 C,  52 Y and  52 Bk through the feeding rollers  53 M,  53 C,  53 Y and  53 Bk and formed into thin layers with the squeezing blades  54 M,  54 C,  54 Y and  54 Bk. The toners thus positively charged develop negatively the electrostatic latent images thus positively charged on the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk in good conditions to form images with significantly high quality. 
   The intermediate transfer material  5  in a belt form (corresponding to the transfer medium) is formed with an electroconductive sheet, such as polycarbonate and polyimide, into a belt form. The intermediate transfer material  5  in a belt form is hung on two driving rollers  60  and  62 , and intermediate transfer rollers  61 M,  61 C,  61 Y and  61 Bk are disposed in the vicinities of the positions facing the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk. The surface of the intermediate transfer material  5  facing the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk is conveyed from top down in the vertical direction, as shown in  FIG. 1 . 
   The intermediate transfer rollers  61 M,  61 C,  61 Y and  61 Bk are applied with a prescribed voltage to transfer the toner images formed on the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk onto the intermediate transfer material  5 . A secondary transfer roller  63  is provided to face the roller  62  provided down below in the vertical direction of the intermediate transfer material  5 , at which the toner images are transferred to paper P (corresponding to the recording medium), and the secondary transfer roller  63  is also applied with a prescribed voltage. As a result, the toner images of four colors carried on the intermediate transfer material  5  in a belt form are transferred to the paper P. 
   A cleaning device,  6  is provided on the side of the intermediate transfer material  5  opposite to the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, as shown in  FIG. 1 . The cleaning device is configured with a scraping member  65  and a casing  66 , and the toner remaining on the intermediate transfer material  5  is scraped with the scraping member  65  to house in the casing  66 . 
   The fixing part  8  is configured with a first heating roller  81  and a second heating roller  82 , and the paper P carrying the toner images of four colors is conveyed by holding between the first heating roller  81  and the second heating roller  82  under heat and pressure to fix the toner images on the paper P. 
   The paper feeding part  9  is provided at the bottom of the apparatus and is configured with a housing tray  91  housing paper P and a pickup roller  92  for delivering the paper P. The paper feeding part  9  feeds the paper P at a prescribed timing corresponding to the image forming step carried out by the exposing unit  72 M,  72 C,  72 Y and  72 Bk, the developing devices  51 M,  51 C,  51 Y and  51 Bk, the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, and the intermediate transfer material  5 . 
   An upper cover  10  is provided rotatably around an axis  10   a  on the top of the apparatus, and a part of the surface cover  10  configures a paper delivery tray  10   b . The paper delivery tray  10   b  is provided on the paper delivery side of the fixing part  8  and houses the paper P thus delivered from the fixing part  8  and conveyed by pairs of rollers  101 ,  102  and  103 . 
   In this embodiment as shown in  FIG. 1 , a front cover  20  is provided rotatably in the direction shown in  FIG. 1  around an axis  20   a . The developing devices  51 M,  51 C,  51 Y and  51 Bk can be replaced by opening the front cover  20 . Spring members  21 M,  21 C,  21 Y and  21 Bk are provided on the front cover  20  at the positions facing the developing devices  51 M,  51 C,  51 Y and  51 Bk to press the developing devices  51 M,  51 C,  51 Y and  51 Bk to the rear (leftward in  FIG. 1 ) upon closing the front cover  20 . 
   As shown in  FIG. 2A , a side view, and  FIG. 2B , a perspective view, supporting members  31  for supporting the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk are fixed on side panels  30  on the right and left sides of the color laser printer  1 . The supporting member  31  has formed therein a substantially vertical guide channel  32  for guiding axes  3   a M,  3   a C,  3   a Y and  3   a Bk of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, substantially horizontal guide channels  33 M,  33 C,  33 Y and  33 Bk for guiding axes  52   a M,  52   a C,  52   a Y and  52   a Bk of the developing rollers  52 M,  52 C,  52 Y and  52 Bk, and substantially horizontal guide channels  34 M,  34 C,  34 Y and  34 Bk for guiding projections  55   a M,  55   a C,  55   a Y and  55   a Bk provided on the casings  55 M,  55 C,  55 Y and  55 Bk of the developing devices. 
   The axes  52   a M,  52   a C,  52   a Y and  52   a Bk of the developing rollers  52 M,  52 C,  52 Y and  52 Bk are positioned at tip ends of the guide channels  33 M,  33 C,  33 Y and  33 Bk, respectively, by pressing the casings  55 M,  55 C,  55 Y and  55 Bk of the developing devices with the spring members  21 M,  21 C,  21 Y and  21 Bk upon closing the front cover  20 . 
   The photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, are retained by a holder  35  and are integrally detachable upon opening the upper cover  10 . As shown in  FIG. 3 , the holder  35  is formed to have a substantially horseshoe shape with an opening downward and retains the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk being in parallel to each other to configure a photoreceptor cartridge  37 . The holder  35  also retains the charging devices  71 M,  71 C,  71 Y and  71 Bk. The holder  35  further retains the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk, which are not shown in the figure. 
   As shown in  FIG. 3 , gears  3   b M,  3   b C,  3   b Y and  3   b Bk provided on the outer peripheries of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk are exposed from the holder  35  and are engaged with gears of a driving system, which is not shown in the figure, upon mounting the photoreceptor cartridge  37  on the color laser printer  1  to enable driving of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk. The lengths of the axes  3   a M,  3   a C,  3   a Y and  3   a Bk of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk are sequentially decreased in this order (i.e., the lower axis is shorter). That is, the protruding lengths of the axes  3   a M,  3   a C,  3   a Y and  3   a Bk from the holder  35  are increased from the upstream to the downstream in the mounting direction of the photoreceptor cartridge  37  on the color laser printer  1 . Corresponding to the configuration, step parts  32 M,  32 C,  32 Y and  32 Bk are formed in the guide channel  32 , as shown in  FIG. 2B , and the axes  3   a M,  3   a C,  3   a Y and  3   a Bk are engaged with the step parts  32 M,  32 C,  32 Y and  32 Bk, respectively, to position the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk. 
   A linking rod  41  is vertically provided on the supporting member  31 , and the linking rod  41  moves vertically associated with closing and opening motion of the upper cover  10 , whereby the developing devices  51 M,  51 C,  51 Y and  51 Bk move upon opening the upper cover  10  as described below. 
     FIG. 4A  is a perspective view showing the configuration of the vicinity of the part of the supporting member  31  where the guide channels  33 M and  34 M, and  FIG. 4B  is a perspective view showing the configuration in the state where an inner panel  31   a  (the guide channels  33 M and  34 M are formed in the inner panel  31   a ) is removed from the supporting member  31 . The vicinity of the guide channels  33 C and  34 C, the vicinity of the guide channels  33 Y and  34 Y, and the vicinity of the guide channels  33 Bk and  34 Bk are configured in the similar manner. 
   As shown in  FIG. 4B , an upper end of the linking rod  41  is swingably connected to one end of a linking rod  42 , the other end of which is swingably connected to the upper cover  10 . According to the configuration, the movement of the upper cover  10  is transferred to the linking rod  41  through the linking rod  42 , and the linking rod  41  moves vertically upon opening and closing the upper cover  10 . The guide channel  34 M has on the outside thereof (on the side of the side panel  30 ) a slider  43  capable of slidingly moving along the guide channel  34 M, and one end of a linking rod  44  is swingably connected to the slider  43 . The other end of the linking rod  44  is swingably connected to the linking rod  41 . The slider  43  is disposed in front of the linking rod  41  (on the near side in  FIGS. 4A and 4B ), and the connecting part of the linking rod  44  and the slider  43  is disposed above the connecting part of the linking rod  44  and the linking rod  41 . According to the configuration, upon opening the upper cover  10 , the linking rod  44  falls down to the substantially horizontal position associated with ascent of the linking rod  41 , whereby the slider  43  moves forward. Upon closing the upper cover  10 , on the other hand, the slider  43  moves backward associated with descent of the linking rod  41 . 
   The slider  43  has on the inner surface thereof a pair of projections  43   a  and  43   b  provided with a certain distance in the back and forth direction. According to the configuration, in the case where the projections  55   a M,  55   a C,  55   a Y and  55   a Bk are disposed between the corresponding projections  43   a  and  43   b , the projections  43   b  push the projections  55   a M,  55   a C,  55   a Y and  55   a Bk forward against the biasing force of the spring members  21 M,  21 C,  21 Y and  21 Bk upon opening the upper cover  10 , so as to move the developing devices  51 M,  51 C,  51 Y and  51 Bk forward (rightward in  FIG. 1 ). Upon closing the upper cover  10 , the projections  43   a  push the projections  55   a M,  55   a C,  55   a Y and  55   a Bk backward, so as to move the developing devices  51 M,  51 C,  51 Y and  51 Bk backward. 
   The back end of the moving range of the slider  43  is the position where the developing rollers  52 M,  52 C,  52 Y and  52 Bk are in substantially contact with the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, respectively, and precise positioning can be attained by receiving the pressing force from the spring members  21 M,  21 C,  21 Y and  21 Bk as described in the foregoing. The front end of the moving range of the slider  43  is the position where the developing devices  51 M,  51 C,  51 Y and  51 Bk are certainly evacuated from the moving path for detaching the photoreceptor cartridge  37 , and the developing devices  51 M,  51 C,  51 Y and  51 Bk can be easily replaced. The projections  55   a M,  55   a C,  55   a Y and  55   a Bk are temporarily retracted into the casings  55 M,  55 C,  55 Y and  55 Bk of the developing devices by operating a prescribed lever (or button), which is not shown in the figure, provided on the casings  55 M,  55 C,  55 Y and  55 Bk of the developing devices. According to the configuration, engagement of the projections  55   a M,  55   a C,  55   a Y and  55   a Bk and the projections  43   a  and  43   b  is released to enable replacement of the developing devices  51 M,  51 C,  51 Y and  51 Bk. 
   The front surface of the projection  43   a  on the near side forms a tapered surface, and the projections  55   a M,  55   a C,  55   a Y and  55   a Bk pressed from the front overstride the projection  43   a  by being retracted as described in the foregoing. According to the configuration, the developing devices  51 M,  51 C,  51 Y and  51 Bk can be mounted without operation of the lever or button. Furthermore, the pressing force necessary for the projections  55   a M,  55   a C,  55   a Y and  55   a Bk to overstride the projection  43   a  is smaller than the pressing force applied by the spring members  21 M,  21 C,  21 Y and  21 Bk. Therefore, the developing devices  51 M,  51 C,  51 Y and  51 Bk can be completely mounted by lightly inserting the developing devices  51 M,  51 C,  51 Y and  51 Bk, followed by closing the front cover  20 . 
   The operation of the color laser printer  1  according to the embodiment will be described. The photosensitive layers of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk are uniformly charged with the charging devices  71 M,  71 C,  71 Y and  71 Bk, respectively, and then the photosensitive layers are exposed corresponding to images of magenta color, cyan color, yellow color and black color, respectively. A magenta toner, a cyan toner, a yellow toner and a black toner are attached to electrostatic latent images formed on the photosensitive layers of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk with the magenta developing device  51 M, the cyan developing device  51 C, the yellow developing device  51 Y and the black developing device  51 Bk, so as to effect development of magenta color, cyan color, yellow color and black color. The toner images of magenta color, cyan color, yellow color and black color thus formed are once transferred onto the intermediate transfer material  5 . 
   The toners remaining on the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk after transferring are temporarily retained by the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk. The toner images of the respective colors are formed with certain time differences among them corresponding to the moving velocity of the intermediate transfer material  5  and the positions of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, and the toner images of the colors are transferred to overlap on the intermediate transfer material  5 . 
   The toner images of the four colors thus formed on the intermediate transfer material  5  are transferred onto the paper P fed from the paper feeding part  9  at the position where the secondary transfer roller  63  and the intermediate transfer material  5  are in contact with each other under pressure. The toner images are fixed on the paper P in the fixing part  8  and delivered to the paper delivery tray  10   b . Thus, a four-color image is formed. 
   In the case where the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk are replaced, the upper cover  10  is opened, whereby the developing devices  51 M,  51 C,  51 Y and  51 Bk are evacuated from the moving path of the photoreceptor cartridge  37 , and the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk can be taken out by withdrawing the photoreceptor cartridge  37  upward. In the case where a new one of a photoreceptor cartridge  37  is mounted, the upper cover  10  is left opening, and the photoreceptor cartridge  37  is brought down with the axes  3   a M,  3   a C,  3   a Y and  3   a Bk of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk being engaged with the guide channel  32 . The axes  3   a M,  3   a C,  3   a Y and  3   a Bk are then positioned at the step parts  32 M,  32 C,  32 Y and  32 Bk. 
   The new photoreceptor cartridge  37  is covered with a covering member  47  as shown in  FIG. 5 , and upon mounting the photoreceptor cartridge  37 , the covering member  47  is removed. Accordingly, the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk are favorably prevented from being damaged until the photoreceptor cartridge  37  is mounted on the color laser printer  1 . Upon opening the upper cover  10 , the developing devices  52 M,  51 C,  51 Y and  51 Bk move to the front side, at which they can be easily taken out, and on this occasion, the developing devices  51 M,  51 C,  51 Y and  51 Bk can be replaced by opening the front cover  20 . Thus, the maintenance property is improved. 
