Patent Publication Number: US-8989622-B2

Title: Developer-accommodating vessel and developing device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Applications No. 2009-027008 filed Feb. 9, 2009, 2009-027009 filed Feb. 9, 2009, and 2009-109614 filed Apr. 28, 2009. The entire contents of these priority applications are incorporated herein by references. 
     TECHNICAL FIELD 
     The present invention relates to a developer-accommodating vessel and a developing device provided in a laser printer or other image-forming device. 
     BACKGROUND 
     A toner box is a conventional example of a developer-accommodating vessel provided in an image-forming device for accommodating developer used in image formation. The toner boxes disclosed in above-mentioned patent applications are detachably mounted in a process unit functioning as the developing device of a laser printer. 
     The developer-accommodating vessel includes an exterior casing having a first toner outlet formed therein, and an interior casing having a second toner outlet formed therein. The interior casing accommodates toner. An agitator is also provided in the interior casing for agitating the toner. The agitator rotates about a rotational shaft passing through the center of the circular interior casing. When the agitator is driven to rotate as the first toner outlet formed in the exterior casing is aligned with the second toner outlet formed in the interior casing, the toner agitated by the agitator in the interior casing is discharged sequentially through the second toner outlet and first toner outlet and is supplied to the process unit side. 
     A tandem type color printer is a type of image-forming device well known in the art having a plurality of photosensitive drums arranged in parallel and juxtaposed horizontally. This color printer includes a photosensitive drum unit comprising the plurality of photosensitive drums in one cohesive unit, a developing unit disposed below the photosensitive drum unit and functioning to form toner images on the photosensitive drums, and an intermediate transfer belt unit disposed above the photosensitive drum unit for receiving toner images transferred from the photosensitive drums. 
     The developing unit is provided with a number of developing devices equivalent to the number of photosensitive drums, which developing devices are arranged parallel to each other and juxtaposed horizontally. Each developing device includes a developer case for accommodating toner, and a developing roller disposed at the top of the developer case. 
     In order to perform maintenance on the photosensitive drum unit and developing unit with the printer, these units can be pulled out from the body of the printer in a horizontal direction following the juxtaposed direction of the photosensitive drums. Once the developing unit has been pulled out of the printer, the individual developing devices can be upwardly extracted from the developing unit. 
     SUMMARY 
     In the toner boxes, toner that is supplied from the interior casing of the toner box to the process unit side initially passes through the second toner outlet. However, the second toner outlet is formed in a circumferential surface of the interior casing and does not lie in the rotating path of the agitator. The rotating agitator conveys toner in the interior casing primarily in a direction along the path of the agitator, i.e., the rotating direction of the agitator. 
     In other words, since the second toner outlet is not provided in the rotating path of the agitator, the agitator cannot efficiently supply toner from the interior casing to the process unit through the second toner outlet. 
     Further, replacing the developer case is a maintenance activity that is frequently performed in the printer. However, since the developer case, which must be accessed most frequently, is disposed in the bottom of each developing device, it is necessary to remove the developing roller and the like disposed in the top of the developing unit before refilling the developer case with toner or replacing the developer case. Thus, maintenance of the developer case is troublesome. 
     Further, when the photosensitive drum unit and developer unit are pulled out of the printer together, the photosensitive drum unit disposed on top of the developing unit must be removed in order to remove the developing roller and the like, as described above, making maintenance of the developer case even more troublesome. 
     Therefore, it is an object of the present invention to provide a developer-accommodating vessel capable of efficiently supplying developer externally, and an image-forming device having a structure in which a plurality of developing units are disposed below a plurality of photosensitive drums juxtaposed in parallel and that is capable of facilitating maintenance of the developer-accommodating vessels disposed in the developing unit for accommodating developer. 
     In order to attain above and other object, the present invention provides a toner cartridge. The toner cartridge includes a first toner accommodating unit and an agitating member. The first toner accommodating unit has a peripheral surface in which an inner hollow space for accommodating a toner is formed. The agitating member is disposed in the first toner accommodating unit and rotates in a rotating direction to agitate the toner. The peripheral surface has a wall portion formed with a first supply-hole at an upstream side in the rotating direction. The wall portion is curved about a reference line as a center of curvature and depressed inward the first toner accommodating unit. The reference line is outside of the first toner accommodating unit in a radial direction of the agitating member and extending in a prescribed direction. 
     According to another aspect, the present invention provides a developing device. The developing device includes a casing, a supply roller, and a toner cartridge. The supply roller is mounted on the casing. The toner cartridge is mountable on the casing. The toner cartridge includes a first toner accommodating unit and an agitating member. The first toner accommodating unit has a peripheral surface in which an inner hollow space for accommodating a toner is formed. The agitating member is disposed in the first toner accommodating unit and rotates in a rotating direction to agitate the toner. The peripheral surface has a wall portion formed with a first supply-hole at an upstream side in the rotating direction. The wall portion is curved about a reference line as a center of curvature and depressed inward the first toner accommodating unit. The reference line is outside of the first toner accommodating unit in a radial direction of the agitating member and extending in a prescribed direction. 
     According to still another aspect, the present invention provides an image forming device. The image forming device includes a casing, a photosensitive drum, a developing device, a toner cartridge, and a support frame. The photosensitive drum rotates about a rotational axis. The developing device is disposed beneath the photosensitive drum. The toner cartridge accommodates the toner and is detachably mounted on the developing device to supply the toner. The support frame is drawable from the casing while integrally supporting the developing device and the toner cartridge. The support frame has an allowing unit that allows the toner cartridge to be detached from the developing device while maintaining the developing device to be supported, when the support frame is drawn from the casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a right side cross-sectional view of a color printer  1  according to a first embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view of the color printer when a first front cover moves upward and a second front cover is open; 
         FIG. 3  is a right side cross-sectional view of the color printer in a state that a drawer unit is extracted; 
         FIG. 4  is an enlarged right side cross-sectional view of the color printer; 
         FIG. 5A  is a right-side cross-sectional view of a process cartridge with a toner shutter and a process shutter open; 
         FIG. 5B  is a right-side view of the process cartridge with the toner shutter and the process shutter open; 
         FIG. 5C  is a right-side view of the process cartridge in which a toner box is dismounted therefrom with the toner shutter and the process shutter open; 
         FIG. 6A  is a right side cross-sectional view of a process cartridge with the toner shutter and the process shutter close; 
         FIG. 6B  is a right-side view of the process cartridge with the toner shutter and the process shutter close; 
         FIG. 6C  is a right-side view of the process cartridge in which the toner box is dismounted therefrom with the toner shutter and the process shutter close; 
         FIG. 7  is a perspective view of a shutter unit as viewed from a right-front side; 
         FIG. 8A  is a perspective view of the toner box with the toner shutter close as viewed from the left-front side; 
         FIG. 8B  is a perspective view of the toner box with the toner shutter open as viewed from the left-front side; 
         FIG. 8C  is a perspective view of the toner box with the toner shutter close as viewed from the right-front side; 
         FIG. 8D  is a perspective view of the toner box with the toner shutter open as viewed from the right-front side; 
         FIG. 8E  is a partial cross-sectional view around a pawl of the toner box; 
         FIG. 9A  is a right side cross-sectional view of the process cartridge while rotating an agitator with the toner shutter and the process shutter open; 
         FIG. 9B  is a right side cross-sectional view of the process cartridge after passing a prescribed time from  FIG. 9A ; 
         FIG. 9C  is a right side cross-sectional view of the process cartridge after passing a prescribed time from  FIG. 9B ; 
         FIG. 9D  is a right side cross-sectional view of the process cartridge after passing a prescribed time from  FIG. 9C ; 
         FIG. 10  is a perspective view of a toner box as viewed from right-front side according to a modification of the first embodiment; 
         FIG. 11  is a schematic cross-sectional view of a laser printer provided with a developing unit according to a second embodiment of the present invention; 
         FIG. 12  is a schematic cross-sectional view of the developing unit; 
         FIG. 13  is an exploded perspective view of a toner cartridge as viewed from right-front side; 
         FIG. 14A  is a schematic illustration of a relationship among a toner shutter, a first supply hole, and a second supply hole in a state in which the toner shutter is close; 
         FIG. 14B  is a schematic illustration of a relationship among the toner shutter, the first supply hole, and the second supply hole in a state in which the toner shutter is open; 
         FIG. 15  is a schematic cross-sectional view of the developing unit showing an each room formed therein as viewed from top; 
         FIG. 16  is a schematic cross-sectional view of the toner cartridge taken along a line in which the second supply hole is formed as viewed from left side; 
         FIG. 17  is an exploded perspective view of the toner cartridge as viewed from left-front side; 
         FIG. 18  is an exploded perspective view of a developing device and a process cartridge; 
         FIG. 19A  is a schematic illustration of the developing unit when the toner shutter and a process shutter are close; and 
         FIG. 19B  is a schematic illustration of the developing unit when the toner shutter and a process shutter are open. 
     
    
    
     DETAILED DESCRIPTION 
     1. General Structure of a Color Printer 
       FIG. 1  is a right side cross-sectional view of a color printer  1  serving as the image-forming device according to a first embodiment of the present invention.  FIG. 2  shows the same view of the color printer  1  in  FIG. 1  when a unit-mounting opening  49  is open.  FIG. 3  shows the color printer  1  in  FIG. 1  when a drawer unit  14  has been pulled out from a main casing  2 .  FIG. 4  is a right side view of the color printer  1  in the state shown in  FIG. 3 . For the sake of description, one process cartridge  16  is shown in  FIG. 4  after being extracted upward from the drawer unit  14 . 
     As shown in  FIG. 1 , the color printer  1  includes a main casing  2  forming the body of the color printer  1 . The main casing  2  is box-shaped and slightly elongated vertically. 
     Within the main casing  2 , the color printer  1  also includes a feeding unit  3  for supplying sheets of a paper P to be printed, an image-forming unit  4  for forming images on the paper P supplied from the feeding unit  3 , and a discharge unit  5  for discharging the paper P from the main casing  2  after an image has been formed thereon. The main casing  2  is provided with a first front cover  31  and a second front cover  50  on a front wall thereof. 
     Unless otherwise specified, the orientation of the color printer  1  and the orientation of the individual components in the color printer  1  will be described in accordance with the directional arrows shown in the drawing. This holds true for  FIG. 1  as well as subsequent drawings. The directions specified herein are defined based on the perspective of a user standing in front of the color printer  1  while facing the color printer  1 . Thus, the left side of the color printer  1  is the far side of the drawing in  FIG. 1 , while the right side is the near side. The left-to-right direction will also be referred to as the width direction, and both the left-to-right (width) direction and the front-to-rear direction are considered horizontal directions. 
     (1) Feeding Unit 
     The feeding unit  3  includes a paper tray  6 , a feeding mechanism  7 , and a pair of registration rollers  8 . 
     The paper tray  6  is disposed in the bottom section of the main casing  2  and extends in a horizontal direction. The paper tray  6  may be removed from the main casing  2  on the front side thereof. The paper tray  6  holds sheets of paper P in a stacked state. The feeding mechanism  7  is disposed at the rear end of the paper tray  6  and functions to feed sheets of paper P from the paper tray  6  toward the image-forming unit  4 . The feeding mechanism  7  includes components well known in the art for which reference numerals have not been assigned in  FIG. 1 , including a feeding roller, a separating roller, a separating pad, a paper dust roller, and a paper-conveying path. With this configuration, the feeding mechanism  7  functions to separate and feed sheets of paper P accommodated in the paper tray  6  upward to the registration rollers  8  one sheet at a time. 
     When the leading edge of the sheet of paper P reaches the registration rollers  8 , the registration rollers  8  adjust the registration of the sheet and subsequently convey the sheet of paper P to the image-forming unit  4  at a prescribed timing. 
     (2) Image-forming Unit 
     The image-forming unit  4  includes a scanning unit  10 , a process unit  11 , a transfer unit  12 , and a fixing unit  13 . 
     (2-1) Scanning Unit 
     The scanning unit  10  is disposed in the bottom section of the main casing  2  above the paper tray  6 . The scanning unit  10  includes various components well known in the art for which reference numerals have not been assigned in  FIG. 1 , including a laser light-emitting unit, a polygon mirror, and a plurality of lenses and reflecting mirrors. The laser light-emitting unit of the scanning unit  10  emits laser beams based on image data inputted from a device external to the color printer  1  or from an image-reading unit  35  described later. The laser beams are guided along paths indicated by dotted lines in  FIG. 1  and are ultimately irradiated onto photosensitive drums  19  described later that are provided in the process unit  11 . 
     (2-2) Process Unit 
     The process unit  11  is disposed above the scanning unit  10  and includes a drawer unit  14 . 
     The drawer unit  14  is detachably mounted in the main casing  2 . As will be described later, the drawer unit  14  is mounted in and removed from the main casing  2  along the front-to-rear direction. The drawer unit  14  includes a hollow, box-shaped drawer frame  15 , and four process cartridges  16  disposed inside the drawer frame  15 . 