   In this embodiment having been described herein, the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk are integrally retained in the photoreceptor cartridge  37  and are detachable separately from the developing devices  51 M,  51 C,  51 Y and  51 Bk. Furthermore, the detaching direction thereof is the substantially vertical direction, which is substantially in parallel to the conveying direction of the intermediate transfer material  5  on the surface facing the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk and to the aligning direction of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk. According to the configuration, the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk can be replaced without interference with the exposing unit  72 M,  72 C,  72 Y and  72 Bk to prevent favorably color drift from occurring. 
   Upon mounting the photoreceptor cartridge  37 , the axes  3   a M,  3   a C,  3   a Y and  3   a Bk are positioned at the step parts  32 M,  32 C,  32 Y and  32 Bk, whereby the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk can be precisely positioned to prevent further favorably color drift from occurring. Upon opening the upper cover  10  for detaching the photoreceptor cartridge  37 , the developing devices  51 M,  51 C,  51 Y and  51 Bk are automatically evacuated from the moving path of the photoreceptor cartridge  37 , whereby the photoreceptor cartridge  37  can be easily detached. 
   In this embodiment, moreover, the charging devices  71 M,  71 C,  71 Y and  71 Bk are integrally retained in the photoreceptor cartridge  37  and can be replaced simultaneously with the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, whereby the maintenance property of the color laser printer  1  is improved. The service life of the charging devices  71 M,  71 C,  71 Y and  71 Bk is longer than that of the developing devices  51 M,  51 C,  51 Y and  51 Bk and is equivalent to or longer than that of the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk. In this embodiment, therefore, the running cost of the color laser printer  1  can be further reduced in comparison to the case where the charging devices  71 M,  71 C,  71 Y and  71 Bk are replaced simultaneously with the developing devices  51 M,  51 C,  51 Y and  51 Bk. 
   In the case where the configuration of this embodiment is changed to dispose assemblies black (Bk), yellow (Y), cyan (C) and magenta (M) in this order from top down, it is possible that the photoreceptor cartridge is configured in the manner shown in  FIG. 6 , whereby the running cost can be further reduced. Specifically, in the photoreceptor cartridge  137  shown in  FIG. 6 , photoreceptor drums  3 M,  3 C and  3 Y, cleaning rollers  70 M,  70 C and  70 Y, and charging devices  71 M,  71 C and  71 Y are retained by a holder  135  having a substantially horseshoe shape with an opening downward, and a photoreceptor drum  3 Bk, a cleaning roller  70 Bk and a charging device  71 Bk are retained by a holder  136  having a substantially horseshoe shape with an opening downward and being capable of connecting to an upper part of the holder  135 . A hook  136   a  is provided at the lower end of the holder  136  and is capable of being engaged with a female hook  135   a  provided at the upper end of the holder  135 . The hook  136   a  temporarily moves outward by operating a lever (or button), which is not shown in the figure, provided in an upper part of the holder  136 , so as to release the engagement with the female hook  135   a.    
   In the case where the photoreceptor cartridge  137  thus configured is withdrawn upward without releasing the engagement, the photoreceptor drums  3 M,  3 C,  3 Y and  3 Bk, the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk, and the charging devices  71 M,  71 C,  71 Y and  71 Bk can be integrally replaced as similar to the case of the photoreceptor cartridge  37 . In the case where the engagement is released, on the other hand, only the photoreceptor drum  3 Bk, the cleaning roller  70 Bk and the charging device  71 Bk can be replaced with the photoreceptor drums  3 M,  3 C and  3 Y, the cleaning roller  70 M,  70 C and  70 Y, and the charging devices  71 M,  71 C and  71 Y being left remaining in the color laser printer  1 . 
   Black color is frequently used in comparison to the other colors, and therefore, the service life of the photoreceptor  3 Bk expires faster in comparison to the photoreceptor drums  3 M,  3 C and  3 Y. In the photoreceptor cartridge  137 , only the photoreceptor drum  3 Bk can be replaced with the photoreceptor drums  3 M,  3 C and  3 Y being left remaining, whereby the running cost of the color laser printer  1  can be further reduced. The photoreceptor cartridge  137  is also covered with a covering member similar to the covering member  47 . 
   In the embodiments described in the foregoing, the photoreceptor cartridge  137  is detached in the direction, which is substantially perpendicular to the axes  3   a M,  3   a C,  3   a Y and  3   a Bk, whereby the configuration of the bearing part and the like can be simplified to reduce the production cost of the color laser printer  1 , in comparison to the case where the photoreceptor cartridge is detached along the axes  3   a M,  3   a C,  3   a Y and  3   a Bk. In the embodiments, furthermore, the photoreceptor  3 M,  3 C,  3 Y and  3 Bk can be replaced as being separated from the developing devices  51 M,  51 C,  51 Y and  51 Bk, whereby the running cost of the color laser printer  1  can be further reduced in comparison to the case where they are integrally replaced. 
   In the embodiments described in the foregoing, the guide channel  32  corresponds to the guide part, the axes  3   a M,  3   a C,  3   a Y and  3   a Bk correspond to the guided part, and the linking rods  41 ,  42  and  43  and the slider  43  correspond to the evacuating unit. The invention is not limited to the embodiments and can be practiced with various embodiments as far as the gist of the invention is not deviated. For example, it is not necessary that the developing devices  51 M,  51 C,  51 Y and  51 Bk are automatically evacuated. 
   Furthermore, it is possible that the spring members  21 M,  21 C,  21 Y and  21 Bk are omitted, and the front cover  20  is provided apart from the developing devices  51 M,  51 C,  51 Y and  51 Bk. In this case, however, it is preferred that the developing rollers  52 M,  52 C,  52 Y and  52 Bk are positioned, for example, by such a manner that spring members for fixing the axes  52   a M,  52   a C,  52   a Y and  52   a Bk are provided at tip ends of the guide channels  33 M,  33 C,  33 Y and  33 Bk. 
   Moreover, while the toners are once transferred to the intermediate transfer material  5  and then further transferred to the paper P in the embodiment, the toners may be directly transferred to the paper P. In this case, the paper P corresponds to both the recording medium and the transfer medium. Still moreover, the guide part may be a rail instead of a channel. 
   The Second Preferred Embodiment 
     FIG. 7  is a cross sectional view showing a second embodiment of a color laser printer as an apparatus for forming an image.  FIG. 8  is a cross sectional view showing the color laser printer shown in  FIG. 7  in a state where a front cover thereof is opened.  FIG. 9  is a cross sectional view showing the color laser printer shown in  FIG. 7  in a state where a rear cover thereof is opened.  FIG. 10  is an enlarged cross sectional view showing a process part of the laser color printer shown in  FIG. 7  in a state where drum cartridges corresponding to a yellow developing cartridge and a magenta developing cartridge are mounted. 
   In  FIG. 7 , the color laser printer  201  is a tandem type vertical standing color laser printer with an intermediate transfer system, which contains, inside a main frame casing  202 , a feeder part  204  for feeding paper  203  as a transfer medium, and an image forming part  205  for forming an image on the paper  203  thus fed. In the following description, the right side in  FIG. 1  having an operating part  202   a  provided is referred to as a front side of the color laser printer  201 , and the left side is referred to as a rear side of the color laser printer  201 . 
   The mainframe casing  202  has a substantially rectangular box shape and has a rear cover  206   r  as a first openable member and a front cover  206   f  as a second openable member on the rear side and the front side thereof, respectively. The mainframe casing  202  has, on an upper part on the front side, an operating panel  202   a  having a liquid crystal panel displaying operation state and the like of the color laser printer  201  and user operable buttons for setting configurations, as an operating part. A paper delivery tray  258  described later is provided on an upper part of the mainframe casing  202 . 
   The front cover  206   f  has a plate shape, and one lower end thereof is connected to the mainframe casing  202  through a hinge  207   f . According to the configuration, the front cover  206   f  is openably and closably swung with respect to the mainframe casing  202  with the hinge  207   f  as a supporting point. In the case where the front cover  206   f  is opened as shown in  FIG. 8 , the front side of the mainframe casing  202  is left open to form a front opening  208   f  as an opening, whereby consumable parts and the like can be replaced through the front opening  208   f . In the case where the front-cover  206   f  is closed as shown in  FIG. 7 , the color laser printer  201  is set up as a printable state. 
   The rear cover  206   r  has a plate shape, and one lower end thereof is connected to the mainframe casing  202  through a hinge  207   r . According to the configuration, the rear cover  206   r  is openably and closably swung with respect to the mainframe casing  202  with the hinge  207   r  as a supporting point. In the case where the rear cover  206   r  is opened as shown in  FIG. 9 , the rear side of the mainframe casing  202  is left open to form a rear opening  208   r  as an opening, and whereby paper jam can be resolved, and consumable parts and the like can be replaced, through the rear opening  208   r . In the case where the rear cover  206   r  is closed as shown in  FIG. 7 , the color laser printer  201  is set up as a printable state. 
   The feeder part  204  is provided at a bottom of the mainframe casing  202  as shown in  FIG. 7 , and contains a paper feeding tray  209  detachable in the cross direction of the mainframe casing  202 , a paper feeding roller  210  disposed at an upper part of one end of the paper feeding tray  209 , a paper feeding path  211 , conveying rollers  212  provided on the paper feeding path  211  on the downstream side of the conveying direction of the paper  203 , with respect to the paper feeding roller  210  (hereinafter, the upstream side and the downstream side of the conveying direction of the paper  203  are sometimes simply referred to as an upstream side and the downstream side, respectively), and resist rollers  213  provided on the paper feeding path  211  on the downstream side of the conveying direction of the paper  203  with respect to the conveying rollers  212 . 
   The paper feeding tray  209  has a box shape having an open upper surface and is equipped with a paper pressing plate  214 , on which the paper  203  is stacked. The paper pressing plate  214  is provided in the paper feeding tray  209  with one end thereof facing the paper feeding roller  210  being vertically swingable. On the paper pressing plate  214 , the paper  203  is stacked. The paper pressing plate  214  is biased from the back surface thereof with a spring, which is not shown in the figure, and the uppermost paper  203  stacked on the paper pressing plate  214  is pressed with the spring onto the paper feeding roller  210  and is fed one by one to the paper feeding path  211  through rotation of the paper feeding roller  210 . 
   The paper feeding path  211  extends upward from the end of the upstream side where the paper feeding roller  210  is provided, along the rear cover  206   r  in the mainframe casing  202 , through a transferring position (a position where a secondary transfer roller  247  and a first intermediate transfer material supporting roller  243 , which will be described later, are in contact with each other), to reach the end of the downstream side where a fixing part, which will be described later, is provided. 
   The conveying rollers  212  and the resist rollers  213  are disposed on the upstream side and the downstream side of the conveying direction, respectively, to face the paper feeding path  211 . 
   The paper  203  fed with the paper feeding roller  210  to the paper feeding path  211  is conveyed with the conveying rollers  212  from the upstream side to the downstream side of the conveying direction, and then conveyed with the resist rollers  213  to the transferring position after resisted. 
   The image forming part  205  contains a process part  215 , an intermediate transfer mechanism part  216  as a transfer unit, and a fixing part  217 . 
   The process part  215  contains a developing cartridge  218  as a developing part, a drum cartridge  219  as an image carrying part, and a scanner unit  220  as an exposing unit, and plural (four) process parts for each of colors are provided. The process parts are disposed in parallel to each other and are aligned with prescribed intervals in a substantially vertical direction. 
   The developing cartridges  218  contain four developing cartridge, a yellow developing cartridge  218 Y, a magenta developing cartridge  218 M, a cyan developing cartridge  218 C and a black developing cartridge  218 K, and the developing cartridges  218 , which will be described in detail later, are independently detachable to the mainframe casing  202  through the front opening  208   f  formed in the state where the front cover  206   f  is opened. 
   The developing cartridges  218  are formed as casings, each contains a toner housing part  221  as a developer housing chamber, a feeding roller  222 , a developing roller  223  as a developer carrying member, and a squeezing plate  224 . 
   The developing cartridge  218  has a grabbing member  225  at the front end thereof, and a first developing side guide projection  226  and a second developing-side guide projection  227  are provided at both side ends in the width direction (a direction perpendicular to the cross direction on the plane view, whereinafter the same) of the developing cartridge  218 . The developing cartridge  218  has an opening on the rear side of the bottom thereof to expose a part of the developing roller  223 . 
   The grabbing member  225  has a substantially arc shape on the side view and protrudes toward the front on the front end of the developing cartridge  218 . Two grabbing members  225  are provided as opposed to each other on both sides in the width direction of the developing cartridge  218 , i.e., both sides in the axial direction of the developing roller  223 . 
   The first developing-side guide projection  226  is configured by extending the axis of the developing roller  223  in the longitudinal direction thereof to protrude from the both side surfaces in the width direction of the developing cartridge  218 . 
   The second developing-side guide projection  227  has a substantially elliptic plate shape on the side view extending in the longitudinal direction of the developing cartridge  218 , and protrudes from the front side of the bottom end of each of the both side surfaces in the width direction of the developing cartridge  218  in the directions extending outward in the width direction. 
   The first developing-side guide projections  226  and second developing-side guide projections  227  are engaged with first guide channels  228  as a guide part formed on side plates  202   b  provided on both inside side surfaces in the width direction of the mainframe casing  202 . 
   The first guide channels  228  are provided in parallel to each other on the mainframe casing  202  with a prescribed interval in the substantially vertical direction, and extend in the cross direction on the mainframe casing  202  at the mounting position of the developing cartridge  218 , i.e., in the detaching direction of the developing cartridge  218 , and more specifically in the direction along an upper surface of a scanner casing  241  of the scanner unit  220  described later. Further specifically, the first guide channel  228  is a channel extending in the substantially horizontal direction, in which the front end thereof is opened toward the front opening  208   f  in the state where the front cover  206   f  is opened, and in the case where the first developing-side guide projection  226  is positioned at the rear end (the deepest part) of the first guide channel  228 , the developing roller  223  faces and in contact with the photoreceptor drum  231 . 