     An opening  15 A is formed in nearly the entire top surface of the drawer frame  15 . The opening  15 A provides communication between the interior of the drawer frame  15  and the region above the drawer frame  15 . A plurality of through-holes  15 B is formed in the bottom wall of the drawer frame  15 . The laser beams irradiated by the laser light-emitting units in the scanning unit  10  pass through corresponding through-holes  15 B before being irradiated on the photosensitive drums  19 . Four circular exposure holes  15 C (see  FIG. 4 ) are formed in the right wall of the drawer frame  15  at positions aligned in the front-to-rear direction. The circular exposure holes  15 C penetrate the right wall of the drawer frame  15  in the width direction so as to provide communication with the interior of the drawer frame  15 . 
     The process cartridges  16  are detachably mounted in the drawer frame  15  via the opening  15 A. When viewed along the width direction, the four process cartridges  16  mounted in the drawer frame  15  are arranged parallel to each other and juxtaposed at substantially equal intervals in the front-to-rear direction (almost horizontally). Each of the process cartridges  16  is disposed at a slight incline to the vertical, with the top positioned farther rearward than the bottom. 
     The following description of the process cartridges  16  will be based on the state of the process cartridges  16  when they are mounted in the drawer frame  15  and oriented at a slight incline to the vertical, with the top farther rearward from the bottom. Since the structure of the four process cartridges  16  is identical, the following description will focus on the forwardmost process cartridge  16  in  FIG. 1 . 
     Each process cartridge  16  is provided with an upper case  17  on the top side, and a lower case  18  on the bottom. Together, the upper case  17  and lower case  18  form a hollow box shape elongated in the width direction. The upper case  17  primarily accommodates a photosensitive drum  19  and a charger  20 . 
     The photosensitive drum  19  has a central shaft  19 A extending in the width direction. Hence, the axial direction of the photosensitive drum  19  corresponds to this width direction, and the photosensitive drum  19  is elongated in the width direction. The widthwise ends of the photosensitive drum  19  are rotatably supported in corresponding side walls constituting the widthwise side walls of the upper case  17 . The top of the upper case  17  is open so that the upper peripheral surface of the photosensitive drum  19  is exposed in the top surface of the upper case  17  from a perspective above the upper case  17  (process cartridge  16 ). The upper peripheral surface of the photosensitive drum  19  is also exposed through the opening  15 A of the drawer frame  15  from a perspective above the drawer frame  15  (drawer unit  14 ). When viewed from the top, the four photosensitive drums  19  are arranged parallel to each other and juxtaposed at substantially equivalent intervals in the front-to-rear direction (nearly horizontal). 
     The charger  20  is held between both widthwise side walls of the upper case  17  and is positioned to oppose the lower rear surface of the corresponding photosensitive drum  19  from a distance. The lower case  18  is coupled to the upper case  17  by means of a coupling shaft  21  inserted through the upper rear corners of the lower case  18  in the width direction as shown in  FIG. 4 . With the coupling shaft  21 , the lower case  18  can move relative to the upper case  17 , and more specifically, can pivot relative to the upper case  17  about the coupling shaft  21 . 
     The lower case  18  primarily accommodates a developing roller  22 , a supply roller  23 , and a toner box  24 . Central shafts for each of the developing roller  22  and supply roller  23  extend in the width direction. The lower case  18  has widthwise side walls in which both widthwise ends of the developing roller  22  and supply roller  23  are rotatably supported. 
     The developing roller  22  is disposed in the upper end of the lower case  18 . The top of the lower case  18  is open so that the upper peripheral surface of the developing roller  22  is exposed in the top surface of the lower case  18  from a perspective above the lower case  18 . 
     The upper case  17  is also open on the bottom so that the upper peripheral surface of the developing roller  22  exposed in the top opening of the lower case  18  opposes and contacts the lower front peripheral surface of the photosensitive drum  19  through the bottom opening in the upper case  17 . More specifically, urging members (not shown) are provided for urging the entire lower case  18  supporting the developing roller  22  upward toward the upper case  17  supporting the photosensitive drum  19  so that the developing roller  22  contacts the photosensitive drum  19 . 
     The supply roller  23  is disposed in contact with the developing roller  22  on the lower front side thereof. The toner box  24  is disposed in an area of the lower case  18  below the supply roller  23  (a box-accommodating chamber  73  described later; see  FIG. 5C ). As will be described later, the toner box  24  is detachably mounted in the lower case  18 . The toner box  24  is formed with an inner hollow space and has a substantially cylindrical shape that is elongated in the width direction. The toner box  24  accommodates toner in the inner hollow space in one of the corresponding colors cyan, magenta, yellow, or black. 
     The lower case  18  and the group of members accommodated in the lower case  18  (the developing roller  22 , supply roller  23 , toner box  24 , and the like) are referred to as a developing unit  37 . The number of developing units  37  is equivalent to the number of process cartridges  16  (four in the preferred embodiment), and each developing unit  37  is disposed below a corresponding upper case  17  (i.e., below the photosensitive drum  19  provided in the upper case  17 ). The drawer unit  14  including the process cartridges  16  and toner boxes  24  is described below in greater detail. 
     When forming images with this process unit  11 , the charger  20  in each process cartridge  16  applies a uniform electrical charge to the peripheral surface of the corresponding photosensitive drum  19 . Subsequently, the scanning unit  10  irradiates a laser beam (indicated by a dotted line in  FIG. 1 ) onto the peripheral surface of the charged photosensitive drum  19  via the corresponding through-hole  15 B formed in the bottom surface of the drawer frame  15 , forming an electrostatic latent image on the peripheral surface of the photosensitive drum  19  that corresponds to an image to be formed on paper P. 
     In the meantime, components in the developing unit  37  of each process cartridge  16  supply toner from the toner box  24  downward to the supply roller  23  (this will be described later in greater detail). The supply roller  23  supplies the toner received from the toner box  24  to the developing roller  22 . The developing roller  22  carries on its peripheral surface a thin layer of toner that has been regulated to a prescribed thickness. 
     As the photosensitive drum  19  rotates, the electrostatic latent image formed on the peripheral surface of the photosensitive drum  19  rotates into a position opposite the developing roller  22 , at which time the toner carried on the surface of the developing roller  22  is supplied to the electrostatic latent image, developing the latent image into a visible toner image of the prescribed color. 
     Thus, each developing unit  37  functions to develop an electrostatic latent image found on the corresponding photosensitive drum  19 . Further, the toner box  24  in each developing unit  37  accommodates toner used to develop the latent image on the corresponding photosensitive drum  19 . 
     (2-3) Transfer Unit 
     The transfer unit  12  includes a follow roller  25 , a drive roller  26 , an intermediate transfer belt  27 , primary transfer rollers  28 , a secondary transfer roller  29 , and a cleaning unit  30 . 
     The follow roller  25  and drive roller  26  are disposed parallel to each other and separated in the front-to-rear direction. The endless intermediate transfer belt  27  is looped over and pulled taut between the follow roller  25  and drive roller  26 . In this state, the intermediate transfer belt  27  is adjacent to the top side of the drawer unit  14 . When the drive roller  26  is driven to rotate, the intermediate transfer belt  27  moves circularly about the follow roller  25  and drive roller  26 , while the follow roller  25  follows the rotation of the drive roller  26 . 
     Four primary transfer rollers  28  are arranged inside the intermediate transfer belt  27  at intervals in the front-to-rear direction. Each primary transfer rollers  28  is positioned opposite a corresponding photosensitive drum  19  so as to pinch the lower portion of the intermediate transfer belt  27  against the photosensitive drum  19 . The position at which the photosensitive drum  19  contacts the intermediate transfer belt  27  is referred to as a primary transfer position. A high voltage circuit board (not shown) applies a first transfer bias to the primary transfer rollers  28 , causing the toner image on each photosensitive drum  19  to be transferred onto the intermediate transfer belt  27  at the corresponding primary transfer position. Toner images in four colors transferred from the four photosensitive drums  19  become superimposed on the intermediate transfer belt  27  to form a color toner image. 
     The secondary transfer roller  29  is disposed on the rear side of the drive roller  26  and applies pressure to the drive roller  26  via the intermediate transfer belt  27 . The position at which the secondary transfer roller  29  contacts the drive roller  26  (intermediate transfer belt  27 ) is referred to as a secondary transfer position. A high voltage circuit board (not shown) applies a second transfer bias to the secondary transfer roller  29 , causing the color toner image formed on the intermediate transfer belt  27  to be transferred onto a sheet of paper P at the secondary transfer position as the registration rollers  8  convey the sheet upward. 
     The cleaning unit  30  is disposed above the intermediate transfer belt  27 . The cleaning unit  30  functions to recover toner remaining on the intermediate transfer belt  27  after the toner image has been transferred to the paper P and to retain the toner within. The cleaning unit  30  may be replaced by opening the first front cover  31  rotatably attached to the front wall of the main casing  2 . 
     (2-4) Fixing Unit 
     The fixing unit  13  is disposed on the downstream side of the transfer unit  12  with respect to the conveying direction of the paper P, and more specifically above the contact position between the drive roller  26  and secondary transfer roller  29  (second transfer position). The fixing unit  13  includes a heating roller  32  and a pressure roller  33  well known in the art. The pressure roller  33  contacts and applies pressure to the heating roller  32 . As a sheet of paper P passes between the heating roller  32  and pressure roller  33 , the toner image transferred onto the sheet is fixed to the sheet by heat and pressure. 
     (3) Discharge Unit 
     The discharge unit  5  has a plurality of conveying rollers (not indicated with reference numerals in  FIG. 1 ) for conveying a sheet of paper P discharged from the fixing unit  13  onto a discharge tray  34  formed on top of the main casing  2 . 
     (4) Other Components 
     An image-reading unit  35  is provided in the main casing  2  at a position covering the top of the discharge tray  34 . The image-reading unit  35  takes in an original document and scans image data from the document. The color printer  1  can form images based on the image data scanned by the image-reading unit  35 . 
     Thus, the color printer  1  can both read and form images and is therefore referred to as a multifunction peripheral. The color printer  1  is also referred to as an in-body paper discharge type device, since the sheets of paper P are discharged onto the discharge tray  34 , which is located vertically midway in the main casing  2  (below the image-reading unit  35 ). 
     Next, the components of the color printer  1  will be described in greater detail. 
     2. Main Casing 
     A unit-accommodating chamber  40  is formed in the main casing  2  for accommodating the drawer unit  14 . The unit-accommodating chamber  40  is a space defined vertically by the intermediate transfer belt  27  and the scanning unit  10 . As shown in  FIG. 3 , a pair of partitioning walls  41  is provided in the main casing  2  for defining the left and right boundaries of the unit-accommodating chamber  40 . Only the left partitioning wall  41  is shown in  FIG. 3 . 
     A guide part  42  is provided inner surfaces of the partitioning walls  41  (the right surface in the case of the left partitioning wall  41  shown in  FIG. 3 ). As shown in  FIGS. 2 and 3 , the guide part  42  includes in order beginning from the front side, a first guiding roller  43 , a first sloped wall  44 , a first guiding rail  45 , a second guiding roller  46 , a second sloped wall  47 , and a second guiding rail  48 . 
     The first guiding roller  43  is rotatably disposed on the front end of the corresponding partitioning wall  41  (see  FIG. 2 ). The first guiding rail  45  is formed in a plate shape that extends evenly in the front-to-rear direction (see  FIG. 3 ). The front end of the first guiding rail  45  is in proximity with the rear side of the first guiding roller  43  (not illustrated in the drawings). The first guiding rail  45  is disposed at a vertical position that is substantially equivalent to the lower portion of the first guiding roller  43  (see  FIGS. 2 and 3 ). 
     When viewed along the width direction, the first sloped wall  44  is substantially triangular in shape, narrowing toward the rear side (see  FIG. 2 ). The top surface of the first sloped wall  44  slopes downward from the top part of the first guiding roller  43  to the top surface of the first guiding rail  45  (see  FIGS. 2 and 3 ). 
     As shown in  FIG. 3 , the second guiding roller  46  is rotatably disposed on the rear end of the corresponding partitioning wall  41  adjacent to the rear end of the first guiding rail  45 . The second guiding roller  46  is positioned so that its upper part is substantially equivalent in position to the first guiding rail  45  relative to the vertical. 
     The second guiding rail  48  is formed in a plate shape that extends evenly in the front-to-rear direction, and then curves upward on the rear end. The second guiding rail  48  is disposed at substantially the same position as the lower portion of the second guiding roller  46  with respect to the vertical. When viewed along the width direction, the second sloped wall  47  has a substantially triangular shape, tapering toward the rear. The top surface of the second sloped wall  47  slopes downward from the top portion of the second guiding roller  46  to the top surface of the second guiding rail  48 . 
     As shown in  FIG. 1 , a unit-mounting opening  49  is formed in the front wall of the main casing  2 . The unit-mounting opening  49  allows communication between the area on the front side of the color printer  1  and the unit-accommodating chamber  40  (see  FIGS. 2 and 3 ). The second front cover  50  is provided on the front wall of the main casing  2  and is capable of opening and closing thereon. In the closed state shown in  FIG. 1  the second front cover  50  is in an erect orientation and blocks the unit-mounting opening  49  on the front side thereof. In this state of the second front cover  50 , a rotational shaft  51  extending in the width direction penetrates the bottom edge of the second front cover  50 . The second front cover  50  is coupled to the front wall of the main casing  2  via the rotational shaft  51  and is capable of pivotally rotating about the rotational shaft  51 . When an operator pulls the second front cover  50  forward from the closed position shown in  FIG. 1 , the second front cover  50  rotates forward and downward about the rotational shaft  51  (see  FIGS. 2 through 4 ). At this time, the second front cover  50  is in the open state and the unit-mounting opening  49  is exposed on the front side of the color printer  1  (see  FIGS. 2 and 3 ). 
     A pivoting unit  52  is provided in the main casing  2  above the second front cover  50  when the second front cover  50  is in the closed state. The pivoting unit  52  is an integrated unit that includes the discharge tray  34  and various parts positioned beneath the discharge tray  34  (specifically the intermediate transfer belt  27 , follow roller  25 , drive roller  26 , primary transfer rollers  28 , and cleaning unit  30 ), as well as the first front cover  31  described earlier. The pivoting unit  52  is capable of pivoting about the drive roller  26  of the transfer unit  12  and a pivoting shaft  53  disposed on the rear side of the discharge tray  34  so that the front of the pivoting unit  52  moves vertically. 
     When the second front cover  50  is in the closed state shown in  FIG. 1 , the lower front end of the pivoting unit  52  is engaged with the top end of the second front cover  50 , restricting forward rotation of the second front cover  50 . Therefore, the second front cover  50  can be maintained in the closed state. 
     When the pivoting unit  52  is pivoted upward from the state shown in  FIG. 1  to the state shown in  FIG. 2 , the lower front end of the pivoting unit  52  disengages from the top end of the second front cover  50 , allowing the second front cover  50  to be rotated forward and downward. By rotating the second front cover  50  downward on the front side, the operator can expose the unit-mounting opening  49  described above. 
     3. Drawer Unit 
     (1) Drawer frame 
     As shown in  FIG. 4 , the drawer frame  15  has a hollow box shape with the opening  15 A formed in the top surface. 
     Four guiding grooves  60  are formed in the inner widthwise surface on each widthwise side walls of the drawer frame  15  (i.e., the widthwise surfaces of the drawer frame  15  facing inwardly). The guiding grooves  60  are formed in the upper end of the side walls at substantially equal intervals in the front-to-rear direction and extend downward along a forward slope from the top edge of the inner surface. The innermost part (bottom end) of each guiding groove  60  is rounded so that each guiding groove  60  has a substantial U-shape when viewed in the width direction. 
     A positioning shaft  61  is integrally provided on the upper front corner of each widthwise wall of the drawer frame  15 , protruding outward in the width direction therefrom. The top edge of each widthwise wall of the drawer frame  15  is bent outward in the width direction to form a flange  62 . 
     As shown in  FIG. 3 , a protruding piece  63  is integrally provided on the upper rear end of each widthwise wall in the drawer frame  15 , protruding rearward. Two wheels  64  juxtaposed in the front-to-rear direction are provided on each protruding piece  63 . Rotational shafts of the wheels  64  extend in the width direction and are supported in the protruding pieces  63 . Thus, the wheels  64  are rotatably supported by the protruding pieces  63 . 
     A rotational shaft  65  and a handle  66  are provided on the front wall of the drawer frame  15 . The rotational shaft  65  extends in the width direction and is supported in the front wall of the drawer frame  15 . The rotational shaft  65  is inserted through one end (the rear end in  FIG. 3 ) of the handle  66 , by which the handle  66  is capable of rotating about the rotational shaft  65  (see  FIGS. 1 through 4 ). The operator grips the handle  66  when mounting the drawer unit  14  in or removing the drawer unit  14  from the main casing  2 . 
     (2) Process Cartridges 
       FIG. 5  shows one of the process cartridges  16  when a process shutter  83  and a toner shutter  112  described later are both in an open position.  FIG. 5A  is a right side cross-sectional view of the process cartridge  16 ;  FIG. 5B  is a right side view of the process, cartridge  16 ; and  FIG. 5C  illustrates the process cartridge  16  when the toner box  24  is separated from all other parts of the process cartridge  16 . 
       FIG. 6  shows the process cartridge  16  when the process shutter  83  and the toner shutter  112  are both in a closed position.  FIG. 6A  is a right side cross-sectional view of the process cartridge  16 ;  FIG. 6B  is a right side view of the process cartridge  16 ; and  FIG. 6C  shows the process cartridge  16  when the toner box  24  is separated from all other parts of the process cartridge  16 . 
     (2-1) Upper Case and Lower Case 
     As shown in  FIG. 5A , the upper case  17  of each process cartridge  16  has a hollow box shape elongated in the width direction and open on the top and bottom surfaces. The upper case  17  supports the photosensitive drum  19  and charger  20 . Both widthwise ends of the central shaft  19 A of the photosensitive drum  19  penetrate the corresponding side walls of the upper case  17  in the width direction and protrude outward from the upper case  17  in the width direction (see  FIG. 5B ). 
     A cleaning roller  70  is rotatably supported in the upper case  17  above the charger  20 . The outer peripheral surface of the cleaning roller  70  contacts the outer surface of the photosensitive drum  19  on the rear side thereof. The cleaning roller  70  functions to remove the toner and other foreign matter from the peripheral surface of the photosensitive drum  19  after a toner image has been transferred from the photosensitive drum  19  to the intermediate transfer belt  27  (see  FIG. 1 ). 
     As described above, the lower case  18  has a hollow box shape elongated in the width direction and is open on the top. While the lower case  18  can move relative to the upper case  17 , which is coupled to the lower case  18  via the coupling shaft  21 , as described above, the lower case  18  in  FIG. 5A  is fixed in position so that the developing roller  22  contacts the lower front side of the photosensitive drum  19 . 
     When viewed along the width direction, the lower case  18  resembles a teardrop that is wider toward the bottom. Accordingly, the interior of the lower case  18  also grows wider in the front-to-rear direction toward the bottom side. The interior of the lower case  18  is partitioned into a developing chamber  71 , a supply chamber  72 , and a box-accommodating chamber  73  in order from top to bottom. 
     When viewed along the width direction, the developing chamber  71  appears to be shaped substantially like a parallelogram tilted downward to the rear. The developing chamber  71  is exposed from above (outside of the lower case  18 ) through the open top surface of the lower case  18 . 
     Viewed in the width direction, the supply chamber  72  is substantially circular in shape and in communication with the lower front side of the developing chamber  71 . 
     An arced wall  74  forming part of the front wall of the lower case  18  corresponding to the supply chamber  72  bulges forward in an arc shape. A first protruding wall  75  integrally provided on the front wall of the lower case  18  protrudes continuously from the lower edge of the arced wall  74  in a rearward direction into the lower case  18  and at the same curvature as the arced wall  74 . Viewed along the width direction, the first protruding wall  75  is substantially shaped like a letter U that has been flattened vertically so as to curve in an arc that is convex on the bottom side. Viewed from the right side, the arced wall  74  and first protruding wall  75  together are shaped like the letter C with an opening in the rear side. The region within the C-shaped portion formed by the arced wall  74  and first protruding wall  75  is the supply chamber  72 . 
     When viewed in the width direction, the box-accommodating chamber  73  is substantially circular in shape and more than twice the size of the supply chamber  72 . More specifically, a portion of the circumference of the circularly shaped box-accommodating chamber  73  in a side view is depressed inwardly to form an arc shape that is concave on the top. The bottom wall of the lower case  18  (including the lower ends of the front wall and rear wall) is formed in an arc shape conforming to the shape of the box-accommodating chamber  73  that is convex on the bottom. A second protruding wall  76  integrally provided on the rear wall of the lower case  18  is formed continuously with the rear end on the bottom wall of the lower case  18  and protrudes upward and forward into the lower case  18  at the same curvature as the bottom wall. The second protruding wall  76  is arc-shaped when viewed in the width direction and extends upward and forward while bulging upward and rearward. The rear end of the first protruding wall  75  described above is connected to the front (top) end of the second protruding wall  76 . The second protruding wall  76  defines the lower rear side of the developing chamber  71 . 
     In a right side view, the bottom wall of the lower case  18  and the second protruding wall  76  are taken together form an approximate C-shape having a cutout part in the top. The first protruding wall  75  is provided in the cutout region of this C-shape and protrudes thereto. The box-accommodating chamber  73  is the region defined by the bottom wall of the lower case  18 , the second protruding wall  76 , and the first protruding wall  75 . The toner box  24  (and the toner shutter  112  and an agitator  111  described later provided in the toner box  24 ) is accommodated in the box-accommodating chamber  73 , as shown in  FIG. 5A . 
     A box-mounting opening  84  that is substantially circular and is approximately the same shape as the box-accommodating chamber  73  when viewed along the width direction is formed in a portion of the right wall in the lower case  18  corresponding to the box-accommodating chamber  73  in the width direction. The box-accommodating chamber  73  is exposed on the right side of the lower case  18  through the box-mounting opening  84  (see  FIG. 5C ). The portion of the left wall of the lower case  18  aligned with the box-accommodating chamber  73  in the width direction is substantially circular in shape and approximately the same shape and size as the box-accommodating chamber  73  in the width direction. A coupling gear  85  is rotatably disposed in the center of this circular region (see  FIG. 5C ). 
     A single through-hole  77  is formed in the upper rear portion of the first protruding wall  75 , which appears to be shaped substantially like the letter U (or a tray) that has been flattened vertically when viewed along the width direction. The through-hole  77  is a slit extending in the width direction that penetrates the first protruding wall  75  in the front-to-rear direction to provide communication between the supply chamber  72  and box-accommodating chamber  73 . Specifically, the box-accommodating chamber  73  communicates with the lower rear portion of the supply chamber  72  via the through-hole  77 . Since the through-hole  77  is formed in the upper rear side of the first protruding wall  75 , part of the first protruding wall  75  is positioned lower than the lower edge defining the through-hole  77 . This part is an accumulating part  75 A. 
     With this configuration of the lower case  18 , the developing roller  22  described earlier is disposed in the developing chamber  71 , and the supply roller  23  is disposed in the supply chamber  72 . The lower peripheral surface of the supply roller  23  opposes the first protruding wall  75  from above and follows the upper curved surface of the first protruding wall  75 . The through-hole  77  formed in the first protruding wall  75  confronts the lower rear portion of the supply roller  23 . 
     A thickness-regulating blade  78  is also disposed in the developing chamber  71 . The thickness-regulating blade  78  is integrally provided with a leaf spring  79  formed in a thin plate shape elongated in the width direction, and a rubber pressing part  80  disposed on the front end of the leaf spring  79 . The leaf spring  79  extends from the rear wall of the lower case  18  in a direction upward and forward along the second protruding wall  76  toward the bottom peripheral surface of the developing roller  22 . The elastic force of the leaf spring  79  presses the pressing part  80  against the bottom peripheral surface of the developing roller  22 . The gap between the leaf spring  79  and the second protruding wall  76  is filled by a seal  81 . The gap between the front peripheral surface of the developing roller  22  and the front wall of the lower case  18  is filled by a seal  82 . 
     The process shutter  83  is provided in the lower case  18  for opening and closing the through-hole  77 . The process shutter  83  has a thin plate shape elongated in the width direction. In the width direction, the process shutter  83  is shaped substantially like a letter U that has been flattened vertically, similar to the first protruding wall  75  and is positioned along the bottom of the first protruding wall  75 . The process shutter  83  can slide in a direction along the curved surface of the first protruding wall  75 . The toner shutter  112  mentioned earlier is positioned immediately below the process shutter  83  in  FIG. 5A . 
     In  FIG. 5A , the entire process shutter  83  is positioned below the first protruding wall  75 . This position is the open position in which the process shutter  83  is shifted downward and forward from the through-hole  77 . When the process shutter  83  is in the open position, the through-hole  77  is open. 
     When the process shutter  83  is slid a prescribed amount from the open position upward and rearward along the lower surface of the first protruding wall  75 , the process shutter  83  arrives in the closed position shown in  FIG. 6A . In the closed position, the process shutter  83  blocks the entire through-hole  77  from the bottom rear side. 
     The process shutter  83  is returned to the open position shown in  FIG. 5A  when slid a prescribed distance from the closed position along a downward and forward arc following the lower surface of the first protruding wall  75 . 
     A support member  86  is disposed in the lower case  18  for supporting the process shutter  83 . The support member  86  is formed integrally with the process shutter  83 . Taken together, the process shutter  83  and support member  86  constitute a shutter unit  87 . 