   The rear end of the first guide channel  228  is provided with a positioning projection  229 . 
   The positioning projection  229  has a substantially triangular shape and provided on the lower surface of the first guide channel  228 . The positioning projection  229  is elastically biased toward the upper surface of the first guide channel  228  with a spring, which is not shown in the figure. 
   In a state where the developing cartridge  218  is mounted in the mainframe casing  202 , the first developing-side guide projection  226  of the developing cartridge  218  is engaged with the first guide channel  228  at the rear end thereof as being restrained from moving to the front with the positioning projection  229 , and the second developing-side guide projection  227  is engaged with the first guide channel  228  at the front end thereof. In this state, the developing roller  223  of the developing cartridge  218  is made in contact under pressure with a photoreceptor drum  231  of a drum cartridge  219  described later. 
   The toner housing part  221  is formed as an interior space of the developing cartridge  218 , in which plural agitators  230  (for example, three agitators in this embodiment) are provided in the cross direction of the toner housing part  221  as shown in  FIG. 7 . The toner housing parts  221  of the respective developing cartridges  218  are charged with a positively charged non-magnetic one-component toner of yellow color for the yellow developing cartridge  218 Y, that of magenta color for the magenta developing cartridge  218 M, that of cyan color for the cyan developing cartridge  218 C, and that of black color for the black developing cartridge  218 K, respectively. The toner used herein may be a substantially spherical polymerized toner obtained by polymerizing a polymerizable monomer, such as a styrene monomer, e.g., styrene, and an acrylic monomer, e.g., acrylic acid, an alkyl (having from 1 to 4 carbon atoms) acrylate ester and an alkyl (having from 1 to 4 carbon atoms) methacrylate ester, by a known polymerization method, such as suspension polymerization. The toner contains a coloring agent corresponding to the respective colors and wax and is improved in flowability by adding an external additive, such as silica. The toner generally has a particle diameter of about from 6 to 10 μm. 
   The toner contained in the toner housing part  221  is discharged toward the feeding roller  222  through a toner feeding outlet opening on the side surface of the toner housing part  221  by rotation and agitation of the agitators  231  provided in the toner housing part  221 . 
   The feeding roller  222  is rotatablly disposed on the side of the toner feeding outlet, and the developing roller  223  is rotatablly disposed on the side of the feeding roller  222  to face the feeding roller  222 . The feeding roller  222  and the developing roller  223  are rotatablly supported on the developing cartridge  218  in a state where they are in contact with each other under compression to certain extent. 
   The feeding roller  222  is configured by covering a metallic roller axis with a roller formed with an electroconductive sponge member. 
   The developing roller  223  is configured by covering a metallic roller axis with a roller formed with an electroconductive rubber material as an elastic member. More specifically, the roller of the developing roller  223  has a two-layer structure containing an elastic roller formed with urethane rubber, silicone rubber, EPDM rubber or the like having electroconductivity with carbon fine particles or the like, having coated on the surface thereof a coating layer mainly containing urethane rubber, a urethane resin, a polyimide resin or the like. The developing roller  223  is disposed in such a manner that a lower part thereof is exposed downward from an opening of the developing cartridge  218  (as shown in  FIG. 10 ), and upon developing the developing roller  223  is applied with a developing bias voltage from an electric power source, which is not shown in the figure. 
   The squeezing blade  224  has a blade mainframe formed with a metallic leaf spring material and a pressing part having a hemicyclic cross section formed with insulating silicone rubber. One end of the blade mainframe is supported by the developing cartridge  218  in the vicinity of the developing roller  223 , and the pressing part is in contact with the developing roller  223  under pressure by the elastic force of the blade mainframe. 
   The toner discharged from the toner feeding outlet is fed to the developing roller  223  through rotation of the feeding roller  222 , and is charged positively through friction between the feeding roller  222  and the developing roller  223 . The toner thus fed to the developing roller  223  intervenes between the pressing part of the squeezing blade  224  and the developing roller  223  associated with rotation of the developing roller  223  and is consequently carried as a thin layer having a constant thickness on the developing roller  223 . 
   The drum cartridges  219  are disposed below the oblique rear parts of the developing cartridges  218  in the respective process parts  215  for each colors, and are independently detachable to the mainframe casing  202  through the rear opening  208   r , which is formed upon opening the rear cover  206   r , while the configurations will be described in detail later. 
   The drum cartridges  219  are formed as casings, each contains a photoreceptor drum  231  as an image carrying member and a scorotron charging device  232 . The drum cartridge  219  is provided with a grabbing member  233  at a rear end thereof as shown in  FIG. 10 , and is provided with drum-side guide projections  234  as an engaging part on both sides in the width direction. The drum cartridge  219  has an opening on the front side of the top thereof to expose a part of the photoreceptor drum  231 . 
   The grabbing member  233  has a substantially arc shape on the side view and protrudes toward the rear on the rear end of the drum cartridge  219 . Two grabbing members  233  are provided as opposed to each other on both sides in the width direction of the drum cartridge  219 , i.e., both sides in the axial direction of the photoreceptor drum  231 . 
   The drum-side guide projection  234  has a substantially cylindrical shape and protrudes from both side surfaces of the drum cartridge  219  at the substantially center part of the drum cartridge  219  in the direction extending outward in the width direction. It is effective for improving the positional accuracy that the drum-side guide projection  234  is used as a rotation axis of the photoreceptor drum  231 . Specifically, bearings are provided on both ends of the photoreceptor drums and are engaged with the drum-side guide projections  234 , whereby the number of components can be decreased, and the positional accuracy can be improved. 
   The drum-side guide projections  234  are engaged with second guide channels  235  formed on side plates  202   b  provided on both sides in the width direction inside the mainframe casing  202 . 
   The second guide channels  235  are provided in parallel to each other on the mainframe casing  202  with a prescribed interval in the substantially vertical direction, and extend in the cross direction on the mainframe casing  202  at the mounting position of the drum cartridge  219 , i.e., in the detaching direction of the drum cartridge  219 . More specifically, the second guide channel  235  is a channel extending in the substantially horizontal direction, in which the rear end thereof is opened toward the rear opening  208   r  in the state where the rear cover  206   r  is opened, and in the case where the drum-side guide projection  234  is positioned at the front end (the deepest part) of the second guide channel  235 , the developing roller  223  faces and in contact with the photoreceptor drum  231 . 
   The front end of the second guide channel  235  is provided with a positioning projection  236 . 
   The positioning projection  236  has a substantially triangular shape and provided on the lower surface of the second guide channel  235 . The positioning projection  236  is elastically biased toward the upper surface of the second guide channel  235  with a spring, which is not shown in the figure. 
   In a state where the drum cartridge  219  is mounted in the mainframe casing  202 , the drum-side guide projection  234  of the drum cartridge  219  is engaged with the second guide channel  235  at the front end thereof as being restrained from moving to the rear with the positioning projection  236 . 
   The photoreceptor drum  231  is rotatablly disposed on the drum cartridge  219  in a state where it is in contact with the developing roller  223  below the oblique rear of the developing roller  223 , and is configured with a grounded drum mainframe having on the surface thereof a photosensitive layer formed with an organic photoreceptor mainly containing polycarbonate. 
   The scorotron charging device  232  is disposed under the photoreceptor drum  231  with a prescribed distance for preventing from being in contact with each other. The scorotron charging device  232  is a scorotron type charging device generating corona discharge from a charging wire, such as tungsten, and is fixed to the drum cartridge  219  for uniformly charging the surface of the photoreceptor drum  231  positively. 
   The scanner unit  220  is configured as a laser scanner, and contains inside a scanner casing  241  as a casing, a laser light emitting part, which is not shown in the figure, a polygonal mirror  237  rotationally driven, a lens  238 , and a reflecting mirrors  239  and  240 . The scanner unit  220  is disposed in the process part  215  under the developing cartridge and on the side (front side) of the photoreceptor drum opposite to the side of the transferring position. 
   The scanner casing  241  has a substantially rectangular shape on the side view and is disposed to overlap horizontally the photoreceptor drum  231  and fixed to the mainframe casing  202 . According to the configuration, the developing cartridges  218  and the scanner units  220  are not horizontally overlapped but are vertically overlapped alternately and the developing cartridge  218  is detachable along the upper surface of the scanner casing  241  of the scanner unit  220 . 
   The scanner unit  220  transmits or reflects a laser beam based on image data emitted from a laser light emitting part by the polygonal mirror  237 , the lens  238  and the reflecting mirrors  239  and  240 , in this order, whereby the surface of the photoreceptor drum  231  is irradiated therewith by rapid scanning. 
   The toners of the respective colors are subjected to exposure and development in the following manner in the process parts  215 . The surface of the photoreceptor drum  231  is uniformly charged positively with the scorotron charging device  232  associated with rotation of the photoreceptor drum  231 , and then exposed by light emitted from the scanner unit  220  to form an electrostatic latent image based on the image data. Subsequently, the toner thus positively charged and carried on the developing roller  223  is electrically moved, upon contacting with the photoreceptor drum  231  associated with rotation of the developing roller  223 , onto the electrostatic latent image formed on the surface of the photoreceptor drum  231 , i.e., such a region on the surface of the photoreceptor drum that has a lower electric potential due to exposure with the scanner unit  220 , and carried thereon, whereby a toner image is formed on the surface of the photoreceptor drum  231 . As a result, reversal development is completed. 
   The intermediate transfer mechanism part  216  is provided in the mainframe casing  202  on the rear side of the process parts  215  on the side of the drum cartridges  219  opposite to the scanner units  220 , and contains an intermediate transfer material driving roller  242 , a first intermediate transfer material supporting roller  243 , a second intermediate transfer material supporting roller  244 , an intermediate transfer belt  245  formed with an endless belt, a primary transfer roller  246 , a secondary transfer roller  247  and a belt cleaner  248 . 
   The intermediate transfer material driving roller  242  is rotatablly disposed in the mainframe casing  202  on the oblique rear of the photoreceptor drum  231  of the drum cartridge  219  provided corresponding to the yellow developing cartridge  218 Y with the intermediate transfer belt  245  intervening therebetween. 
   The first intermediate transfer material supporting roller  243  is rotatablly disposed below the oblique rear of the intermediate transfer material driving roller  242 , and disposed to face the secondary transfer roller  247  with the intermediate transfer belt  245  intervening therebetween. 
   The second intermediate transfer material supporting roller  244  is rotatablly disposed in the mainframe casing  202  below the intermediate transfer material driving roller  242  in the substantially vertical direction. 
   The intermediate transfer material driving roller  242 , the first intermediate transfer material supporting roller  243  and the second intermediate transfer material supporting roller  244  are disposed to make a substantially triangular arrangement, around which the intermediate transfer belt  245  is wound. 
   The intermediate transfer belt  245  is formed with an electroconductive resin, such as polycarbonate and polyimide, having electroconductive particles, such as carbon, dispersed therein, and is disposed in parallel to the photoreceptor drums  231  in such a manner that the contact surface on the front side of the wound intermediate transfer belt  245  faces and is in contact with all the photoreceptor drums  231  of the drum cartridges  219 . 
   In the intermediate transfer mechanism part  216 , a driving force is transmitted from a main motor, which is not shown in the figure, to the intermediate transfer material driving roller  242  to rotate the intermediate transfer material driving roller  242 , and the first intermediate transfer material supporting roller  243  and the second intermediate transfer material supporting roller  244  are driven thereby, whereby the intermediate transfer belt  245  wound on the intermediate transfer material driving roller  242 , the first intermediate transfer material supporting roller  243  and the second intermediate transfer material supporting roller  244  is rotationally driven in the same direction as the photoreceptor drums  231  of the drum cartridges  219  on the contact surface facing and in contact with the photoreceptor drums  231 . 
   The primary transfer rollers  246  are disposed inside the intermediate transfer belt  245  thus wound to face the photoreceptor drums  231  of the drum cartridges  219  with the intermediate transfer belt  245  intervening therebetween. The primary transfer roller  246  is configured by covering a metallic roller axis with a roller formed with an elastic member, such as an electroconductive rubber material. The primary transfer roller  246  is rotatablly disposed to rotate in the same direction as the rotational moving direction of the intermediate transfer belt  245  on the contact surface facing and in contact with the intermediate transfer belt  245 , and is applied with a transfer bias voltage upon transferring from an electric power source, which is not shown in the figure. 
   The secondary transfer roller  247  is disposed to be in, contact with the intermediate transfer belt  245  and to face the first intermediate transfer material supporting roller  243  with the intermediate transfer belt  245  intervening therebetween. The secondary transfer roller  247  is configured by covering a metallic roller axis with a roller formed with an elastic member, such as an electroconductive rubber material. The secondary transfer roller  247  is rotatablly disposed to rotate in the same direction as the rotational moving direction of the intermediate transfer belt  245  on the contact surface facing and in contact with the intermediate transfer belt  245 , and is applied with a transfer bias voltage upon transferring from an electric power source, which is not shown in the figure. 
   The belt cleaner  246  is disposed to face the intermediate transfer material driving roller  242  of the intermediate transfer mechanism part  216  with the intermediate transfer belt  245  intervening therebetween, and has, in a cleaner casing  249  disposed on the way from the first intermediate transfer material supporting roller  243  to the intermediate transfer material driving roller  242 , a cleaner brush  250 , a recovering roller  251 , a recovering box  252  and a scraper  253 . 