       FIG. 7  is a perspective view of the shutter unit  87  from a front right viewpoint. Next, the components of the shutter unit  87  will be described with reference to  FIG. 7 . The following description will be based on the orientation of the shutter unit  87  in  FIG. 7 . The support member  86  is integrally provided with a pair of left and right rotating plates  88  connected to corresponding widthwise ends of the process shutter  83 , and a beam member  89  spanning between the top portions of the left and right rotating plates  88 . 
     The rotating plates  88  are formed in thin plate shapes with the thin dimension corresponding to the width direction and appear substantially circular in shape when viewed along the width direction. A through-hole  90  is formed in the circular center position of each rotating plate  88 . 
     A left protruding part  91  is integrally formed on the peripheral surface of the left rotating plate  88  at one location in the circumferential direction (the bottom of the rotating plate  88  in  FIG. 7 ) and protrudes radially outward from the left rotating plate  88  (downward in  FIG. 7 ). As with the rotating plates  88 , the protruding part  91  is also formed thin in the width direction and is substantially shaped like the letter U following the lower peripheral edge of the left rotating plate  88  when viewed along the width direction (see  FIG. 5C ). Two left recess parts  92  are formed in the right surface of the protruding part  91  at positions along the circumferential direction of the left rotating plate  88  (see  FIG. 5C ). 
     A right protruding part  93  is integrally provided on the right side surface of the right rotating plate  88 . The right protruding part  93  has a thin plate shape with the thin dimension corresponding to the width direction and has a circular shape when viewed in the width direction with a diameter smaller than that of the right rotating plate  88 . The right protruding part  93  is concentric with the right rotating plate  88 . The circular center of the right protruding part  93  is aligned with the circular center of the right rotating plate  88  in the width direction. Further, the through-hole  90  described above that is formed in the right rotating plate  88  penetrates both the right rotating plate  88  and the right protruding part  93  at the center thereof. 
     As shown in  FIG. 5C , a protruding part  94  is integrally provided on the right side surface of the right rotating plate  88  at one location along the outer circumference thereof (the part on the rear side of the right protruding part  93  in  FIG. 7 ). The protruding part  94  protrudes radially outward from the right rotating plate  88  (rearward in  FIG. 7 ). The protruding part  94  protrudes farther outward than the peripheral surface of the right rotating plate  88  on the right side thereof (see  FIG. 5C ). 
     Two right recessed parts  95  are formed in the right surface of the right rotating plate  88  along the peripheral edge thereof and at positions different from the protruding part  94  with respect to the circumferential direction of the right rotating plate  88  (positions lower than the right protruding part  93  in  FIG. 7 ). The right recessed parts  95  are arranged along the circumferential direction of the rotating plate  88  (see  FIG. 5C ). 
     A rack gear  96  is formed on the outer peripheral surface of the right protruding part  93  in a position corresponding to the right recessed parts  95  with respect to the circumferential direction of the right protruding part  93  (the rotating plate  88 ; near the bottom end of the right protruding part  93  in  FIG. 7 ; see also  FIG. 5C ). The rack gear  96  has a plurality of gear teeth that are arranged along the circumference of the right protruding part  93 . 
     The left end of the process shutter  83  is connected to a part of the left rotating plate  88  (the lower end of the left rotating plate  88  in  FIG. 7 ) on which the protruding part  91  (and specifically the left recess parts  92 ) is provided, while the right end is connected to a part of the right rotating plate  88  (the lower end of the right rotating plate  88  in  FIG. 7 ) in which the right recessed parts  95  are formed. In  FIG. 7 , the process shutter  83  spans between the lower ends of the left and right rotating plates  88 . The two right recessed parts  95  are continuously formed in the process shutter  83  from the right rotating plate  85  (see  FIG. 5C ). In other words, it may be said that the right recessed parts  95  are provided in the process shutter  83 . 
     If the protruding part  91  is treated as part of the process shutter  83 , then it may be considered that the left recess parts  92  formed in the protruding part  91  are provided in the process shutter  83 . 
     The beam member  89  has a plate shape extending in the width direction and spans between outer peripheral surfaces of the left and right rotating plates  88  at a different position along the circumference of the rotating plates  88  than the process shutter  83 . When viewed along the width direction, the beam member  89  is curved in an arc that follows the outer peripheral surfaces of the left and right rotating plates  88 . 
     The shutter unit  87  having a construction described-above is rotatably supported by the lower case  18 , as shown in  FIG. 5C . More specifically, in the shutter unit  87  having this construction, the left rotating plate  88  is positioned farther leftward than the left side surface of the lower case  18  (not shown), while the right rotating plate  88  is positioned farther rightward than the right side surface of the lower case  18 . In this state, the left and right rotating plates  88  are aligned in the width direction with the entire supply chamber  72  and the lower left portion of the developing chamber  71  formed in the lower case  18  (see  FIG. 5A ). 
     Further, the process shutter  83  of the shutter unit  87  is accommodated in the box-accommodating chamber  73  of the lower case  18 , while the beam member  89  is positioned farther forward from the front side surface of the lower case  18  (the front wall portion of the lower case  18  in the vicinity of the arced wall  74 ; see  FIG. 5A ). 
     Hence, only the process shutter  83  of the shutter unit  87  is positioned within the lower case  18 , while all other parts of the shutter unit  87  are disposed outside of the lower case  18  (or outside of the process cartridge  16 ). 
     A support shaft  97  is integrally provided on the outer surface of each widthwise side wall of the lower case  18  at a position aligned with the center of curvature of the first protruding wall  75  (see  FIG. 5A ). The support shafts  97  protrude outward in the width direction from their corresponding outer surfaces. Each support shaft  97  is inserted into the through-hole  90  formed in the rotating plate  88  on the same widthwise side of the shutter unit  87 . The support shaft  97  is inserted from the inside of the through-hole  90  in the width direction. 
     With this construction, the entire shutter unit  87  can freely rotate about the support shafts  97  provided on widthwise side walls of the lower case  18  in either a clockwise or counterclockwise direction when viewing the shutter unit  87  on the right side. Thus, the support shafts  97  on the lower case  18  serve as the rotational center of the shutter unit  87 . The shutter unit  87  can rotate freely within a range in which the process shutter  83  slides between the open position and closed position described above. 
     When the process shutter  83  is in the open position shown in  FIG. 5A , the beam member  89  contacts a portion of the front wall of the lower case  18  near the upper end of the arced wall  74  from the front side thereof. This contact restricts the shutter unit  87  from rotating clockwise in a right side view. 
     When the process shutter  83  is in this open position, the shutter unit  87  is rotated counterclockwise in a right side view until the beam member  89  contacts the front wall of the lower case  18  near the lower end of the arced wall  74  from the upper front side, as shown in  FIG. 6A . This contact restricts the shutter unit  87  from rotating farther in the counterclockwise direction in a right side view. At this point, the process shutter  83  is in the closed position. 
     (2-2) Toner Box 
       FIG. 8A  is a perspective view from the left front side of the toner box  24  when the toner shutter  112  is in the closed position.  FIG. 8B  is a perspective view from the left front side of the toner box  24  when the toner shutter  112  is in the open position.  FIG. 8C  is a perspective view from the right front side of the toner box  24  when the toner shutter  112  is in the closed position.  FIG. 8D  is a perspective view from the right front side of the toner box  24  when the toner shutter  112  is in the open position.  FIG. 8E  is a front cross-sectional view showing relevant parts of the toner box  24 . 
     The toner box  24  is hollow with a substantially cylindrical shape elongated in the width direction, as described above. As shown in  FIG. 8 , a box casing  100  forms the outer shell of the toner box  24  and has a hollow, substantially cylindrical shape elongated in the width direction. Hence, a cross section of the box casing  100  viewed along the width direction is substantially circular in shape (see  FIG. 5A ). The box casing  100  is closed on both widthwise ends and accommodates toner therein. The box casing  100  is integrally provided with a circumferential wall  101  forming the outer circumferential (peripheral) surface thereof, and a pair of side walls  102  covering both widthwise ends of the circumferential wall  101 . 
     A curved wall  103  is formed along one peripheral portion (upper portion) of the circumferential wall  101 , extending across the entire width of the circumferential wall  101  (box casing  100 ). The curved wall  103  is depressed inward toward the circular center of the circumferential wall  101  (toward the interior of the box casing  100 ) to form an arc-shaped curve. When viewed along the width direction, the curved wall  103  is substantially U-shaped. In other words, as shown in  FIG. 5C , the curved wall  103  is curved in an arc shape whose center of curvature is a reference line X extending in the width direction. The reference line X positions outside (above) the box casing  100  in a radial direction of the circular center of the circumferential wall  101  as viewed along width direction (a radial to an agitator shaft  107  described later). The curvature of the curved wall  103  is substantially identical to that of the first protruding wall  75  in the lower case  18  of the process cartridge  16  (see  FIG. 5A ). The curved wall  103  functions to guide the toner after passing through a supply hole  104  described later. 
     As shown in  FIG. 8B , the side walls  102  have the same shape as the circumferential wall  101  when viewed along the width direction. That is, each side wall  102  is substantially circular in shape, with one portion of the outer peripheral edge cut out in an arc. Thus, the box casing  100  comprising the circumferential wall  101  and side walls  102  is hollow and substantially cylindrical in shape, with one region on the outer peripheral surface depressed inwardly in an arc that extends across the entire width direction. Consequently, the interior space of the box casing  100  is also substantially circular in shape, but has one portion in the outer peripheral edge cut out in an arc shape when viewed along the width direction. 
     A supply hole  104  elongated in the width direction is formed in and penetrates the outer surface of the curved wall  103  (the surface exposed on the outside) near the rear edge of the curved wall  103 . The width dimension of the supply hole  104  is slightly smaller than that of the curved wall  103 . The supply hole  104  provides communication between the interior and exterior of the box casing  100 . 
     A right guide groove  105  is formed in the outer surface of the curved wall  103  at a position farther rightward than the supply hole  104  and is recessed toward the circular center of the circumferential wall  101  (toward the interior of the box casing  100 ). When viewed along the width direction, the right guide groove  105  forms an arc shape that follows the outer curved surface of the curved wall  103 . A cross section of the right guide groove  105  forms a convex shape that grows narrower toward the outer surface of the curved wall  103 . In other words, the width of the right guide groove  105  is wider at its deepest point than at the surface of the curved wall  103 . 
     A left guide groove  106  is formed in the left side surface of the left side wall  102  in an area following the curved wall  103  and is recessed rightward. The left guide groove  106  has an arc shape that follows the curved wall  103  and is substantially U-shaped when viewed along the width direction. The agitator shaft  107  extending along the width direction spans between the center parts of the circular left and right side walls  102  and is rotatably supported in these side walls  102 . The left end of the agitator shaft  107  protrudes farther leftward than the left side surface of the left side wall  102 . An input gear  108  is integrally provided on the left end of the agitator shaft  107  and protrudes leftward from this left end. 
     As shown in  FIG. 5A , a support part  109  is integrally provided on the outer peripheral surface of the agitator shaft  107  within the box casing  100  and protrudes radially outward from the agitator shaft  107 . The support part  109  is formed of a hard material (hard resin or the like). When viewed in the width direction, the support part  109  grows narrower away from the agitator shaft  107  in the radial direction. 
     An agitating blade  110  is attached to the support part  109 . The agitating blade  110  is formed of a flexible material, such as film. The agitating blade  110  is supported on the support part  109  and extends outward from the support part  109  along the radial direction of the agitator shaft  107  to the inner surface of the circumferential wall  101  forming the box casing  100 . The natural length of the agitating blade  110  with respect to the radial direction of the agitator shaft  107  is greater than the distance between the support part  109  and the inner surface of the circumferential wall  101 . Accordingly, the agitating blade  110  contacts the inner surface of the circumferential wall  101  and is slightly bowed in the box casing  100 . When viewed from the right side, as in  FIG. 5A , the side of the agitating blade  110  toward the inner surface side of the circumferential wall  101  is bowed in a clockwise direction with respect to the support part  109 . 
     The agitator shaft  107 , support part  109 , and agitating blade  110  together configure the agitator  111 . Excluding the left end part of the agitator shaft  107 , the agitator  111  is disposed entirely in the toner box  24  (and specifically the box casing  100 ). In a right side view, the agitator  111  can rotate in the counterclockwise direction about the agitator shaft  107 . 
     In one rotation of the agitator  111 , the agitating blade  110  passes over and contacts the entire inner surface of the circumferential wall  101  (including the curved wall  103 ). Since the agitator  111  rotates (and the agitating blade  110  passes over the circumferential wall  101 ) in the counterclockwise direction when viewed from the right side, the supply hole  104  formed at the rear end of the curved wall  103  is on the upstream side of the curved wall  103  with respect to the rotating direction of the agitator  111 . 
     As shown in  FIG. 8B , the toner shutter  112  is provided in the toner box  24 . The toner shutter  112  is plate-shaped and elongated in the width direction. When viewed along the width direction, the toner shutter  112  is arc-shaped with approximately the same curvature as the curved wall  103  of the box casing  100  (see  FIG. 5A ). The widthwise dimension of the toner shutter  112  is approximately the same as that of the curved wall  103 , while the dimension of the toner shutter  112  in the circumferential direction (the direction of the curved surface) is about half that of the curved wall  103 . 
     A folded part  113  is integrally provided on the left end of the toner shutter  112 . The folded part  113  is bent in a direction orthogonal to the width direction and away from the center of curvature of the toner shutter  112  (downward in  FIG. 8B ). The folded part  113  has a plate shape that is thin in the width direction and is connected to the left end of the toner shutter  112  along the entire periphery thereof. 
     Two left protrusions  114  are integrally provided on the left side surface of the folded part  113 . The left protrusions  114  are spaced from each other in the circumferential direction of the toner shutter  112  and protrude leftward. A left guide rib  115  is integrally provided on the right side surface of the folded part  113 . The left guide rib  115  protrudes rightward and extends along the circumferential direction of the toner shutter  112 . 
     Two right protrusions  116  are integrally provided on the outer surface of the toner shutter  112  (the surface exposed externally from the toner box  24 ; the top surface in  FIG. 8B ) near the right end thereof. The right protrusions  116  are spaced an interval in the circumferential direction of the toner shutter  112  and protrude in a direction toward the center of curvature of the toner shutter  112  (upward in  FIG. 8B ). 
     A right guide rib  117  is integrally provided on the underside surface of the toner shutter  112  (although not shown in  FIG. 8B , the bottom surface of the toner shutter  112  the same drawing) and extends along the circumferential direction of the toner shutter  112  (see  FIGS. 8A and 8C ). The right guide rib  117  protrudes in a direction orthogonal to the width direction and away from the center of curvature of the toner shutter  112  (downward in  FIG. 8A ). 
     The toner shutter  112  is supported in the curved wall  103  of the box casing  100 . More specifically, the left guide rib  115  of the toner shutter  112  is fitted into the left guide groove  106  formed in the box casing  100  from the left side, while the right guide rib  117  is fitted into the right guide groove  105  of the box casing  100  from the top in  FIG. 8B . With this construction, the toner shutter  112  is supported by the box casing  100  at multiple points along the width direction (i.e., at the left guide rib  115  and right guide rib  117 ). 
     In this state, the left guide rib  115  can freely slide within the left guide groove  106  along the circumferential direction of the curved wall  103 , and the right guide rib  117  can freely slide within the right guide groove  105  along the circumferential direction of the curved wall  103 . 
     Using the drawing of  FIG. 8A  as a reference, the bottom surface of the toner shutter  112  is disposed over the outer surface (top surface) of the curved wall  103  and opposes this outer surface along substantially the entire width direction. In this state, the toner shutter  112  can freely slide along the circumferential direction of the curved wall  103  between the open position shown in  FIGS. 8B and 8D  and the closed position shown in  FIGS. 8A and 8C . 
     In the open position, the toner shutter  112  is shifted to the front side of the supply hole  104  so that the entire supply hole  104  is exposed (open) on the outside (the top; see  FIGS. 8B and 8D ). In the closed position, the toner shutter  112  is aligned with the supply hole  104 , covering the entire supply hole  104  on the outside (see  FIGS. 8A and 8C ). Thus, the supply hole  104  is opened and closed by shifting the toner shutter  112  along the circumferential direction of the curved wall  103 . 
     As shown in  FIG. 8D , an operating part  118  is provided in the toner box  24  on the right side surface of the right side wall  102 . The operating part  118  is plate-shaped with its thin dimension corresponding to the width direction. The operating part  118  is circular-shaped in a right side view, with a smaller diameter than the side wall  102 . The center of the circular operating part  118  is aligned with the center of the circular right side wall  102  (the agitator shaft  107 ); in the width direction (see  FIGS. 5A and 5C ). 
     A grip part  119  is integrally provided on the right side surface of the operating part  118 . The grip part  119  has a narrow square columnar shape that extends along a straight line passing through the center of the circular operating part  118  in a right side view. A rack gear  120  is formed on the outer peripheral surface of the operating part  118  at one location on the circumference thereof. With respect to  FIG. 8D  in which the grip part  119  extends vertically, the rack gear  120  is formed at a position shifted slightly counterclockwise from the top end of the grip part  119  in a right side view. The rack gear  120  has a plurality of gear teeth arranged along the circumferential direction of the operating part  118 . 
     The operating part  118  is supported on the right side wall  102  and is capable of rotating freely about its own center. Bosses  121  are integrally provided on the right side surface of the right side wall  102  adjacent to the operating part  118  and at positions on either side of the curved wall  103  with respect to the circumferential direction of the substantially circular side wall  102  (see  FIG. 5C ). The bosses  121  are cylindrical in shape and protrude rightward. 
     With the operating part  118  rotatably supported by the right side wall  102 , as described above, the rack gear  120  of the operating part  118  is disposed between the two bosses  121  with respect to the circumferential direction of the side wall  102  (or operating part  118 ). Accordingly, the operating part  118  may be rotated between a position in which the rack gear  120  contacts the forward boss  121 , as shown in  FIG. 8D  (hereinafter referred to as the “forward position”), and a position in which the rack gear  120  contacts the rearward boss  121 , as shown in  FIG. 8C  (hereinafter referred to as the “rearward position”). An operator rotates the operating part  118  by gripping and twisting the grip part  119 . 
     When in the forward position, the operating part  118  can be moved to the rearward position by rotating the grip part  119  all the way clockwise in a right side view. From the rearward position, the operating part  118  can be returned to the forward position by rotating the operating part  118  all the counterclockwise in a right side view. 
     When looking at the toner box  24  by itself, the toner shutter  112  and operating part  118  are not mechanically coupled together, and the toner shutter  112  slides independent of the rotation of the operating part  118 . Therefore, the toner shutter  112  does not slide when the operating part  118  is rotated in a standalone toner box  24 . The toner shutter  112  is in the closed position in this state (see  FIG. 8C ). 
     As shown in  FIG. 8E , a recessed part  122  is formed at one location in the left peripheral edge of the operating part  118 . A cylindrical pawl  123  elongated in the width direction is fitted into the recessed part  122 . A spring  124  has one end coupled to the right end of the pawl  123  and another end coupled to the portion of the operating part  118  defining the bottom of the recessed part  122 . The spring  124  urges the pawl  123  leftward. 
     When the operating part  118  is in the rearward position shown in  FIG. 8C , the left end of the pawl  123  protrudes leftward from the recessed part  122 , as shown in  FIGS. 8A and 8E . The left end of the pawl  123  contacts the right outer surface of the curved wall  103  near the rearward boss  121  from the upstream side with respect to the counterclockwise direction in a right side view. While the pawl  123  remains in contact with the curved wall  103  in this state, the operating part  118  cannot be rotated counterclockwise in a right side view. 
     On the other hand, when the operating part  118  is in the forward position shown in  FIG. 8D , the pawl  123  does not catch on the curved wall  103 , as described above, but is almost completely accommodated in the recessed part  122  (see  FIG. 8E ) while opposing the right side surface of the right side wall  102 . 
     (2-3) Mounting and Removal of the Toner Box with Respect to the Process Cartridge 
     Next, the procedure for mounting the toner box  24  in and removing the toner box  24  from the process cartridge  16  (and specifically the developing unit  37 ) will be described. 
     Before mounting the toner box  24  in the process cartridge  16 , the process shutter  83  is in the closed position in the lower case  18  of the process cartridge  16 , as shown in  FIG. 6C  (see also  FIG. 6A ). In the toner box  24 , the toner shutter  112  is in the closed position and the operating part  118  is in the rearward position, as shown in  FIGS. 8A and 8C . 
     In this state, the toner box  24  is oriented with the curved wall  103  on the top side and disposed on the right side of the box-mounting opening  84  formed in the right side wall of the lower case  18  (the near side in  FIG. 6 ). From this position, the toner box  24  is pushed leftward so that the left end (the end with the input gear  108 ) of the toner box  24  enters the box-mounting opening  84  first. The toner box  24  is pushed through the box-mounting opening  84  until fully accommodated in the box-accommodating chamber  73 , as shown in  FIG. 6B . 
     When the toner box  24  is fully accommodated in the box-accommodating chamber  73 , the operation for mounting the toner box  24  in the process cartridge  16  (in the lower case  18  of the developing unit  37 ) is complete. 
     As shown in  FIG. 6A , the toner box  24  is almost a perfect fit in the box-accommodating chamber  73  at this time and is positioned lower than the supply roller  23  when viewed in the width direction. Further, the curved wall  103  of the toner box  24  opposes the bottom of the first protruding wall  75  provided in the lower case  18  and closely conforms to the shape of the first protruding wall  75 . When both the process shutter  83  and the toner shutter  112  are in the closed position, the top surface (outer surface) of the toner shutter  112  closely contacts the bottom surface of the process shutter  83  in the closed position from the lower rear side. Further, the support shaft  97  of the lower case  18  (rotational center of the shutter unit  87 ) is aligned with the reference line X (a line extending in the width direction and passing through the center of curvature of the curved wall  103  provided in the toner box  24 ) in, the width direction (see  FIG. 6B ). 
     At the same time, the right side surface of the toner box  24  (and particularly the operating part  118 ) is exposed through the box-mounting opening  84  (see  FIG. 6C ) in the right side of the lower case  18  (see  FIG. 6B ). Since the toner box  24  is mounted leftward into the developing unit  37 , the operating part  118  on the right side surface of the toner box  24  is disposed on the upstream side of the toner box  24  relative to the mounting direction. However, since the input gear  108  (see  FIG. 8A ) is provided on the left side surface of the toner box  24 , the input gear  108  is on the downstream side of the toner box  24  relative to the mounting direction. 
     The supply hole  104  of the curved wall  103  confronts the through-hole  77  in the first protruding wall  75  from the lower rear side with the process shutter  83  and toner shutter  112  interposed therebetween, since both the process shutter  83  and the toner shutter  112  are in the closed position. In this state, the supply hole  104  and through-hole  77  are not in communication with each other. 
     The input gear  108  of the toner box  24  (see  FIG. 8A ) is coupled with the coupling gear  85  provided on the left side wall of the lower case  18  (see  FIG. 6C ). When the process cartridge  16  is mounted in the main casing  2 , the coupling gear  85  is coupled with a drive source (not shown) provided in the main casing  2 . By coupling the input gear  108  to the coupling gear  85  in this case, the input gear  108  can be coupled to the main casing  2  via the coupling gear  85 . Consequently, the input gear  108  can receive a drive force from the drive source in the main casing  2  and transfer this drive force to the agitator shaft  107  (and thereby to the agitator  111 ). 
     The left protrusions  114  provided on the toner shutter  112  (see  FIG. 8A ) are fitted from the right into the corresponding left recess parts  92  (see  FIG. 6C ) formed in the protruding part  91 , which is disposed on the left rotating plate  88  (see  FIG. 7 ) of the process shutter  83  (and specifically the shutter unit  87 ). Further, the right protrusions  116  provided on the toner shutter  112  (see  FIG. 8A ) are fitted from the right into the right recessed parts  95  provided on the right side of the process shutter  83  (see  FIG. 6C ). 
     When the toner box  24  is mounted in the lower case  18 , the left protrusions  114  are engaged in the left recess parts  92  and the right protrusions  116  are engaged in the right recessed parts  95  within the developing unit  37 . Through these engagements, the toner shutter  112  is coupled with the process shutter  83 . 
     Further, when the toner box  24  is mounted in the corresponding process cartridge  16  (developing unit  37 ), the rack gear  120  provided on the operating part  118  of the toner box  24  meshes with the rack gear  96  provided on the right protruding part  93  of the shutter unit  87  from the bottom side thereof, as shown in  FIG. 6B . Hence, the operating part  118  is engaged with the shutter unit  87  of the corresponding developing unit  37 , thereby coupling the operating part  118  with the shutter unit  87  (process shutter  83 ). 
     Since the toner shutter  112  is coupled with the process shutter  83 , as described above (see  FIG. 6A ), the operating part  118  is coupled with the toner shutter  112  via the process shutter  83  when the toner box  24  is mounted in the process cartridge  16 . 
     The pawl  123  provided in the left surface of the operating part  118  (see  FIG. 8E ) contacts the right side surface of the right rotating plate  88  provided in the shutter unit  87  at a position lower than the right protruding part  93  so that the pawl  123  is completely accommodated in the recessed part  122  (see  FIG. 8E ) and does not catch on the curved wall  103  (not illustrated in the drawings). In this state, the operating part  118  in the rearward position can be rotated counterclockwise in a right side view. 
     When the operating part  118  is in the rearward position and the toner box  24  is mounted in the process cartridge  16 , the operating part  118  can be rotated counterclockwise in a right side view. At this time, the shutter unit  87  coupled with the operating part  118  receives a drive force produced by rotating the operating part  118  via the rack gear  96 . As a result, the shutter unit  87  rotates clockwise in a right side view about the support shaft  97  of the lower case  18  (the reference line X described above). 