   The cleaner brush  250  has a cylindrical mainframe having brush formed radially, and is rotationally disposed in such a state that the brush faces and is in contact with the intermediate transfer belt  245  in the course of from the first intermediate transfer material supporting roller  243  to the intermediate transfer material driving roller  242 . The cylindrical mainframe of the cleaner brush  250  is applied with a cleaning bias voltage to form an electric potential difference with respect to the intermediate-transfer belt  245  from an electric power source, which is not shown in the figure, upon cleaning. 
   The recovering roller  251  is formed with a metallic roller and is rotatablly disposed on the side of the cleaner brush  250  to face and be in contact with the cleaner brush  250 . The recovering roller  251  is applied with a recovering bias voltage to form an electric potential difference with respect to the cleaner brush  250  from an electric power source, which is not shown in the figure, upon cleaning. 
   The recovering box  252  is disposed on the side of the recovering roller  251  and has an opening on a part facing the recovering roller  251 . The scraper  253  is provided in the vicinity of the opening and is in contact with the recovering roller  251  under pressure. 
   The monochrome toner images formed on the photoreceptor drums  231  are transferred to the intermediate transfer belt  245  at the time when they face the intermediate transfer belt  245 , and are sequentially accumulated on the intermediate transfer belt  245  to form a color image. 
   Specifically, a yellow toner image formed on the photoreceptor drum  231  of the yellow developing cartridge  218 Y is transferred on the intermediate transfer belt  245 , and then a magenta toner image formed on the photoreceptor drum  231  of the magenta developing cartridge  218 M is transferred to the intermediate transfer belt  245  to overlap the yellow toner image having been transferred. A cyan toner image formed by the cyan developing cartridge  218 C and a black toner image formed by the black developing cartridge  218 K are then sequentially transferred in the same procedures to form a color image on the intermediate transfer belt  245 . 
   The color image thus formed on the intermediate transfer belt  245  is transferred to paper  203  at once during a period when the paper  203  passes through between the intermediate transfer belt  245  and the secondary transfer roller  247  in contact therewith. 
   The toner attached to the intermediate transfer belt  245  after transferring the color image to the paper  203  is scraped with and attached to the cleaner brush  250  by the cleaning bias voltage applied to the cleaner brush  250  when the toner faces the cleaner brush  250 . Thereafter, the toner attached to the cleaner brush  250  is attached to the recovering roller  251  by the recovering bias voltage applied to the recovering roller  251  when the toner faces the recovering roller  251 , and then the toner is scraped with the scraper  253  and recovered into the recovering box  252 . 
   In the color laser printer  201 , the remaining toner is recovered by the so-called cleanerless developing system, in which the toner remaining on the surface of the photoreceptor drum  231  is recovered with the developing roller  223 . Specifically, the photoreceptor drum  231  is charged with the scorotron charging device  232  at the charging position associated with rotation of the photoreceptor drum  231 , and then exposed with the scanner unit  220  at the exposing position. When the remaining toner on the surface of the photoreceptor drum  231  reaches the developing position where the developing roller  223  and the photoreceptor drum  231  are in contact with each other, the remaining toner present on a non-exposed region is moved to and recovered by the developing roller  223  through the developing bias voltage. In the case where the remaining toner is present on an exposed region, it forms a toner image along with a toner that is newly moved from the developing roller  223  to the exposed region. 
   The peripheral velocity of the developing roller  223  is set at 1.6 times the peripheral velocity of the photoreceptor drum  231 . The difference in peripheral velocity facilitates recovery of the remaining toner from the photoreceptor drum  231  to the developing roller  223 . The polymerized toner remains in a small amount owing to the good flowability thereof and is easily recovered by the developing roller  223 . 
   According to the formation of a color image described in the foregoing, the color laser printer  201  has a tandem type system having plural photoreceptor drums  231  for respective colors, and therefore, toner images of respective colors are formed at a speed equivalent to formation of a monochrome image, so as to attain rapid formation of a color image. 
   The fixing part  217  is disposed above the intermediate transfer mechanism part  216  and contains a first heating roller  254  in contact with the surface of the paper having a color image transferred thereon, a second heating roller  255  disposed to face the first heating roller  254  with the paper  203  intervening therebetween and being in contact with the back surface of the paper  203 , and a pair of conveying rollers  256  disposed above the first heating roller  254  and the second heating roller  255 . 
   The first heating roller  254  contains a cylindrical metallic tube, such as aluminum, a halogen heater for heating provided therein, and an elastic layer formed outside the tube. The second heating roller  255  contains, as similar to the first heating roller  254 , a cylindrical metallic tube, such as aluminum, a halogen heater for heating provided therein, and an elastic layer formed outside the tube, and is disposed to press the first heating roller  254 . 
   The color image thus transferred on the paper  203  is thermally fixed thereon with the first heating roller  254  and the second heating roller  255  during a period when the paper  203  passes through between the first heating roller  254  and the second heating roller  255 , and then the paper  203  is conveyed with the conveying rollers  256  to paper delivery rollers  257 . 
   The paper delivery rollers  257  are disposed above the conveying roller  256  and in the vicinity of a paper delivery outlet, and the paper  3  thus conveyed with the conveying roller  256  is delivered in the direction from the rear side to the front side to the paper delivery tray  258  with the paper delivery rollers  257 . 
   In the color laser printer  201 , the paper feeding path  211 , the conveying rollers  212 , the resist rollers  213 , the intermediate transfer mechanism part  216 , the fixing part  217  and the paper delivery rollers  257  are integrally retained by the rear cover  206   r . According to the configuration, in the case where the rear cover  206   r  is opened as shown in  FIG. 9 , the paper feeding path  211 , the conveying rollers  212 , the resist rollers  213 , the intermediate transfer mechanism part  216 , the fixing part  217  and the paper delivery rollers  257  are integrally moved along with the rear cover  206   r  thus opened, whereby the intermediate transfer belt  245  of the intermediate transfer mechanism part  216  is substantially horizontally disposed. In the case where the rear cover  206   r  is opened as shown in FIG.  7 , the intermediate transfer belt  245  of the intermediate transfer mechanism part  216  is substantially vertically disposed, and the intermediate transfer belt  245  is in contact with the photoreceptor drums  231  under pressure. 
   In the color laser printer  201  having the aforementioned configuration, the developing cartridges  218  and the drum cartridges  219  can be independently detached. For example, in the case where only the developing cartridge  218  is to be detached, the front cover  206   f  is opened as shown in  FIG. 8 , so as to form a front opening  208   f , through which the developing cartridges  218  and the scanner units  220  are exposed on the front side of the mainframe casing  202 . 
   In the case where the developing cartridge  218  mounted in the mainframe casing  202  is removed from the mainframe casing  202 , the grabbing member  225  of the developing cartridge  218  is grasped as shown in  FIG. 10 , and the developing cartridge  218  is withdrawn from the mainframe casing  202  to the front side. The first developing-side guide projection  226  disposed at the rear end of the first guide channel  228  overstrides the positioning projection  229  by pressing down the positioning projection  229  against the bias force of the positioning projection  229 , and is guided to the front side along the first guide channel  228 . The developing cartridge  218  is moved to the front side with the second developing-side guide projection  227  is guided to the front side along the first guide channel  228 , and then the developing cartridge  218  is removed from the mainframe casing  202  and taken out through the front opening  208   f , as shown by a virtual image shown by broken lines in  FIG. 8 . 
   In the case where the developing cartridge  218  is mounted on the mainframe casing  202 , the front cover  206   f  is opened, and while the developing cartridge  218  is inserted into the mainframe casing  202  through the front opening  208   f  by grasping the grabbing member  225  of the developing cartridge  218 , the first developing-side guide projection  226  is engaged with the first guide channel  228 , and then the second developing-side guide projection  227  is engaged with the first guide channel  228 . Thereafter, the developing cartridge  218  is pressed onto the rear side of the mainframe casing  202 , whereby the developing cartridge  218  is moved to the rear side with the first developing-side guide projection  226  and the second developing-side guide projection  227  being guided to the rear side along the first guide channel  228 . Subsequently, the first developing-side guide projection  226  overstrides the positioning projection  229  by pressing down the positioning projection  229  against the bias force of the positioning projection  229 , and is retained at the rear end of the first guide channel  228  in a state where it is restrained from moving to the front with the positioning projection  229 . In this state, the developing roller  223  of the developing cartridge  218  is in contact with the photoreceptor drum  231  under pressure. 
   In the case where the drum cartridge  219  is detached, for example, the rear cover  206   r  is opened, and the intermediate transfer belt  245  of the intermediate transfer mechanism part  216  is substantially horizontally disposed as shown in  FIG. 9 , so as to form the rear opening  208   r , through which the drum cartridges  219  are exposed on the rear side of the mainframe casing  202 . 
   In the case where the drum cartridge  219  mounted in the mainframe casing  202  is removed from the mainframe casing  202 , the grabbing member  233  of the drum cartridge  219  is grasped as shown in  FIG. 10 , and the drum cartridge  218  is withdrawn from the mainframe casing  202  to the rear side. The drum-side guide projection  234  disposed at the front end of the second guide channel  235  overstrides the positioning projection  236  by pressing down the positioning projection  236  against the bias force of the positioning projection  236 , and is guided to the rear side along the second guide channel  235 , and the drum cartridge  219  is moved to the rear side. The drum cartridge  219  is then removed from the mainframe casing  202  and taken out through the rear opening  208   r , as shown by a virtual image shown by broken lines in  FIG. 9 . 
   In the case where the drum cartridge  219  is mounted on the mainframe casing  202 , the rear cover  206   r  is opened, and while the drum cartridge  219  is inserted into the mainframe casing  202  through the rear opening  208   r  by grasping the grabbing member  233  of the drum cartridge  219 , the drum-side guide projection  234  is engaged with the second guide channel  235 . Thereafter, the drum cartridge  218  is pressed onto the front side of the mainframe casing  202 , whereby the drum cartridge  218  is moved to the front side with the drum-side guide projection  234  being guided to the front side along the second guide channel  235 . Subsequently, the drum-side guide projection  234  overstrides the positioning projection  236  by pressing down the positioning projection  236  against the bias force of the positioning projection  236 , and is retained at the front end of the second guide channel  235  in a state where it is restrained from moving to the rear with the positioning projection  236 . In this state, the photoreceptor drum  231  of the drum cartridge  219  is in contact with the developing roller  223  under pressure. 
   Upon detaching the developing cartridge  218  or the drum cartridge  219  as described in the foregoing, the developing roller  223  and the photoreceptor drum  231  having been in contact with each other are not in friction with each other, whereby they can be effectively prevented from being damaged. 
   In the color laser printer  201 , accordingly, the developing cartridges  218  and the drum cartridges  219  can be detached from the mainframe casing  202  in the directions substantially opposite to each other, whereby in the case where. the toner is emptied out, only the developer cartridge  218  is replaced, but the expensive drum cartridge  219  can be used until the service life thereof is expired. Therefore, the running cost can be reduced. Furthermore, only the developing cartridge  218  is necessarily discarded to reduce the amount of industrial waste, whereby the color laser printer  201  good for the environment can be provided. In the color laser printer  201 , moreover, the developing cartridges  218  and the drum cartridges  219  can be detached in the directions substantially opposite to each other, whereby the developing cartridges  218  and the drum cartridges  219  can be independently replaced while interference between them. Furthermore, the degrees of freedom in detaching directions of the developing cartridges  218  and the drum cartridges  219  upon replacement are increased, whereby the developing cartridges  218 .and the drum cartridges  219  can be replaced without moving the scanner units for exposing the drum cartridges  219 , and the developing cartridges  218  with a high replacement frequency can be replaced from the front side provided with the operating panel  202   a , which is easily accessed in comparison to the photoreceptor drums  231 . 
   In the color laser printer  201 , the developing cartridges  218  can be smoothly replaced through the front opening  208   f  formed upon opening the front cover  206   f . Accordingly, the developing cartridges  218  with a high replacement frequency can be detached from the front side provided with the operating panel  202   a , whereby the operationality can be improved. In the color laser printer  201 , particularly, all the detaching direction of the paper feeding tray  209 , the delivery direction of the paper  203 , the operation direction of the operating panel  202   a  and the detaching direction of the developing cartridges  218  are the same direction, and thus, the operationality is further improved. 
   In the color laser printer  201 , the developing cartridges  218  and the scanner units  220  do not horizontally overlap each other and are disposed alternately in the vertical direction, and the developing cartridge  218  can be detached along the upper surface of the scanner casing  241  through engagement of the first guide channel  228  with the first developing-side guide projection  226  and the second developing-side guide projection  227 . Accordingly, the developing cartridges  218  can be detached without interfering with the scanner units  220 , and thus the developing cartridges  218  can be easily replaced. 
   In the color laser printer  201 , the scanner unit  220  is provided on the side of the photoreceptor drum  231  opposite to the transferring position to overlap the photoreceptor drum  231  horizontally, whereby the scanner units  220  can be disposed, and the apparatus can be miniaturized, without impairing replacement of the developing cartridges  218  and the drum cartridges  219 . 
   In the color laser printer  201 , the intermediate transfer mechanism part  216  is supported by the rear cover  206   r , whereby the drum cartridges  219  can be smoothly detached through the rear opening  208   r  formed upon opening the rear cover  206   r.    