     As the shutter unit  87  rotates clockwise in a right side view, the process shutter  83  of the shutter unit  87  and the toner shutter  112  coupled to the process shutter  83  rotate (slide) clockwise in a right side view. When the operating part  118  reaches the forward position shown in  FIG. 5B , the process shutter  83  and the toner shutter  112  have rotated as far as possible clockwise in a right side view and have arrived in their respective open positions, as shown in  FIG. 5A . When the process shutter  83  and toner shutter  112  are both in the open position, the supply hole  104  and the through-hole  77  are in communication with each other. 
     With the operating part  118  in the forward position, the protruding part  94  provided on the right rotating plate  88  of the shutter unit  87  is adjacent to the right side wall  102  of the toner box  24  from the right side thereof. Hence, the toner box  24  mounted in the process cartridge  16  cannot be pulled rightward (cannot be removed), thereby maintaining the mounted state of the toner box  24  in the process cartridge  16 . 
     However, when the operating part  118  in the forward position is rotated clockwise in a right side view, the shutter unit  87  (process shutter  83 ) and the toner shutter  112  rotate (slide) counterclockwise in a right side view about the support shaft  97  of the lower case  18  (i.e., the reference line X) and are thus returned to their respective closed positions shown in  FIG. 6A . As a result, the supply hole  104  and the through-hole  77  are no longer in communication. 
     Further, the protruding part  94  of the shutter unit  87  rotates together with the rotation of the shutter unit  87  and is thus shifted upward from the toner box  24  in a right side view, as illustrated in  FIG. 6B . Consequently, the protruding part  94  is not in the way of the toner box  24  when the toner box  24  is moved rightward, allowing an operator to remove the toner box  24  from the process cartridge  16 , as described above. 
     As described above, the operations of the toner shutter  112  to open and close the supply hole  104  are associated with the operations of the process shutter  83  to open and close the through-hole  77 . The image-forming operation described above can be implemented when the toner box  24  is mounted in the process cartridge  16 , as shown in  FIG. 5A , and the process shutter  83  and toner shutter  112  are both in the open position. 
       FIG. 9  is a right side cross-sectional view showing the process cartridge  16  when both the process shutter  83  and toner shutter  112  are in the open position, where  FIGS. 9A through 9D  show the various rotational states of the agitator  111  over time. As shown in  FIG. 9 , during an image-forming operation, a drive force from the main casing  2  side is transmitted to the agitator  111  via the input gear  108  (see  FIG. 8A ) and the coupling gear  85  (see  FIG. 5C ). This drive force serves to rotate the agitator  111  counterclockwise in a right side view so that the agitator  111  agitates the toner (the dot filled regions indicated in  FIG. 9 ). The rotating path of the support part  109  provided in the agitator  111  is indicated by a dotted line in  FIG. 1 . 
     Specifically, the agitating blade  110  of the rotating agitator  111  contacts the inner peripheral surface of the circumferential wall  101  constituting the box casing  100  in a somewhat bowed state and scrapes toner accumulated in the bottom of the box casing  100  upward along the rotating direction of the agitator  111 , as illustrated in  FIG. 9A . As the agitator  111  continues to rotate, the toner scraped upward by the agitating blade  110  approaches the supply hole  104  from the upstream side (rear side in  FIG. 9B ) with respect to the rotating direction of the agitator  111  (counterclockwise in a right side view), as illustrated in  FIG. 9B . 
     As the agitator  111  continues to rotate, the agitating blade  110  pushes the toner confronting the supply hole  104 , as described above, into the supply hole  104  from the upstream side in the rotating direction of the agitator  111 , as illustrated in  FIG. 9C . The toner pushed through the supply hole  104  is pushed further inward by successive quantities of toner that the agitating blade  110  pushes through the supply hole  104 , as shown in  FIG. 9D . Consequently, toner pushed through the supply hole  104  passes through the through-hole  77  in communication with the supply hole  104  enters the supply chamber  72  of the lower case  18  provided in the process cartridge  16 , and accumulates on the accumulating part  75 A, which is a portion on the top surface of the first protruding wall  75  positioned lower than the through-hole  77 . 
     Thus, in addition to supplying toner into the supply chamber  72 , the rotations of the agitator  111  suitably agitate the toner collected in the bottom of the box casing  100 . Toner collected in the accumulating part  75 A in the supply chamber  72 , as described above, is subsequently supplied onto the supply roller  23  provided in the supply chamber  72 . Thereafter, the supply roller  23  rotating clockwise in a right side view conveys this toner upward and supplies the toner to the developing roller  22  at the point of contact between the supply roller  23  and developing roller  22 . 
     As the developing roller  22  rotates counterclockwise in a right side view, the toner supplied onto the developing roller  22  is conveyed between the outer peripheral surface of the developing roller  22  and the pressing part  80  on the thickness-regulating blade  78 . The pressing part  80  regulates the thickness of toner carried on the outer peripheral surface of the developing roller  22  at a thin layer. This thin layer of toner is subsequently supplied to the photosensitive drum  19 . 
     When more than the required amount of toner collects on the accumulating part  75 A of the first protruding wall  75 , the excess toner is shifted from the accumulating part  75 A to the through-hole  77 , sequentially passes through the through-hole  77  and the supply hole  104  and returns to the toner box  24 . Hence, the toner circulates between the toner box  24  and the accumulating part  75 A of the first protruding wall  75 . 
     When both the process shutter  83  and the toner shutter  112  are in the open position, as described above, toner can move between the toner box  24  and the corresponding photosensitive drum  19 . However, when the process shutter  83  and toner shutter  112  are both in the closed position, as shown in  FIG. 6A , toner cannot move between the toner box  24  and the photosensitive drum  19  since both the supply hole  104  and the through-hole  77  are closed. 
     In this way, the process shutter  83  and the toner shutter  112  open and close between the toner box  24  and the corresponding photosensitive drum  19  to allow or restrict the movement of toner therebetween. The operating part  118  of the toner box  24  (see  FIG. 6B ) is operated in order to open and close the process shutter  83  and toner shutter  112 . 
     (3) Mounting and Removal of the Process Cartridges (Toner Boxes) with Respect to the Drawer Frame 
     Next, operations for mounting the process cartridges  16  in and removing the process cartridges  16  from the drawer frame  15  of the drawer unit  14  will be described. The following description will assume that the drawer unit  14  is outside of the main casing  2 . 
     The process cartridge  16  is placed above the drawer frame  15 , as shown in  FIG. 4 . The front-to-rear position of the process cartridge  16  with respect to the drawer frame  15  (see  FIG. 1 ) differs according to the color of toner accommodated in the toner box  24  of the process cartridge  16 . Therefore, the process cartridge  16  is disposed at a prescribed position in the front-to-rear direction above the drawer frame  15 . 
     At this time, the corresponding left and right guiding grooves  60  in the drawer frame  15  are positioned below the process cartridge  16 . The process cartridge  16  is inserted into the drawer frame  15  by lowering the process cartridge  16  through the opening  15 A formed in the top of the drawer frame  15 . When the process cartridge  16  has been lowered a certain amount, the guiding grooves  60  receive the corresponding widthwise ends of the central shaft  19 A provided in the photosensitive drum  19  of the process cartridge  16 . As the central shaft  19 A of the photosensitive drum  19  is guided in the guiding grooves  60 , the process cartridge  16  proceeds downward at a forward slant that follows the extended direction of the guiding grooves  60 . 
     When the central shaft  19 A of the photosensitive drum  19  arrives at the deepest point (bottom) of the guiding grooves  60 , as illustrated by the forwardmost process cartridge  16  in  FIG. 4 , the downward movement of the process cartridge  16  stops and the operation for mounting the process cartridge  16  in the drawer frame  15  is complete. Assembly of the drawer unit  14  is complete when all process cartridges  16  have been mounted in the drawer frame  15  according to the procedure described above. In this state, the drawer frame  15  supports the four process cartridges  16  (i.e., the four photosensitive drums  19  and developing units  37 ). 
     When mounting of the process cartridge  16  in the drawer frame  15  is complete, the entire right side surface of the toner box  24  in the process cartridge  16  is exposed outside the drawer frame  15  through the circular exposure holes  15 C formed in the right wall of the drawer frame  15  at the same position as the toner box  24  with respect to the front-to-rear direction. Hence, the operator can manipulate the grip part  119  on the operating part  118  of the toner box  24  exposed through the circular exposure holes  15 C when the process cartridge  16  is mounted in the drawer frame  15  in order to move the process shutter  83  and toner shutter  112  between the open and closed positions, as described above (see  FIGS. 5 and 6 ). 
     Further, by gripping the grip part  119  provided on the operating part  118  that is exposed in the circular exposure holes  15 C, the operator can pull the toner box  24  through the circular exposure holes  15 C and extract the toner box  24  from the process cartridge  16  (i.e., from the drawer frame  15 ). 
     Conversely, by inserting the toner box  24  through the circular exposure holes  15 C formed on the right side of the drawer frame  15  and thus through the box-mounting opening  84  in the corresponding process cartridge  16  (see  FIG. 6C ) so that the toner box  24  is accommodated in the box-accommodating chamber  73 , the operator can mount the toner box  24  in the process cartridge  16  (i.e., the drawer frame  15 ). 
     In this way, the operator can move the toner box  24  along the width direction through the circular exposure holes  15 C while the process cartridge  16  is mounted in the drawer frame  15  in order to remove only the toner box  24  from or mount only the toner box  24  in the drawer unit  14 . 
     Further, when the operator pulls a process cartridge  16  that is mounted in the drawer frame  15  upward until the entire process cartridge  16  is lifted above the opening  15 A formed in the top of the drawer frame  15 , removal of the process cartridge  16  from the drawer frame  15  is complete. 
     4. Mounting and Removal of the Drawer Unit Relative to the Main Casing 
     Next, the procedure for mounting the drawer unit  14  in and removing the drawer unit  14  from the main casing  2  will be described. 
     While the drawer unit  14  is not mounted in the main casing  2 , as illustrated in  FIG. 3 , the operator first pivots the pivoting unit  52  upward and subsequently rotates the second front cover  50  downward on the front side to expose the unit-mounting opening  49 , as described above. Next, the operator inserts the drawer unit  14  through the unit-mounting opening  49  into the unit-accommodating chamber  40 . The drawer unit  14  is inserted substantially horizontally along the guide parts  42 . 
     When inserting the drawer unit  14  into the unit-accommodating chamber  40 , the wheels  64  on the rear end of the drawer unit  14  roll over the first guiding roller  43  (see  FIG. 2 ), first sloped wall  44  (see  FIG. 2 ), and first guiding rail  45  of the corresponding guide part  42  (i.e., the guide part  42  on the same widthwise side) as the drawer unit  14  progresses rearward. When the wheels  64  reach the corresponding first guiding rails  45 , the flanges  62  of the drawer unit  14  (see  FIG. 4 ) come to rest on the corresponding first guiding rollers  43  (see  FIG. 2 ). 
     Thus, as the drawer unit  14  progresses rearward, the wheels  64  roll over the first guiding rails  45  while the flanges  62  (see  FIG. 4 ) slide rearward over the rolling first guiding rollers  43  (see  FIG. 2 ). Accordingly, the drawer unit  14  moves rearward in a stable state. Subsequently, the wheels  64  on the drawer unit  14  roll over the second guiding rollers  46 , second sloped walls  47 , and second guiding rails  48  before coming to a halt at the curved rear ends of the second guiding rails  48 . Thus, the drawer unit  14  moves diagonally downward and rearward while remaining in a level state before halting in the position shown in  FIG. 2 . 
     Through this process, the drawer unit  14  is mounted in the main casing  2 . At this time, the positioning shafts  61  on the drawer frame  15  contact the main casing  2  (more specifically the partitioning wall  41 ), which contact further fixes the position of the drawer unit  14  relative to the main casing  2 . 
     Next, the operator closes the second front cover  50  by rotating the second front cover  50  upward and closes the pivoting unit  52  by pivoting the pivoting unit  52  downward, as shown in  FIG. 1 . Through this operation, the intermediate transfer belt  27  of the transfer unit  12  contacts the tops of the photosensitive drums  19  provided in the drawer unit  14 . 
     To remove the drawer unit  14  from the main casing  2 , the operator sequentially opens the pivoting unit  52  and second front cover  50  to expose the unit-mounting opening  49 , as shown in  FIG. 2 , and subsequently pulls the drawer unit  14  forward through the unit-mounting opening  49  (see  FIG. 3 ). 
     The drawer unit  14  is completely removed from the main casing  2  when the operator has pulled the drawer unit  14  forward in a substantially level state until the four circular exposure holes  15 C formed in the right side wall of the drawer frame  15  are exposed on the right side, as shown in  FIG. 4 . Thus, the drawer unit  14  (drawer frame  15 ) can be removed from the main casing  2  along the front-to-rear direction (the juxtaposed direction of the four photosensitive drums  19  shown in  FIGS. 1 through 3 ). 
     While the drawer unit  14  (drawer frame  15 ) is pulled out of the main casing  2  in this way, the operator can remove toner boxes  24  from or insert toner boxes  24  into the process cartridges  16  (developing units  37 ) supported in the drawer unit  14  through the corresponding circular exposure holes  15 C of the drawer frame  15  along the width direction, as described above. Further, by manipulating the operating part  118  of each toner box  24  in this state, the operator can move the process shutter  83  and toner shutter  112  between their respective open and closed positions (see  FIGS. 5 and 6 ). 
     5. Effects of the first embodiment 
     (1) In the toner box  24  shown in  FIG. 9 , the agitator  111  is disposed inside the box casing  100 , which serves to accommodate toner. The agitator  111  agitates toner in the box casing  100  by rotating about its agitator shaft  107  extending in the width direction. 
     As described above, the box casing  100  has the curved wall  103 . The curved wall  103  curves in an arc shape whose convex side faces the interior of the box casing  100  (see  FIG. 5C ). The center of curvature of the curved wall  103  is the reference line X. Therefore, when the agitator  111  rotates inside the box casing  100 , the agitating blade  110  can contact and pass over the curved wall  103  protruding into the box casing  100 . 
     The supply hole  104  is formed in the upstream side of the curved wall  103  with respect to the rotating direction of the agitator  111  (the counterclockwise direction in a right side view) to provide communication between the interior and exterior of the box casing  100 . Accordingly, the supply hole  104  through which toner accommodated in the box casing  100  passes when expelling the toner outside of the toner box  24  can be disposed along the rotating path of the agitator  111  so as to oppose the rotating agitator  111  on the downstream side with respect to the rotating direction of the agitator  111 . 
     As a result, toner conveyed by the rotating agitator  111  in the rotating direction thereof is efficiently supplied to the supply hole  104 . Hence, the configuration of the toner box  24  enables toner accommodated in the box casing  100  to be efficiently expelled therefrom. 
     (2) Since the toner shutter  112  moves along the circumferential direction of the curved wall  103  in order to open and close the supply hole  104  (see  FIGS. 8A through 8D ), the toner shutter  112  can appropriately allow or restrict the passage of toner through the supply hole  104 . 
     (3) The agitator  111  includes the support part  109  formed of a hard material and extending radially outward from the agitator shaft  107 , and the flexible agitating blade  110  that is supported on the support part  109  and extends radially outward therefrom. 
     With this construction, the agitating blade  110  supported on the support part  109  bows a suitable amount when the agitator  111  rotates in order to convey a large amount of toner in the rotating direction of the agitator  111 . Since the agitating blade  110  also flexes a suitable amount when passing over the curved wall  103  (see  FIG. 9D ), the agitating blade  110  does not get caught on the curved wall  103 , enabling the agitator  111  to rotate smoothly. 
     (4) Since the cross section of the box casing  100  when viewed along the width direction is substantially circular in shape and substantially matches the rotating path of the agitator  111 , the agitator  111  can convey toner within the box casing  100  in the rotating direction of the agitator  111  without toner leaking around the agitator  111 . Accordingly, the agitator  111  can convey toner to the supply hole  104  with even better efficiency and can agitate all of the toner within the box casing  100  thoroughly. 
     (5) As shown in  FIG. 8 , the toner shutter  112  is supported by the box casing  100  at a plurality of locations along the width direction. Therefore, the toner shutter  112  can open and close the supply hole  104  in a more stable state than when the toner shutter  112  is supported at only one location. 
     (6) As shown in  FIG. 9 , the developing unit  37  includes the toner box  24 , and the lower case  18  in which the toner box  24  is detachably mounted, as described above. 
     The first protruding wall  75  is disposed in the lower case  18  opposite the curved wall  103  of the toner box  24  and curves in an arc shape that conforms with the shape of the curved wall  103 . That is, the first protruding wall  75  protrudes in an arc shape toward the curved wall  103  of the toner box  24 . 
     The lower case  18  supports the developing roller  22 , which carries toner to be supplied to an electrostatic latent image, and the supply roller  23 , which supplies toner from the toner box  24  to the developing roller  22  in the developing unit  37  described above. 
     The supply roller  23  is supported in the lower case  18  such that the outer peripheral surface of the supply roller  23  follows the first protruding wall  75  and the curved wall  103 . The supply roller  23  can be compactly disposed in the developing unit  37  so as to fit within the inner region of the first protruding wall  75 , thereby enabling the entire developing unit  37  to be made more compact (smaller vertically in this case). 
     (7) In the developing unit  37  described above, the through-hole  77  is formed in the first protruding wall  75  at a position confronting the supply roller  23  opposite the supply hole  104  of the toner box  24 . The process shutter  83  is provided to open and close the through-hole  77 . Hence, when the process shutter  83  opens the through-hole  77  while the supply hole  104  is open, toner is allowed to move between the toner box  24  and supply roller  23  (see  FIG. 5A ). When the process shutter  83  closes the through-hole  77 , toner is restricted from moving between the toner box  24  and supply roller  23  (see  FIG. 6A ). 
     (8) The support member  86  is provided in the lower case  18  of the developing unit  37  (see  FIG. 7 ). The support member  86  can freely rotate about the reference line X that extends in the width direction and passes through the center of curvature of the curved wall  103  (see  FIGS. 5B and 6B ). 
     The rotational center of the support member  86  is the reference line X. Since the process shutter  83  is integrally formed with the support member  86 , the rigidity of the entire shutter unit  87  is enhanced. Since the process shutter  83  is supported on the support member  86  (see  FIG. 7 ), the process shutter  83  can be rotated to open and close the through-hole  77  through the simple operation of rotating the support member  86  (see  FIGS. 5A and 6A ) 
     (9) As shown in  FIG. 8 , the operating part  118  engaged with the support member  86  is provided on the toner box  24  (see  FIGS. 5B and 6B ). Hence, the operator can easily rotate the support member  86  and process shutter  83  by operating the operating part  118  in order to open and close the through-hole  77  (see  FIGS. 5A and 6A ). 
     (10) When the toner box  24  is mounted in the lower case  18 , as shown in  FIG. 6 , the left protrusions  114  of the toner shutter  112  are engaged in the left recess parts  92  of the process shutter  83  and the right protrusions  116  of the toner shutter  112  are engaged in the right recessed parts  95  of the process shutter  83  (see  FIGS. 6C ,  8 A, and  8 B). In this way, the operations of the toner shutter  112  to open and close the supply hole  104  are associated with the operations of the process shutter  83  to open and close the through-hole  77 . Hence, both the supply hole  104  and through-hole  77  can be opened and closed together, thereby simplifying the operation. 
     (11) As shown in  FIG. 9 , the supply roller  23  is disposed at a higher position than the toner box  24 . Accordingly, toner accommodated in the toner box  24  must be conveyed upward to be supplied to the supply roller  23 . 
     However, with the toner box  24  of the preferred embodiment, the toner accommodated in the box casing  100  can be efficiently supplied to the supply roller  23  through the supply hole  104  formed in the toner box  24 , even though the toner is being supplied upward, because the supply hole  104  is formed at a position upstream of the curved wall  103  with respect to the rotating direction of the agitator  111 . 
     (12) The first protruding wall  75  provided in the developing unit  37  is arc-shaped with the convex side on the bottom. With this configuration, the accumulating part  75 A is formed in the top of the first protruding wall  75  at a position lower than the bottom end of the through-hole  77 . 
     Therefore, when toner accommodated in the box casing  100  of the toner box  24  is conveyed upward through the supply hole  104  and arrives at the through-hole  77 , rather than falling back into the box casing  100 , the toner accumulates in the accumulating part  75 A of the first protruding wall  75 , which is adjacent to the supply roller  23  and positioned lower than the bottom edge of the through-hole  77  (see  FIG. 9D ). Accordingly, toner can be efficiently supplied to the supply roller  23 . 
     (13) In the color printer  1  shown in  FIG. 1 , a plurality of developing units  37  is disposed beneath the plurality of photosensitive drums  19  juxtaposed horizontally for developing electrostatic latent images formed on the corresponding photosensitive drums  19 . Thus, each of the developing units  37  is disposed beneath a corresponding photosensitive drum  19 . The drawer frame  15  provided for supporting the plurality of photosensitive drums  19  and developing units  37  is disposed in the main casing  2  of the color printer  1  and can be pulled out from the main casing  2  along the juxtaposed direction of the photosensitive drums  19  (see  FIG. 3 ). 
     The toner box  24  is also disposed in each developing unit  37  for accommodating toner needed to develop the electrostatic latent image on the corresponding photosensitive drum  19 . 
     As shown in  FIG. 4 , the toner boxes  24  can be mounted in and removed from the corresponding developing units  37  supported in the drawer frame  15  along the axial direction of the photosensitive drums  19  (width direction) while the drawer frame  15  is pulled out of the main casing  2 . In this way, the toner boxes  24  can be easily mounted in and removed from the developing units  37  without troubling the operator to perform such operations as removing the photosensitive drums  19  above the developing units  37 . 
     This construction has the effect of improving the ease of maintaining the toner boxes  24  provided in the developing units  37  when the developing units  37  are disposed beneath the photosensitive drums  19  juxtaposed in a horizontal direction. 
     (14) With the color printer  1  of the preferred embodiment, the process shutter  83  and toner shutter  112  shown in  FIGS. 5A and 6A  allow or restrict the movement of toner by opening and closing the passage between the toner boxes  24  and corresponding photosensitive drums  19 . 
     The operating part  118  is provided on the upstream side (right side surface) of the toner box  24  with respect to the mounting direction in which the toner box  24  is mounted in the corresponding developing unit  37  (leftward and also referred as the “mounting direction” hereafter) in order to open and close the process shutter  83  and toner shutter  112 . Hence, by operating the operating part  118  disposed on the upstream side of the toner box  24  with respect to the mounting direction, i.e., in an easily accessible location, the operator can easily open the shutters (see  FIGS. 5 and 6 ). 
     The input gear  108  is provided on the downstream side (left side surface) of the toner box  24  with respect to the mounting direction (see  FIGS. 8A and 8B ). The input gear  108  is coupled to the main casing  2  (more accurately, to the coupling gear  85  provided on the left side wall of the lower case  18 ; see  FIG. 6C ) in order to receive a drive force from the main casing  2  and transmit this force to the agitator  111 . 
     Accordingly, as illustrated in  FIG. 6 , the input gear  108  can be coupled with the main casing  2  (strictly speaking, the coupling gear  85 ; see  FIG. 6C ) when the toner box  24  is completely mounted in the developing unit  37 . With this construction, the toner box  24  can be more smoothly mounted in the developing unit  37  than when the input gear  108  couples with the main casing  2  while the toner box  24  is partially mounted in the developing unit  37 . Further, by providing the input gear  108  on the downstream side of the toner box  24  with respect to the mounting direction, sufficient space can be allocated for the operating part  118  provided on the upstream side of the toner box  24  with respect to the mounting direction (see  FIG. 8A ). 
     (15) The first protruding wall  75  provided in each developing unit  37  arcs downward so that the top portion of the first protruding wall  75  (the accumulating part  75 A) is positioned lower than the bottom edge of the through-hole  77 . 
     Therefore, when toner accommodated in the box casing  100  of the toner box  24  is conveyed upward through the supply hole  104  and arrives at the through-hole  77 , rather than falling back down into the box casing  100 , the toner accumulates in the accumulating part  75 A of the first protruding wall  75  at a position adjacent to the supply roller  23  and lower than the bottom edge of the through-hole  77  (see  FIG. 9D ). Accordingly, toner can be efficiently supplied to the supply roller  23 . 
     6. Variations of the First Embodiment 
       FIG. 10  shows a modification of the first embodiment applied to the toner box  24  in  FIG. 8D . 
     In the preferred embodiment described above, a single supply hole  104  is formed in the curved wall  103  of the toner box  24  (see  FIG. 8D ). However, a plurality of supply holes  104  may be formed in the curved wall  103 , as in the variation shown in  FIG. 10 . In this case, the through-hole  77  in the process cartridge  16  (see  FIG. 9 ) should also be formed as a plurality of through-holes  77  equivalent to the number of supply holes  104  and formed at a position where the plurality of supply holes  104  confronts when the toner box  24  is mounted in the process cartridge  16 . In the example shown in  FIG. 10 , three supply holes  104  are aligned in the width direction. The widthwise dimension of each supply hole  104  is approximately one-third that of the single supply hole  104  provided in the first embodiment (see  FIG. 8D ). 
     With this construction, toner within the toner box  24  (box casing  100 ) is supplied into the supply chamber  72  of the process cartridge  16  (see  FIG. 9 ) via the center supply hole  104  and the center through-hole  77 . Toner supplied into the supply chamber  72  that is not supplied onto the supply roller  23  falls by its own weight sequentially through the two through-holes  77  on the widthwise ends and the two supply holes  104  on the widthwise ends, and is returned to the toner box  24 . 