   The color laser printer  201  employs the cleanerless developing system, in which the remaining toner remaining on the photoreceptor drum  231  after transferring the toner image carried on the photoreceptor drum  231  to the intermediate transfer belt  245  is recovered by the developing roller  223 , whereby a waste toner storage for recovering and storing the remaining toner is not necessary. Accordingly, the configuration of the apparatus can be simplified, and the apparatus can be miniaturized. As a result, the developing cartridge  218  and the scanner unit  220  can be disposed to overlap substantially vertically owing to the absence of the space for providing a waste toner storage, whereby the footprint of the color laser printer  201  can be reduced. 
   In the second embodiment of the invention described in the foregoing, the scanner unit  220  is provided as an exposing unit, but an LED array  260  maybe provided-instead of the scanner unit  220  as shown in  FIG. 11 . In  FIG. 11 , the same configurational members as those in  FIG. 7  are attached with the same symbols as in  FIG. 7  to omit the descriptions thereof. 
   The LED array  260  is disposed at the same position as the scanner unit  220  in the above embodiment, i.e., below the developing cartridge  218  in the process part  215  to overlap the photoreceptor drum  231  horizontally on the side (front side) of the photoreceptor drum  231  opposite to the transferring position. According to the configuration, the developing cartridges  218  and the LED arrays  260  do not horizontally overlap each other but alternately overlap in the vertical direction. The LED array  260  is configured by arranging a large number of LEDs and exposes by irradiating with light the surface, of the photoreceptor drum  231  through light emission of the LEDs. 
   The color laser printer  201  according to this embodiment is equipped with the LED array  260  instead of the scanner unit  220 , whereby the color laser printer  201  can be miniaturized. While the LED array  260  has a short focal length and is necessarily disposed closely to the photoreceptor drum  231 , in the color laser printer  201 , the drum cartridge  219  is detached in a direction opposite to the developing cartridge  218 , whereby the LED array  260  can be disposed closely to the photoreceptor drum  231  without impairing replacement of the developing cartridge  218  and the drum cartridge  219 . 
   In the color laser printer  201  according to this embodiment, the developing cartridges  218  and the drum cartridges  219  can be detached from the mainframe casing  202  in the directions substantially opposite to each other, whereby in the case where the toner is emptied out, only the developer cartridge  218  is replaced, but the expensive drum cartridge  219  can be used until the service life thereof is expired. Therefore, the running cost can be reduced. Furthermore, only the developing cartridge  218  is necessarily discarded to reduce the amount of industrial waste, whereby the color laser printer  201  good for the environment can be provided. 
   While the invention has been described with reference to the above embodiments, the invention can be practices with other embodiments than the aforementioned embodiments. For example, while the color laser printer  201  with an intermediate transfer system has been described as the vertical standing color laser printer  201 , the invention can be applied to a color laser printer with a direct transfer system. In such a configuration, in the case where the paper  203  is jammed between the photoreceptor drum  231  and a paper conveying belt as a transfer unit, the jammed paper  203  can be easily removed from the rear opening  208   r  formed by opening the rear cover  206   r.    
   While the vertical standing color laser printer  201  has been exemplified in the aforementioned embodiments, the invention can be applied to a horizontally laid color laser printer. 
   In the aforementioned embodiments, the developing cartridge  218  and the drum cartridge  219  are detached from the front side and the rear side, respectively, i.e., the developing cartridge  218  and the drum cartridge  219  are detached in the cross direction. It is also possible in the embodiments that the developing cartridge  218  and/or the drum cartridge  219  may be detached in the width direction. The detachment in the cross direction is preferred in the apparatus because the detachment in the width direction is practically associated with movements of bearings of the photoreceptor drum  231  and the developing roller  223 . 
   In the aforementioned embodiments, the paper feeding path  211 , the conveying rollers  212 , the resist rollers  213 , the intermediate transfer mechanism part  216 , the fixing part  217  and the paper delivery rollers  257  are supported by the rear cover  206   r . It is also possible that the paper feeding path  211  is released upon opening the rear cover  206   r , and the intermediate transfer mechanism part  216  is supported by an openable member other than the rear cover  206   r . In this case, the openable member corresponds to the first openable member. 
   According to an aspect of the invention, the running cost is reduced, and the amount of industrial waste can be decreased to provide an apparatus for forming an image good for the environment. Furthermore, the developing part and the image carrying member can be replaced without moving the exposing unit for exposing the image carrying member, and the developing part with a high replacement frequency can be replaced in a direction with easier access than that for the image carrying member. 
   According to another aspect of the invention, the developing part and the image carrying part can be easily replaced independently from each other. 
   According to another aspect of the invention, the image carrying part can be smoothly detached. 
   According to another aspect of the invention, the exposing unit can be disposed while the apparatus is downsized without inhibiting replacement of the developing part and the image carrying part. 
   According to another aspect of the invention, the developing part and the image carrying member can be easily replaced while formation of an electrostatic latent image on the image carrying member is ensured. 
   According to another aspect of the invention, the developing part with a high replacement frequency can be easily replaced without interfering with the laser scanner. 
   According to another aspect of the invention, the developing part can be detached with preventing interference with the laser scanner. 
   According to another aspect of the invention, the apparatus can be miniaturized, and the LED array can be disposed closely to the image carrying member without interfering replacement of the developing part and the image carrying part. 
   According to another aspect of the invention, the developing part can be smoothly detached to improve the operationality. 
   According to another aspect of the invention, both the simplification of the configuration of the apparatus and the miniaturization of the apparatus can be simultaneously attained. 
   The Third Preferred Embodiment 
     FIG. 12  is a cross sectional view showing an embodiment of a color laser printer as an apparatus for forming an image.  FIG. 13  is a cross sectional view showing an important part of the color laser printer shown in  FIG. 12  in a state where an openable cover thereof is opened.  FIG. 14  is an enlarged cross sectional view showing an important part of a process part of the laser color printer shown in  FIG. 12  in a state where only a black developing cartridge and a yellow developing cartridge corresponding thereto are mounted. 
   In  FIG. 12 , the color laser printer  301  is a tandem type vertical standing color laser printer with a direct transfer system, which contains, inside a mainframe casing  302 , a feeder part  304  for feeding paper  303  as a transfer medium, and an image forming part  305  for forming an image on the paper  303  thus fed. In the following description, the left side in  FIG. 12  having an operating part  2   a  provided is referred to as a front side of the color laser printer  301 , and the right side is referred to as a rear side of the color laser printer  301 . 
   The mainframe casing  302  has, on the front side thereof, a substantially rectangular box shape and has an openable cover  306 , and has, on an upper surface on the front side, an operating panel  302   a  having a liquid crystal panel displaying operation state and the like of the color laser printer  301  and user operable buttons for setting configurations. A paper delivery tray  359  described later is provided on an upper part of the mainframe casing  302 . 
   The openable cover  306  has a plate shape, and one lower end thereof is connected to the mainframe casing  302  through a hinge  307 . According to the configuration, the openable cover  306  is openably and closably swung with respect to the mainframe casing  302  with the hinge  307  as a supporting point. In the case where the openable cover  306  is opened as shown in  FIG. 13 , the front side of the mainframe casing  302  is left open to form an opening  308 , whereby treatment of jammed paper and replacement of consumable parts can be carried out through the opening  308 . In the case where the openable cover  306  is closed as shown in  FIG. 12 , the color laser printer  301  is set up as a printable state. 
   The feeder part  304  is provided at a bottom of the mainframe casing  302 , and contains a paper feeding tray  309  provided detachably, a paper feeding roller  310  disposed at an upper part of one end of the paper feeding tray  309 , and a conveying roller  10   a  provided above the paper feeding roller  310  to face a transfer driving roller  345  described later with a conveying belt  347  intervening therebetween. The paper feeding tray  309  is equipped with a paper pressing plate  311 , one end of which facing the paper feeding roller  310  is vertically swingable. On the paper pressing plate  311 , the paper  303  is stacked. The paper pressing plate  311  is biased from the back surface thereof with a spring, which is not shown in the figure, and the uppermost paper  303  stacked on the paper pressing plate  311  is pressed with the spring onto the paper feeding roller  310  and is fed one by one through rotation of the paper feeding roller  310 . The paper  303  thus fed by the paper feeding roller  310  is conveyed by the conveying roller  10   a  to the transferring position, at which the photoreceptor drum  334  and the conveying belt  347  are in contact with each other. 
   The image forming part  305  contains a process part  312 , a transfer part  313  as a transfer unit, and a fixing part  314 . 
   The process part  312  contains a developing cartridge  315  as a developing part, a drum cartridge  316  as an image carrying part, and a scanner unit  317  as an exposing unit, and plural (four) process parts for each of colors are provided. The process parts are disposed in parallel to each other and are aligned with prescribed intervals in a substantially vertical direction. 
   The developing cartridges  315  contain four developing cartridge, a yellow developing cartridge  15 Y, a magenta developing cartridge  15 M, a cyan developing cartridge  15 C and a black developing cartridge  15 K, and the developing cartridges  315 , which will be described in detail later, are independently detachable to the mainframe casing  302  through the opening  308  formed in the state where the openable cover  306  is opened. 
   The developing cartridges  315  each contains a toner housing part  318  as a developer housing chamber, a feeding roller  319 , a developing roller  320  as a developer carrying member, and a squeezing plate  321 . 
   As shown in  FIG. 14 , the developing cartridge  315  has a grab handle  322  as the grabbing member at the front end thereof, and a developing-side guide projection  323  and a pressure contact projection  324  are provided at both side ends in the width direction (a direction perpendicular to the cross direction on the plane view, hereinafter the same) of the developing cartridge  315 . The developing cartridge  315  has an opening on the front side of the top thereof to expose a part of the developing roller  320 , as shown in  FIG. 12 . 
   The grab handle  322  has a substantially arc shape on the side view and protrudes toward the front on the front end of the developing cartridge  315 . Two grab handles  322  are provided as opposed to each other on both sides in the width direction of the developing cartridge  315 , i.e., both sides in the axial direction of the developing roller  320 . 
   The developing-side guide projection  323  is formed to have a substantially cylindrical shape and protrudes from the front side along the top surface of each of the both side surfaces in the width direction of the developing cartridge  315  in the directions extending outward in the width direction. 
   The pressure contact projection  324  is formed to have a substantially cylindrical shape and protrudes from the rear side at a substantially center part in the vertical direction (a direction perpendicular to the cross direction on the side view, hereinafter the same) of the developing cartridge  315  in the directions extending outward in the width direction. 
   The developing-side guide projections  323  are engaged with first guide channels  325 , as a guide part formed on side plates  2   b  provided on both inside side surfaces in the width direction of the mainframe casing  302 . 
   The first guide channels  325  are provided in parallel to each other on the mainframe casing  302  with a prescribed interval in the substantially vertical direction, and extend in the cross direction on the mainframe casing  302  at the mounting position of the developing cartridge  315 , i.e., in the detaching direction of the developing cartridge  315 . More specifically, the first guide channel  325  is a channel extending in the substantially horizontal direction, in which the front end thereof is opened toward the opening  308  in the state where the openable cover  306  is opened, and in the case where the developing-side guide projection  323  is positioned at the rear end (the deepest part) of the first guide channel  325 , the developing roller  320  faces and in contact with the photoreceptor drum  334  through rotation of the developing cartridge  315  with the developing-side guide projection  323  as the center. 
   The rear end of the first guide channel  325  is provided with positioning projections  326 . 
   The positioning projection  326  has a substantially triangular shape, and two thereof are provided to face each other in the vertical direction of the first guide channel  325 . The positioning projections  326  are elastically biased in the direction, in which they approach each other, with springs, which are not shown in the figure, and position the developing-side projection  323  to rotate the developing cartridge  315  with the developing-side guide projection  323  as the center. 
   The pressure contact projections  324  are engaged with second guide channels  327  as a guide part formed on side plates  2   b  provided on both sides in the width direction of the mainframe casing  302 . 
   The second guide channels  327  are provided in parallel to each other in the mainframe casing  302  with a prescribed interval in the substantially vertical direction, and extend in the cross direction on the mainframe casing  302  at the mounting position of the developing cartridge  315 , i.e., in the detaching direction of the developing cartridge  315 . More specifically, the second guide channel  327  is a channel extending in the substantially horizontal direction, in which the front end thereof is opened toward the opening  308  in the state where the openable cover  306  is opened, and in the case where the pressure contact projection  324  is positioned at the rear end (the deepest part) of the second guide channel  327 , the developing roller  320  faces the photoreceptor drum  334 . 
   The rear end of the second guide channel  327  is provided with a floating channel  27   a  that is formed to have a substantially circular shape on the side view having a larger diameter than the pressure contact projection  324 , in which the pressure contact projection  324  is floatable. 
   The rear end of the second guide channel  327  is provided with a biasing member  328  as a biasing unit. The biasing member  328  contains a swinging arm  329  and a spring  330 . 
   The swinging arm  329  has a front part having a substantially horseshoe shape on the side view capable of receiving the pressure contact projection  324  by holding in the vertical direction, and a rear part swingably supported by a swinging axis  331  provided on the mainframe casing  302 . 
   According to the configuration, the swinging arm  329  swings with the rear part thereof supported by the swinging axis  331  as supporting point, whereby the front part thereof, swings in the vertical direction at the position facing the floating channel  27   a.    
   The spring  330  has one end in contact with the swinging arm  329  at the midstream of the longitudinal direction thereof, and the other end, above the one end, fixed to a fixing plate  332  provided on the mainframe casing  302 . 
   According to the configuration, the swinging arm  329  is constantly biased downward with the spring  330  and is in contact with a pin, which is not shown in the figure, so as to be retained at a position where an upper member  29   a  of the substantially horseshoe shape member of the front part thereof faces the second guide channel  327 . 
   The upper member  29   a  of the front part of the swinging arm  329  has a slanted front end surface capable of guiding the pressure contact projection  324  into the substantially horseshoe shape member of the front part, as described later. 
   In a state where the developing cartridge  315  is mounted in the mainframe casing  302 , the developing-side guide projection  323  of the developing cartridge  315  is restrained from moving to the front at the rear end of the first guide channel  325  with the positioning projection  326 , and the pressure contact projection  324  is held by the substantially horseshoe shape member of the front part of the swinging arm  329  in the floating channel  27   a  and pressed downward by the biasing force of the spring  330 . According to the configuration, the rear side of the developing cartridge  315  is biased downward, whereby the front side thereof is biased upward with the developing-side guide projection  323  as a supporting point. Consequently, the developing roller  320  of the developing cartridge  315  is made in contact under pressure with the photoreceptor drum  334  of the drum cartridge  316  described later. 
   The toner housing part  318  is formed as an interior space of the developing cartridge  315 , in which plural agitators  333  (for example, three agitators in this embodiment) are provided in the cross direction of the toner housing part  318  as shown in  FIG. 12 . The toner housing parts  318  of the respective developing cartridges  315  are charged with a positively charged non-magnetic one-component toner of yellow color for the yellow developing cartridge  15 Y, that of magenta color for the magenta developing cartridge  15 M, that of cyan color for the cyan developing cartridge  15 C, and that of black color for the black developing cartridge  15 K, respectively. The toner used herein may be a substantially spherical polymerized toner obtained by polymerizing a polymerizable monomer, such as a styrene monomer, e.g., styrene, and an acrylic monomer, e.g., acrylic acid, an alkyl (having from 1 to 4 carbon atoms) acrylate ester and an alkyl (having from 1 to 4 carbon atoms) methacrylate ester, by a known polymerization method, such as suspension polymerization. The toner contains a coloring agent corresponding to the respective colors and wax and is improved in flowability by adding an external additive, such as silica. The toner generally has a particle diameter of about from 6 to 10 μm. 
   The toner contained in the toner housing part  318  is discharged toward the feeding roller  319  through a toner feeding outlet opening on the side surface of the toner housing part  318  by rotation and agitation of the agitators  333  provided in the toner housing part  318 . 
   The feeding roller  319  is rotatablly disposed on the side of the toner feeding outlet, and the developing roller  320  is rotatablly disposed on the side of the feeding roller  319  to face the feeding roller  319 . The feeding roller  319  and the developing roller  320  are rotatablly supported on the developing cartridge  315  in a state where they are in contact with each other under compression to certain extent. 
   The feeding roller  319  is configured by covering a metallic roller axis with a roller formed with an electroconductive sponge member. 
   The developing roller  320  is configured by covering a metallic roller axis with a roller formed with an electroconductive rubber material as an elastic member. More specifically the roller of the developing roller  320  has a two-layer structure containing an elastic roller formed with urethane rubber, silicone rubber, EPDM rubber or the like having electroconductivity with carbon fine particles or the like, having coated on the surface thereof a coating layer mainly containing urethane rubber, a urethane resin, a polyimide resin or the like. The developing roller  320  is disposed in such a manner that an upper part thereof is exposed upward from an opening of the developing cartridge  315  (as shown in  FIG. 14 ), and upon developing, the developing roller  320  is applied with a developing bias voltage from an electric power source, which is not shown in the figure. 
   The squeezing blade  321  has a blade mainframe formed with a metallic leaf spring material and a pressing part having a hemicyclic cross section formed with insulating silicone rubber. One end of the blade mainframe is supported by the developing cartridge  315  in the vicinity of the developing roller  320 , and the pressing part is in contact with the developing roller  320  under pressure by the elastic force of the blade mainframe. 
   The toner discharged from the toner feeding outlet is fed to the developing roller  320  through rotation of the feeding roller  319 , and is charged positively through friction between the feeding roller  319  and the developing roller  320 . The toner thus fed to the developing roller  320  intervenes between the pressing part of the squeezing blade  321  and the developing roller  320  associated with rotation of the developing roller  320  and is consequently carried as a thin layer having a constant thickness on the developing roller  320 . 
   The drum cartridges  316  are disposed above the developing cartridges  315  in the respective process parts  312  for each colors, and are independently detachable to the mainframe casing  302  through the opening  308 , which is formed upon opening the openable cover  306 , while the configurations will be described in detail later. The drum cartridges  316  each contains a photoreceptor drum  334  as an image carrying member and a scorotron charging device  335 . The drum cartridge  316  is provided with a grabbing member  336  at a front end thereof as shown in  FIG. 14 , and is provided with drum-side guide projections  337  as an engaging part on both sides in the width direction. The drum cartridge  316  has an opening on the front side of the lower part thereof to expose a part of the photoreceptor drum  334 , as shown in  FIG. 12 . 
   The grabbing member  336  has a substantially arc shape on the side view and protrudes toward the front on the front end of the drum cartridge  316 . Two grabbing members  336  are provided as opposed to each other on both sides in the width direction of the drum cartridge  316 , i.e., both sides in the axial direction of the photoreceptor drum  334 . 
   The drum-side guide projection  337  has a substantially cylindrical shape and protrudes from both side surfaces of the drum cartridge  316  at the substantially center part of the drum cartridge  316  in the direction extending outward in the width direction. 
   The drum-side guide projections  337  are engaged with third guide channels  338  formed on side plates  2   b  provided on both sides in the width direction inside the mainframe casing  302 . 
   The third guide channels  338  are provided in parallel to each other on the mainframe casing  302  with a prescribed interval in the substantially vertical direction, and extend in the cross direction on the mainframe casing  302  at the mounting position of the drum cartridge  316 , i.e., in the detaching direction of the drum cartridge  16 . More specifically, the third guide channel  338  is a channel extending in the substantially horizontal direction, in which the front end thereof is opened toward the opening  308  in the state where the openable cover  306  is opened, and in the case where the drum-side guide projection  337  is positioned at the rear end (the deepest part) of the third guide channel  338 , the developing roller  320  faces and in contact with the photoreceptor drum  334 . 
   The front end of the third guide channel  338  is provided with a positioning projection  339 . 
   The positioning projection  339  has a substantially triangular shape, and two thereof are provided to face each other in the vertical direction of the third guide channel  338 . The positioning projections  339  are elastically biased in the direction, in which they approach each other, with springs, which are not shown in the figure. 
   In a state where the drum cartridge  316  is mounted in the mainframe casing  302 , the drum-side guide projection  337  of the drum cartridge  316  is engaged with the third guide channel  338  at the front end thereof as being restrained from moving to the front with the positioning projection  339 . 
   The photoreceptor drum  334  is rotatablly disposed on the drum cartridge  316  in a state where it is in contact with the developing roller  320  above the developing roller  320 . According to the configuration, the developing rollers  320  and the photoreceptor drums  334  are alternately arranged, for each of colors, in the substantially vertical direction in the mainframe casing  302 , i.e., in the direction perpendicular to the detaching direction of the developing rollers  320  and the photoreceptor drums  334 , and more specifically, they are alternately arranged in parallel to the conveying direction of the paper  303  conveyed by the conveying belt  347  described later. 
   In the aforementioned arrangement, for each of colors, as shown in  FIG. 15 , the photoreceptor drum  334  and the developing roller  320  are disposed in such a manner that the line X 1  connecting the rotation center P of the photoreceptor drum  334  and the developing position D of the photoreceptor drum  334  facing the developing roller  320  is substantially perpendicular to the line X 2  connecting the rotation center P of the photoreceptor drum  334  and the transferring position T of the photoreceptor drum  334  facing the paper  303  conveyed by the conveying belt  347 . 
   In the aforementioned arrangement, the developing cartridge  315  and the drum cartridge  316  are disposed as not overlapping each other in the detaching direction except for the contact part of the developing roller  320  and the photoreceptor drum  334  (i.e., a part where the surface of the developing roller  320  dented along the surface of the photoreceptor drum  334  by contacting the photoreceptor drum  334  with the developing roller  320  under pressure). 
   In this embodiment, there is such a possibility that the developing roller  320  and the photoreceptor drum  334  overlap each other at the contact part thereof since the developing roller  320  and the photoreceptor drum  334  are in contact with each other. In the case of a jumping developing system with the developing roller  320  and the photoreceptor drum are not in contact with each other, however, there is no part where the developing roller  320  and the photoreceptor drum  334  overlap each other. 
   The photoreceptor drum  334  is configured with a grounded drum mainframe having on the surface thereof a photosensitive layer formed with an organic photoreceptor mainly containing polycarbonate, and is rotatablly supported by the drum cartridge  316 . 
   The scorotron charging device  335  is disposed on the side of the photoreceptor drum  334  opposite to the conveying belt  347  described later with a prescribed interval to the photoreceptor drum  334 , so as not to be in contact therewith, as shown in  FIG. 12 . The scorotron charging device  334  is a scorotron type charging device generating corona discharge from a charging wire, such as tungsten, and is fixed to the drum cartridge  316  for uniformly charging the surface of the photoreceptor drum  334  positively. 
   The scanner unit  317  is disposed, for each of colors, in the process part  312  above the developing cartridge  315  on the side (rear side) of the photoreceptor drum  334  opposite to the transferring position, and is fixed to the mainframe casing  302 . The scanner unit  317  is formed as a casing and contains a laser light emitting part, which is not shown in the figure, a polygonal mirror  340  rotationally driven, lenses  341  and  342 , and a reflecting mirrors  343  and  344 . 
   The scanner unit  317  transmits or reflects a laser beam based on image data emitted from a laser light emitting part by the polygonal mirror  340 , the lens  341 , the reflecting mirrors  343  and  344 , and the lens  342 , in this order, whereby the surface of the photoreceptor drum  334  is irradiated therewith by rapid scanning. 
   The toners of the respective colors are subjected to exposure and development in the following manner in the process parts  312 . The surface of the photoreceptor drum  334  is uniformly charged positively with the scorotron charging device  335  associated with rotation of the photoreceptor drum  334 , and then exposed by light emitted from the scanner unit  317  to form an electrostatic latent image based on the image data. Subsequently, the toner thus positively charged and carried on the developing roller  320  is electrically moved, upon contacting with the photoreceptor drum  334  associated with rotation of the developing roller  320 , onto the electrostatic latent image formed on the surface of the photoreceptor drum  334 , i.e., such a region on the surface of the photoreceptor drum  334  that has a lower electric potential due to exposure with the scanner unit  317 , and selectively carried thereon, whereby a toner image is formed on the surface of the photoreceptor drum  334 . As a result, reversal development is completed. 
   The transfer part  313  is disposed in the mainframe casing  302  in the substantially vertical direction to face the side (front side) of the photoreceptor drums  334  arranged in the substantially vertical direction opposite to the scanner units  317 . The transfer part  313  contains a transfer driving roller  345 , a transfer driven roller  346 , a conveying belt  347  formed with an endless belt, a transfer roller  348 , and a belt cleaner  349 . 
   The transfer driving roller  345  is disposed below the photoreceptor drum  334  of the yellow developing cartridge  15 Y and above the oblique front of the paper feeding roller  310 . The transfer driven roller  346  is disposed above the photoreceptor drum  334  of the black developing cartridge  15 K and below the oblique front of the fixing part  314 . 
   The conveying belt  347  is formed with an electroconductive resin, such as polycarbonate and polyimide, having electroconductive particles, such as carbon, dispersed therein, and is wound on the transfer driving roller  345  and the transfer driven roller  346 . The conveying belt  347  is disposed in such a manner that the contact surface as a back of the wound surface is in contact with all the photoreceptor drums  334  of the drum cartridges  316 . 
   The transfer driven roller  346  is driven by driving the transfer driving roller  345 , whereby the conveying belt  347  is circularly moved between the transfer driving roller  345  and the transfer driven roller  346  in the same direction as the photoreceptor drums  334  of the drum cartridges  316  on the contact surface where the conveying belt  347  faces and is in contact with the photoreceptor drums  334 . 
   The transfer rollers  348  are disposed inside the conveying belt  347  thus wound to face the photoreceptor drums  334  of the drum cartridges  316  with the conveying belt  347  intervening therebetween. The transfer roller  348  is configured by covering a metallic roller axis with a roller formed with an elastic member, such as an electroconductive rubber material. The transfer roller  348  is rotatablly disposed to rotate in the same direction as the rotational moving direction of the conveying belt  347  on the contact surface facing and in contact with the conveying belt  347 , and is applied with a transfer bias voltage upon transferring from an electric power source, which is not shown in the figure. 
   The belt cleaner  349  is disposed on the side (front side) of the conveying belt  347  opposite to the photoreceptor drums  334 , and has, in a cleaner casing  350  disposed on the way from the transfer driving roller  345  to the transfer driven roller  346 , a cleaner brush  351 , a recovering roller  352 , a recovering box  353  and a scraper  354 . 
   The cleaner brush  350  has a cylindrical mainframe having brush formed radially, and is rotationally disposed in such a state that the brush faces and is in contact with the contact surface on the front side of the conveying belt  347 . The cylindrical mainframe of the cleaner brush  351  is applied with a cleaning bias voltage to form an electric potential difference with respect to the conveying belt  347  from an electric power source, which is not shown in the figure, upon cleaning. 
   The recovering roller  352  is formed with a metallic roller and is rotatablly disposed below the cleaner brush  351  to face and be in contact with the cleaner brush  351 . The recovering roller  352  is applied with a recovering bias voltage to form, an electric potential difference with respect to the cleaner brush  351  from an electric power source, which is not shown in the figure, upon cleaning. 
   The recovering box  353  is disposed below the recovering roller  352  and has an opening on a part facing the recovering roller  352 . The scraper  354  is provided in the vicinity of the opening and is in contact with the recovering roller  352  under pressure. 
   The transfer part  313  is supported integrally with the openable cover  306  of the mainframe casing  302 . According to the configuration, in the case where the openable cover  306  is opened, the transfer part  313  moves integrally with the openable cover  306  thus opened, so as to be disposed substantially horizontally. In the case where the openable cover  306  is closed, the transfer part  313  is disposed substantially vertically as shown in  FIG. 12 , and the conveying belt  347  is made in contact with the photoreceptor drums  334  under pressure. 
   The paper  303  fed from the feeder part  304  is conveyed by the conveying belt  347  circularly moving through driving of the transfer driving roller  345  and driven of the transfer driven roller  346  from the lower side toward the upper side under guiding of the conveying roller  10   a , and passes between the conveying belt  347  and the photoreceptor drums  334  of the drum cartridges  316  (i.e., the transferring positions) sequentially. The toner images of the respective colors formed on the photoreceptor drums  334  of the drum cartridges  316  are sequentially transferred to the paper  303  during the period where the paper  303  passes the transferring positions, so as to form a color image on the paper  303 . 
   Specifically, a yellow toner image formed on the photoreceptor drum  334  of the yellow developing cartridge  15 Y is transferred on the paper  303 , and then a magenta toner image formed on the photoreceptor drum  334  of the magenta developing cartridge  15 M is transferred to the paper  303  to overlap the yellow toner image having been transferred. A cyan toner image formed by the cyan developing cartridge  15 C and a black toner image formed by the black developing cartridge  15 K are then sequentially transferred in the same procedures to form a color image on the paper  303 . 
   The toner attached to the conveying belt  347  after transferring the color image to the paper  303  is scraped with and attached to the cleaner brush  351  by the cleaning bias voltage applied to the cleaner brush  351  when the toner faces the cleaner brush  351 . Thereafter, the toner attached to the cleaner brush  351  is attached to the recovering roller  352  by the recovering bias voltage applied to the recovering roller  352  when the toner faces the recovering roller  352 , and then the toner is scraped with the scraper  354  and recovered into the recovering box  353 . 
   In the color laser printer  301 , the remaining toner is recovered by the so-called cleanerless developing system, in which the toner remaining on the surface of the photoreceptor drum  334  is recovered with the developing roller  320 . Specifically, the photoreceptor drum  334  is again charged with the scorotron charging device  335  associated with rotation of the photoreceptor drum  334 , and then exposed with the scanner unit  317 . Upon contacting the developing roller  320  with the photoreceptor drum  334 , the remaining toner present on a non-exposed region on the surface of the photoreceptor drum  334  is moved to and recovered by the developing roller  320  through the developing bias voltage. The toner remaining on an exposed region forms a toner image along with a toner that is newly moved from the developing roller  320  to the exposed region. 
   The peripheral velocity of the photoreceptor drum  334  and the peripheral velocity of the developing roller  320  are differentiated from each other to provide a peripheral velocity difference to facilitate recovery of the remaining toner from the photoreceptor drum  334  to the developing roller  320  through the electric potential difference. Specifically, the peripheral velocity of the developing roller  320  is 1.6 times the peripheral velocity of the photoreceptor drum  334 . The polymerized toner causes a small amount of the remaining toner and is easily movable through an electric potential difference, and therefore, the remaining toner can be favorably recovered by the developing roller  320 . 
   According to the formation of a color-image described in the foregoing, the color laser printer  301  has a tandem type system having plural photoreceptor drums  334  for respective colors, and therefore, toner images of respective colors are formed at a speed equivalent to formation of a monochrome image, so as to attain rapid formation of a color image. 
   The fixing part  314  is disposed above the transfer part  313  and contains a first heating roller  355  in contact with the surface of the paper having a color image transferred thereon, a second heating roller  356  disposed to face the first heating roller  355  with the paper  303  intervening therebetween and being in contact with the back surface of the paper  303 , and a pair of conveying rollers  357  disposed above the first heating roller  355  and the second heating roller  356 . 
   The first heating roller  355  contains a cylindrical metallic tube, such as aluminum, a halogen heater for heating provided therein, and an elastic layer formed outside the tube. The second heating roller  356  contains, as similar to the first heating roller  355 , a cylindrical metallic tube, such as aluminum, a halogen heater for heating provided therein, and an elastic layer formed outside the tube, and is disposed to press the first heating roller  355 . 
   The color image thus transferred on the paper  303  is thermally fixed thereon with the first heating roller  355  and the second heating roller  356  during a period when the paper  303  passes through between the first heating roller  355  and the second heating roller  356 , and then the paper  303  is conveyed with the conveying rollers  357  to paper delivery rollers  358 . 
   The paper delivery rollers  358  are disposed above the conveying roller  357  and in the vicinity of a paper delivery outlet, and the paper  303  thus conveyed with the conveying roller  357  is delivered to the paper delivery tray  359  with the paper delivery rollers  358 . 
   In the color laser printer  301 , one of the developing cartridge  315  and the drum cartridge  316  can be independently detached on the mainframe casing  302 , irrespective to the presence or absence of the other attached to the mainframe casing  302 . For example, in the case where only the developing cartridge  315  is detached, the openable cover  306  is opened to dispose the transfer part  313  in the substantially horizontal direction, so as to form the opening  308 , through which the developing cartridges  315  and the drum cartridges  316  are exposed on the front side of the mainframe casing  302 . 
   In the case where the developing cartridge  315  mounted in the mainframe casing  302  is removed from the mainframe casing  302 , the grabbing member  322  of the developing cartridge  315  is grasped, and the developing cartridge  315  is slightly pushed down. Thus, the rear side of the developing cartridge  315  is biased upward with the developing-side guide projection  323  as a supporting point, whereby the pressure contact projection  324 , which is disposed in the floating channel  27   a  of the second guide channel  327  and held inside the substantially horseshoe shape member at the front side of the swinging arm  329 , presses upward the upper member  29   a  at the front side of the swinging arm  329 , so as to swing upward the front end of the swinging arm  329  against the bias force of the spring  330  with the swinging axis  331  as a supporting point. Accordingly, the pressure contact projection  324  moves upward inside the floating channel  27   a.    
   Subsequently, while the grabbing member  322  of the developing cartridge  315  is still grasped, the developing cartridge  315  is withdrawn to the front side from the mainframe casing  302 . Thus, the developing-side guide projection  323  disposed at the rear end of the first guide channel  325  overstrides the positioning projection  326  by pressing down the positioning projection  326  against the bias force of the positioning projection  326 , and is guided to the front side along the first guide channel  325 , and the pressure contact projection  324  is guided to the front side along the second guide channel  327 , so as to move the developing cartridge  315  to the front side. Thereafter, the developing cartridge  315  is removed from the mainframe casing  302  and taken out through the opening  308 , as shown by a virtual image shown by broken lines in  FIG. 13 . 
   In the case where the developing cartridge  315  is mounted on the mainframe casing  302 , the openable cover  306  is opened, and while the developing cartridge  315  is inserted into the mainframe casing  302  through the opening  308  by grasping the grabbing member  322  of the developing cartridge  315 , the pressure contact projection  324  is engaged with the second guide channel  327 , and then the developing-side guide projection  323  is engaged with the first guide channel  325 . Thereafter, the developing cartridge  315  is pressed onto the rear side of the mainframe casing  302 , whereby the developing cartridge  315  is moved to the rear side with the developing-side guide projection  323  being guided to the rear side along the first guide channel  325 , and the pressure contact projection  324  being guided to the rear side along the second guide channel  327 . Subsequently, the developing-side guide projection  323  overstrides the positioning projection  326  by pressing down the positioning projection  326  against the bias force of the positioning projection  326 , and is retained at the rear end of the first guide channel  325  in a state where it is restrained from moving to the front with the positioning projection  326 . The pressure contact projection  324  is made in contact with the upper member  29   a  at the front side of the swinging arm  329  and guided by  259  the substantially horseshoe shape member at the front side of the swinging arm  329 , whereby the front side of the swinging arm  329  once swings upward with the swinging axis  331  as a supporting point, and when the pressure contact projection  324  is disposed in the floating channel  27   a , the swinging arm  329  is then biased downward with the bias force of the spring  330 , and thus, the front part thereof is moved downward to move the pressure contact projection  324  downward. Consequently, the rear side of the developing cartridge  315  is biased downward, and thus the front side thereof is biased upward with the developing-side guide projection  323  as a supporting point, whereby the developing roller  320  is in contact with the photoreceptor drum  334  under pressure. 
   In the case where the drum cartridge  316  is detached, for example, the openable cover  306  is opened, and the transfer part  313  is substantially horizontally disposed as shown in  FIG. 13 , so as to form the opening  308 , through which the developing cartridges  315  and the drum cartridges  316  are exposed on the front side of the mainframe casing  302 . 
   In the case where the drum cartridge  316  mounted in the mainframe casing  302  is removed from the mainframe casing  302 , the grabbing member  336  of the drum cartridge  316  is grasped, and the drum cartridge  316  is withdrawn from the mainframe casing  302  to the front side. The drum-side guide projection  337  disposed at the rear end of the third guide channel  338  overstrides the positioning projection  339  by pressing down the positioning projection  339  against the bias force of the positioning projection  339 , and is guided to the front side along the third guide channel  338 , and the drum cartridge  316  is moved to the front side. The drum cartridge  316  is then removed from the mainframe casing  302  and taken out through the opening  308 , as shown by a virtual image shown by broken lines in  FIG. 13 . 
   In the case where the drum cartridge  316  is mounted on the mainframe casing  302 , the openable cover  306  is opened, and while the drum cartridge  316  is inserted into the mainframe casing  302  through the opening  308  by grasping the grabbing member  336  of the drum cartridge  316 , the drum-side guide projection  337  is engaged with the third guide channel  338 . Thereafter, the drum cartridge  316  is pressed onto the rear side of the mainframe casing  302 , whereby the drum cartridge  316  is moved to the rear side with the drum-side guide projection  337  being guided to the rear side along the third guide channel  338 . Subsequently, the drum-side guide projection  337  overstrides the positioning projection  339  by pressing down the positioning projection  339  against the bias force of the positioning projection  339 , and is retained at the rear end of the third guide channel  338  in a state where it is restrained from moving to the rear with the positioning projection  339 . 
   Upon detaching the developing cartridge  315  or the drum cartridge  316  as described in the foregoing, the developing roller  320  and the photoreceptor drum  334  having been in contact with each other are not in friction with each other, whereby they can be effectively prevented from being damaged. 
   In the color laser printer  301 , accordingly, one of the developing cartridges  315  and the drum cartridges  316  can be independently detached from the mainframe casing  302 , irrespective to the presence or absence of the other attached to the mainframe casing  302 , whereby only the developing cartridge  315  can be replaced when the toner is emptied out, but the expensive drum cartridge  316  can be used until the service life thereof is expired. As a result, the running cost can be reduced. Furthermore, only the spent developing cartridge  315  may be discarded, and therefore, the amount of industrial waste can be decreased to provide an apparatus for forming an image good for the environment. 
   In the color laser printer  301 , the developing cartridges  315  and the drum cartridges  316  can be smoothly detached through the opening  308  formed on the front side of the mainframe casing  302  upon opening the openable cover  306 . Furthermore, the transfer part  313  is provided in the detaching direction of the developing cartridges  315  and the drum cartridges  316 , whereby treatment of jammed paper and replacement of the developing cartridge  315  and the drum cartridge  316  can be carried out from the same side to provide improved operationality. 
   In the color laser printer  301 , the developing rollers,  320  and the photoreceptor drums  334  are disposed in such a manner that the line X 1  connecting the rotation center P of the photoreceptor drum  334  and the developing position D of the photoreceptor drum  334  facing the developing roller  320  is substantially perpendicular to the line X 2  connecting the rotation center P of the photoreceptor drum  334  and the transferring position T of the photoreceptor drum  334  facing the paper  303  conveyed by the conveying belt  347 , whereby the developing rollers  320  and the photoreceptor drums  334  can be easily replaced independently from each other. In the color laser printer  301 , furthermore, the scanner unit  317  is disposed on the side of the photoreceptor drum  334  opposite to the transferring position, whereby the scanner unit  317  can be disposed while the apparatus is miniaturized without impairing replacement of the developing roller  320  and the photoreceptor drum  334 . 
   In the color laser printer  301 , upon replacing the developing cartridge  315  and the drum cartridge  316 , the developing cartridge  315  and the drum cartridge  316  can be detached from the same side (front side) of the color laser print  1 , whereby the operationality can be improved. 
   In the color laser printer  301 , furthermore, upon replacing the developing cartridge  315  and the drum cartridge  316 , the developing cartridge  315  and the drum cartridge  316  is detached to the mainframe casing  1  by grasping the grab handle  322  of the developing cartridge  315  or the grab handle  336  of the drum cartridge  316 , whereby the developing roller  320  or the photoreceptor drum  334  are not damaged, for example, by accidentally touching them, by grasping the grab handle  322  or  336 , so as to facilitate replacement of the developing cartridge  315  and the drum cartridge  316 . 
   Moreover, upon replacing the developing cartridge  315  and the drum cartridge  316 , the developing cartridge  315  and the drum cartridge  316  are detached to the mainframe casing  302  by grasping the grab handles  322  and  336 , whereby smooth replacement thereof is facilitated to ensure smooth detaching operation, and thus the operationality of the detaching operation can be improved. 
   In the color laser printer  301 , upon mounting the developing cartridge  315  and the drum cartridge  316 , the developing cartridge  315  and the drum cartridge  316  can be guided by the first guide channel  325  and the third guide channel  338 , respectively, provided on the side plate  2   b  of the mainframe casing  302 , and when the developing cartridge  315  and the drum cartridge  316  reach the rear ends thereof, they are positioned with the positioning projection  326  and the positioning projection  339 , respectively, whereby the developing cartridge  315  and the drum cartridge  316  can be thus mounted and positioned. Consequently, a color image with less color drift can be formed while the detaching operation of the developing cartridge  315  and the drum cartridge  316  is facilitated. 
   In the color laser printer  3301 , furthermore, the developing roller  320  of the developing cartridge  315  is biased with the biasing member  328  toward the photoreceptor drum  334  of the drum cartridge  316  having been positioned with the positioning projection  339 , whereby the developing roller  320  and the photoreceptor drum  334  can be in contact with each other while they are positioned. Accordingly, a color image with high quality can be formed while formation failure of an image is prevented from occurring. 
   The color laser printer  301  employs the cleanerless developing system, in which the remaining toner remaining on the photoreceptor drum  334  after transferring the toner image carried on the photoreceptor drum  334  to the paper  303  is recovered by the developing roller  320 , whereby a waste toner storage for recovering and storing the remaining toner is not necessary. Accordingly, the configuration of the apparatus can be simplified, and the apparatus can be miniaturized. As a result, the developing cartridge  315  and the scanner unit  316  can be disposed to overlap substantially vertically owing to the absence of the space for providing a waste toner storage, whereby the footprint of the color laser printer  301  can be reduced. 
   While the vertical standing direct transfer tandem type color laser printer  301  as one embodiment has been described with reference to  FIGS. 1 to 4 , the invention can be applied to a horizontally laid intermediate transfer color laser printer  301  as another embodiment shown in  FIGS. 5 and 6 . In  FIGS. 5 and 6 , the same configurational members as those in  FIGS. 1 and 2  are attached with the same symbols as in  FIGS. 1 and 2 , and this embodiment has the same configuration as in the above embodiment except for those described below. In the following description, the right side in  FIG. 16  having an operating panel  302   a  provided is referred to as a front side of the color laser printer  301 , and the left side is referred to as a rear side of the color laser printer  301 . 
   The color laser printer  301  shown in  FIG. 16  has an intermediate transfer mechanism part  371  as a transfer unit disposed in the cross direction of the mainframe casing  302 , and has an LED array  372  as an exposing unit instead of the scanner unit  317  used in the above embodiment. 
   The mainframe casing  302  has an openable cover  306  as an openable member. 
   The openable cover  306  covers the upper surface of the mainframe casing  302 , and the rear end thereof is connected to the mainframe casing  302  through hinge  307 . The openable cover  306  is openably and closably swung with respect to the mainframe casing  302  with the hinge  307  as a supporting point. In the case where the openable cover  306  is opened as shown in  FIG. 17 , the upper side of the mainframe casing  302  is left open to form an opening  308 , and in the case where the openable cover  306  is closed as shown in  FIG. 16 , the color laser printer  301  is set up as a printable state. 
   The developing cartridges  315  each contains a toner housing part  318 , a feeding roller  319 , a developing roller  320 , a squeezing plate  321  and an agitator  333 , and are arranged in parallel to each other in the cross direction of the mainframe casing  302 . 
   The developing cartridges  315  are detachable along the first guide channels and the second guide channels as similar to the above embodiment, which are provided in parallel to each other on the side plates of the mainframe casing  302  with a prescribed interval in the substantially horizontal direction, and extend in the vertical direction on the mainframe casing  302  at the mounting positions of the developing cartridges  315 , i.e., in the detaching direction of the developing cartridges  315 . 
   The drum cartridges  316  each contains a photoreceptor drum  334  and a scorotron charging device  335  as similar to the drum cartridge  316  in the above embodiment, and arranged in the mainframe casing  302  in parallel to each other in the cross direction of the mainframe casing  302  on the back side of the developing cartridges  315  along the intermediate transfer mechanism part  371  described later. The drum cartridges  316  are detachable along the third guide channels as similar to the above embodiment, which are provided in parallel to each other on the side plates of the mainframe casing  302  with a prescribed interval in the substantially horizontal direction, and extend in the vertical direction on the mainframe casing  302  at the mounting positions of the drum cartridges  316 , i.e., in the detaching direction of the drum cartridges  316 . 
   According to the arrangement, the developing rollers  320  and the photoreceptor drums  334  are alternately arranged in parallel to the circular movement direction of an intermediate transfer belt  375  as a transfer medium described later in a region where it faces the photoreceptor drums  334 . 
   The LED array  372  is disposed in the mainframe casing  302  on the rear side of the developing cartridge  315  on the side (lower side) of the photoreceptor drum  334  opposite to the transferring position facing the intermediate transfer belt  375  described later. The LED array  372  is configured by arranging a large number of LEDs and exposes by irradiating with light the surface of the photoreceptor drum  334  through light emission of the LEDs. 
   The intermediate transfer mechanism part  371  is disposed in the mainframe casing  302  along the cross direction of the mainframe casing  302  to face the photoreceptor drums  334 . The intermediate transfer mechanism part  371  contains an intermediate transfer belt driving roller  373  provided on the rear side of the mainframe casing  302 , an intermediate transfer belt driven roller  374  provided on the front side of the mainframe casing  302 , an intermediate transfer belt  375 , which is wound on the outer peripheries of the intermediate transfer belt driving roller  373  and the intermediate transfer belt driven roller  374  and extends in the substantially horizontal direction, a primary transfer roller  376 , a secondary transfer roller  377 , and a belt cleaner  349 . 
   In the intermediate transfer mechanism part  371 , the intermediate transfer belt driven roller  374  is driven by driving the intermediate transfer belt driving roller  373 , whereby the transfer surface of the intermediate transfer belt  375  facing the photoreceptor drums  334  is circularly moved from the front side toward the rear side of the color laser printer  301 . 
   The primary transfer rollers  376  are disposed to face the photoreceptor drums  334  with the intermediate transfer belt  375  intervening therebetween. The primary transfer roller  376  is configured by covering a metallic roller axis with a roller formed with as an electroconductive rubber material, and is applied with a transfer bias voltage from an electric power source, which is not shown in the figure. 
   The secondary transfer roller  377  is rotatablly disposed at a position facing the intermediate transfer belt driving roller  373  of the intermediate transfer mechanism part  371  with the intermediate transfer belt  375  intervening therebetween. The secondary transfer roller  377  is configured by covering a metallic roller axis with a roller formed with as an electroconductive rubber material, and is applied with a transfer bias voltage from an electric power source, which is not shown in the figure. 
   The belt cleaner  349  is disposed on the side (upper side) of the intermediate transfer belt  375  opposite to the photoreceptor drums  334  and on the way from the intermediate transfer belt driving roller  373  to the intermediate transfer belt driven roller  374 , and contains a cleaner brush  351 , a recovering roller  352 , a recovering box  353  and a scraper  354 , as similar to the above embodiment. 
   In the intermediate transfer mechanism part  371 , the intermediate transfer belt  375  is circularly moved associated with rotation of the intermediate transfer belt driving roller  373  to the intermediate transfer belt driven roller  374  to face the photoreceptor drums  334  sequentially, whereby toner images of respective colors formed on the photoreceptor drums  334  are sequentially accumulated on the intermediate transfer belt  375  to form a color image ion the intermediate transfer belt  375 . The color image thus formed on the intermediate transfer belt  375  is transferred to the paper  303  at once during a period when the paper  303  passes through between the intermediate transfer belt  375  and the secondary transfer roller  377 . 
   The fixing part  314  is disposed above the secondary, transfer roller  377  and contains a first heating roller  355 , a second heating roller  356  and a conveying rollers  357 , as similar to the above embodiment. 
   In the color laser printer  301  according to this embodiment, the intermediate transfer mechanism part  371 , the fixing part  314 , the paper delivery rollers  358  and the operation panel  302   a  are integrally supported by the openable cover  306 . According to the configuration, in the case where the openable cover  306  is opened, the intermediate transfer mechanism part  371 , the fixing part  314 , the paper delivery rollers  358  and the operation panel  302   a  are integrally moved along with the openable cover  306  thus opened, whereby the intermediate transfer belt  375  of the intermediate transfer mechanism part  371  is disposed in the substantially vertical direction, as shown in  FIG. 17 , and in the case where the openable cover  306  is closed, the intermediate transfer belt  375  of the intermediate transfer mechanism part  371  is disposed in the substantially horizontal direction, and the intermediate transfer belt  375  is made in contact with the photoreceptor drums  334  under pressure, as shown in  FIG. 16 . 
   In the color laser printer  301  according to this embodiment, the openable cover  306  is opened to dispose the intermediate transfer mechanism part  371  in the substantially vertical direction, whereby one of the developing cartridge  315  and the drum cartridge  316  can be independently detached as shown by a virtual image shown by broken lines, irrespective to the presence or absence of the other attached to the mainframe casing  302 , as shown in  FIG. 17 . 
   According to the configuration, as similar to the above embodiment, only the developing cartridge  315  can be replaced when the developer is emptied out, but the expensive drum cartridge  316  can be used until the service life thereof is expired. As a result, the running cost can be reduced. Furthermore, only the spent developing cartridge  315  may be discarded, and therefore, the amount of industrial waste can be decreased to provide a color laser printer  301  good for the environment. 
   The color laser printer  301  according to this embodiment is equipped with the LED array  372  instead of the scanner unit  317 , whereby the color laser printer  301  can be miniaturized. While the LED array  372  has a short focal length and is necessarily disposed closely to the photoreceptor drum  334 , in the color laser printer  301 , the LED array  372  is disposed on the side of the photoreceptor drum  334  opposite to the transferring position facing the intermediate transfer belt  375 , whereby the LED array  372  can be disposed closely to the photoreceptor drum  334  without impairing replacement of the developing cartridge  315  and the photoreceptor drum  334 . 
   While the invention has been described with reference to the two embodiments, the invention can be practices with other embodiments than the aforementioned embodiments. For example, while the color laser printer with a direct transfer system has been described as the vertical standing color laser printer  301 , the invention can be applied to a vertical standing color laser printer with an intermediate transfer system. Furthermore, while the scanner unit  317  is exemplified as an exposing unit in the above embodiment, an LED array  372  may be used in the above embodiment. 
   While the color laser printer with an intermediate transfer system has been described as the horizontally laid color laser printer  301  in this embodiment, the intention may be applied to a horizontal laid color laser printer with a direct transfer system. Furthermore, while the LED array  372  is exemplified as an exposing unit in this embodiment, a scanner unit  371  may be used in this embodiment. 
   While the scanner unit  317  is fixed with the developing cartridge  315  and the drum cartridge  316  being detached in the above embodiments, it is possible, for example, that the scanner unit  317  is movably provided, and after evacuating the scanner unit  317 , the developing cartridge  315  and the drum cartridge  316  are detached. 
   While the developing cartridge  315  and the drum cartridge  316  are detached from the front side or the upper side of the color laser printer  301  in the above embodiments, the developing cartridge  315  and/or the drum cartridge  316  may be detached in the width direction in the invention. The detachment in the cross direction or the vertical direction of the color laser printer  301  is preferred as the configuration of the apparatus because the detachment in the width direction is practically associated with movements of bearings of the developing roller  320  and the photoreceptor drum  331 . 
   According to one aspect of the invention, the running cost is reduced, and the amount of industrial waste can be decreased to provide an apparatus for forming an image good for the environment. 
   According to another aspect of the invention, the operationality is improved. 
   According to another aspect of the invention, the replacement of the developing part and the image carrying part can be easily attained independently from each other. 
   According to another aspect of the invention the developing part and the image carrying part can be smoothly detached, and treatment of jammed paper and replacement of the developing part and the image carrying part can be can be carried out from the same direction, so as to improve the operationality. 
   According to another aspect of the invention, the exposing unit can be disposed while the apparatus is miniaturized without impairing replacement of the developing part and the image carrying part. 
   According to another aspect of the invention, the apparatus can be miniaturized, and the LED array can be disposed closely to the image carrying member without interfering replacement of the developing part and the image carrying part. 
   According to another aspect of the invention, the developer carrying member and the image carrying member can be easily replaced independently from each other. 
   According to another aspect of the invention, the developer carrying member and the image carrying member can be easily replaced independently from each other. 
   According to another aspect of the invention, the operationality of the detaching operation can be improved. 
   According to another aspect of the invention, replacement of the developing part and the image carrying part can be smoothly carried out to ensure smooth detaching operation. 
   According to another aspect of the invention, a color image with less color drift can be formed while the detaching operation of the developing part and the image carrying part is facilitated. 
   According to another aspect of the invention, a color image with high quality can be formed while formation failure of an image is prevented from occurring. 
   According to another aspect of the invention, the configuration of the apparatus can be simplified, and the apparatus can be miniaturized. 
   While the invention has been described in conjunction with the specific embodiments described above, many equivalent alternatives, modifications and variations may become apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention as set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments maybe made without departing from the spirit and scope of the invention.