     In this way, toner is circulated between the toner box  24  and supply chamber  72 . In this example, a seal (not shown) may also be provided in the toner box  24  to prevent toner from leaking through the supply holes  104  between the curved wall  103 , in which the supply holes  104  are formed, and the toner shutter  112 . Generally, the seal is provided to fringe the edges of the supply holes  104  (not illustrated in the drawings). 
     If the size of each supply hole  104  is reduced by providing a plurality of supply holes  104 , as in the present modification, the amount of sealing material required to fringe the supply holes  104  can be reduced. Reducing the amount of required sealing material reduces the resistance generated when the toner shutter  112  slides against the seal. Thus, the toner shutter  112  slides more smoothly. 
     7. Second Embodiment 
     Next, a laser printer  201  according to a second embodiment of the present invention will be described while referring to  FIGS. 11 through 19 . Before describing the features of the present invention, a brief description will be given for the general structure of the laser printer  201 . 
     8. General Structure of the Laser Printer 
     As shown in  FIG. 11 , the laser printer  201  includes a main casing  202 , a feeding unit  203 , and an image-forming unit  204 . 
     The main casing  202  is formed in a hollow box shape and includes an opening  202 A formed in the front wall thereof, and a front cover  221  disposed on the front wall and capable of pivoting forward and rearward to open and close the opening  202 A. The top surface of the main casing  202  is configured as a discharge tray  222  for receiving and holding sheets of a paper P after printing. 
     The feeding unit  203  includes a paper tray  231  and a feeding mechanism  232 . The feeding mechanism  232  of the feeding unit  203  separates and conveys the paper P accommodated in the paper tray  231  one sheet at a time to the image-forming unit  204 . The image-forming unit  204  includes a scanning unit  241 , a process cartridge  242 , and a fixing unit  243 . 
     The scanning unit  241  has a well-known construction that includes primarily a laser light-emitting unit (not shown), as well as a polygon mirror and a plurality of lenses and reflecting mirrors (not denoted with reference numerals in the drawings). The laser beam emitted from the laser light-emitting unit follows a path within the scanning unit  241  indicated by a two-dot chain line in  FIG. 11  and is irradiated in a high-speed scan over the surface of a photosensitive drum  251  provided in the process cartridge  242 . 
     The process cartridge  242  is detachably mounted in the main casing  202  when the front cover  221  is opened. The process cartridge  242  includes a process cartridge  205 , and a developing unit  206  pivotably held by the process cartridge  205 . 
     The process cartridge  205  includes the photosensitive drum  251 , a transfer roller  252 , and a Scorotron charger  253 . 
     As shown in  FIG. 12 , the developing unit  206  includes a developing device  207 , and a toner cartridge  208  that is detachably mounted on the developing device  207 . When mounted on the process cartridge  205 , the developing unit  206  is supported on the side walls of the process cartridge  205  and is capable of moving in the front-to-rear direction within elongated holes  205 A formed in the side walls. The developing unit  206  is also pressed against the photosensitive drum  251  by springs  205 B that urge the developing unit  206  rearward via the toner cartridge  208  (see  FIG. 12 ). 
     The developing device  207  includes a developing roller  271 , a thickness-regulating blade  272 , and a supply roller  273 . The toner cartridge  208  accommodates toner internally and is capable of conveying this toner to the developing device  207 . The developing device  207  and toner cartridge  208  will be described later in greater detail. 
     With the process cartridge  242  having this construction, the toner cartridge  208  conveys toner into the developing device  207 , and the supply roller  273  of the developing device  207  supplies this toner onto the developing roller  271 , at which time the toner is positively tribocharged between the supply roller  273  and developing roller  271 . As the developing roller  271  rotates, the toner carried on the surface of the developing roller  271  passes between the thickness-regulating blade  272  and developing roller  271 , at which time the thickness-regulating blade  272  regulates the layer of toner carried on the surface of the developing roller  271  to a prescribed thickness. 
     In the meantime, in the process cartridge  205  shown in  FIG. 11 , the Scorotron charger  253  applies a uniform positive charge to the surface of the photosensitive drum  251 , after which the scanning unit  241  irradiates a laser beam in a high-speed scan to form an electrostatic latent image on the surface of the photosensitive drum  251  based on image data by reducing the electric potential in exposed areas. Next, the rotating developing roller  271  supplies toner to the electrostatic latent image formed on the surface of the photosensitive drum  251  as the toner carried on the developing roller  271  rotates into and contacts the photosensitive drum  251 . 
     In this way, toner selectively attracted to the surface of the photosensitive drum  251  develops the latent image into a visible toner image through reverse development. Thereafter, the toner image carried on the surface of the photosensitive drum  251  is transferred onto a sheet of paper P as the sheet is conveyed between the photosensitive drum  251  and transfer roller  252 . 
     The fixing unit  243  includes a heating roller  243 A and a pressure roller  243 B. The fixing unit  243  functions to fix toner transferred to a sheet of paper P with heat as the sheet passes between the heating roller  243 A and pressure roller  243 B. After the toner image has been fixed to the sheet in the fixing unit  243 , discharge rollers  244  disposed downstream of the fixing unit  243  discharge the sheet onto the discharge tray  222 . 
     9. Structure of the Developing Unit 
     Next, the developing device  207  and toner cartridge  208  constituting the developing unit  206  will be described in greater detail. First, a description of the toner cartridge  208  will be given. 
     10. Structure of the Toner Cartridge 
     As shown in  FIG. 12 , the toner cartridge  208  is provided with a first accommodating unit  281 , a second accommodating unit  282 , and a third accommodating unit  283  that each accommodate toner. The toner cartridge  208  also has a grip part  283 A that protrudes forward from the third accommodating unit  283 . 
     When the toner cartridge  208  is mounted on the developing device  207  (and specifically when the developing unit  206  is mounted in the main casing  202 ), the first accommodating unit  281  is positioned adjacent to the developing device  207  in a horizontal direction. The first accommodating unit  281  is formed in a substantially hollow cylindrical shape. Among the walls constituting the first accommodating unit  281  is an adjoining wall  281 A adjoined with the developing device  207 . The adjoining wall  281 A has a concave shape that is depressed toward the interior of the first accommodating unit  281 . 
     More specifically, the adjoining wall  281 A is curved in an arc shape in a cross-sectional view. The center of the arc shape is a reference line BL (the rotational center of a process shutter S 2  described later with reference to  FIGS. 18 and 19 ) provided rearward (outside) of the first accommodating unit  281  and extending in the left-to-right direction (parallel to the rotational axis of an agitating member described later. For convenience, the adjoining wall  281 A will also be referred to as a “first arc wall W 1 ” in the following description. 
     As shown in  FIG. 13 , a first supply hole W 11  and second supply holes W 12  are formed in the first arc wall W 1  so as to penetrate the interior of the first accommodating unit  281 . 
     A single first supply hole W 11  is formed in the upper half of the first arc wall W 1  at a position in the left-to-right center thereof. The two second supply holes W 12  are formed in the lower half of the first arc wall W 1 , with one positioned at each of the left and right ends thereof. Hence, the second supply holes W 12  are offset from the first supply hole W 11  both in the left-to-right direction and the vertical direction. 
     By forming the first supply hole W 11  in the top half of the first arc wall W 1 , the first supply hole W 11  is disposed above the second supply hole W 12  and provides communication in a downward slope from the first accommodating unit  281  to the developing device  207 . Further, by forming the second supply holes W 12  in the lower half of the first arc wall W 1 , the second supply holes W 12  are disposed lower than the first supply hole W 11  and provide communication in an upward slope from the first accommodating unit  281  to the developing device  207 . 
     Further, the second supply holes W 12  are both positioned outside of a printing area PA (see  FIG. 14A ). The printing area PA denotes the maximum width (length in a direction orthogonal to the conveying direction of the paper P) for a toner image to be transferred onto the paper P. Providing the second supply holes W 12  outside of the printing area PA in this way suppresses the accumulation of toner in the developing device  207  outside of the printing area PA. 
     As shown in  FIG. 13 , a toner shutter S 1  is disposed on the rear side of the first arc wall W 1 . The toner shutter S 1  opens and closes the first supply hole W 11  and second supply holes W 12  by moving along the circumference of the first arc wall W 1 . The toner shutter S 1  includes a thin metal plate S 11  formed in an arc shape that conforms to the first arc wall W 1 , and a pair of sliding pieces S 12  disposed one on each of the left and right ends of the metal plate S 11 . 
     An opening S 13  that is larger than each of the second supply holes W 12  is formed one on each of the left and right sides of the metal plate S 11  in the upper half thereof. When the toner shutter S 1  is in the closed position shown in  FIG. 14A , the first supply hole W 11  is closed by a region of the metal plate S 11  between the openings S 13 , and the second supply holes W 12  are closed by regions of the metal plate S 11  below the respective openings S 13 . 
     When the toner shutter S 1  is moved downward from this closed position to the open position, as shown in  FIG. 14B , the top edge of the S 11  is moved lower than the first supply hole W 11 , thereby opening the first supply hole W 11 , and the openings S 13  are positioned overlapping the second supply holes W 12 , thereby opening the second supply holes W 12 . 
     As shown in  FIG. 13 , the sliding pieces S 12  are slidably supported on the toner cartridge  208 . A plurality of protrusions S 14  is formed on each sliding piece S 12 . The protrusions S 14  protruding frontward engage with a plurality of depressions S 23  (see  FIG. 18 ) formed in engaging plates S 22  of a process shutter S 2  described later. 
     As shown in  FIG. 12 , a single first agitator A 1  is provided in the first accommodating unit  281 . The first agitator A 1  rotates in the first accommodating unit  281  so as to scrape over the first arc wall W 1  in a direction from top to bottom (counterclockwise in  FIG. 12 ). The first agitator A 1  is configured of a rotational shaft A 11  rotatably supported in side walls  284  constituting the left and right walls of the toner cartridge  208  (first accommodating unit  281 ), a support part A 12  extending radially outward from the rotational shaft A 11 , and an agitating blade A 13  supported on the support part A 12 . 
     As shown in  FIG. 15 , the agitating blade A 13  is configured of a first film A 14  disposed in the left-to-right center thereof, and second films A 15  disposed two on each of the left and right sides of the first film A 14 . Overall, the agitating blade A 13  has a width capable of extending over both the first supply hole W 11  and the second supply holes W 12  (a sufficient width for sliding over both openings). 
     The first film A 14  is a rectangular elastically deformable film formed at a width approximately equivalent to the width (left-to-right dimension) of the first supply hole W 11 . As shown in  FIG. 12 , the first film A 14  is in a flexed state while contacting an opposed wall  281 C formed between the first accommodating unit  281  and second accommodating unit  282  and having an arc-shaped cross section; and the first arc wall W 1  (first supply hole W 11 ), a top wall  281 D, and a bottom wall  281 E forming the first accommodating unit  281 . 
     As shown in  FIG. 15 , the opposed wall  281 C is erected for blocking a wide central opening that provides communication between the first accommodating unit  281  and second accommodating unit  282  (see also  FIGS. 12 and 16 ). More specifically, the opposed wall  281 C is disposed opposite the first supply hole W 11  horizontally and is formed with a greater width than that of the first film A 14 . With this construction, toner can be suitably conveyed by the first film A 14  upward between the first film A 14  and opposed wall  281 C and, thereafter, can be suitably conveyed toward the first supply hole W 11  between the first film A 14  and top wall  281 D. 
     The second films A 15  are elastically deformable films, each of which has a leading edge A 16  that slides in contact with the inner surface of the first accommodating unit  281 . The second films A 15  are configured so that their leading edges A 16  slope toward the rotational center of the first agitator A 1  (rotational shaft A 11 ) as the leading edges A 16  approach the first film A 14  side. Specifically, by forming the second films A 15  of a suitable shape and setting the orientation mentioned-above, the second films A 15  can slide over the inner surface of the first accommodating unit  281  in such a way that the inner portion of the leading edge A  16  in the left-to-right direction slides behind the outer portion in the left-to-right direction. 
     With the inner portions being behind the outer portions with respect to the left-to-right direction when the leading edges A 16  of the second films A 15  slide over the inner surface of the first accommodating unit  281 , the second films A 15  can convey toner from the second supply holes W 12  formed in the left and right outer sides toward the first supply hole W 11  formed in the center. Each of the second films A 15  has a hole A 17  formed in the center region thereof. The holes A 17  are provided to ensure that the second films A 15  do not push back more than the necessary amount of toner into the second accommodating unit  282  from among toner that was returned into the first accommodating unit  281  via the second supply holes W 12 . 
     As shown in  FIG. 12 , the second accommodating unit  282  is positioned adjacent to the first accommodating unit  281  in a horizontal direction when the toner cartridge  208  is mounted on the developing device  207  (and specifically when the developing unit  206  is mounted in the main casing  202 ). The main casing  202  has a substantially hollow, cylindrical shape. The rear end of the second accommodating unit  282  is in communication with the first accommodating unit  281  in all regions other than the opposed wall  281 C described above, while substantially the entire front side is in communication with the third accommodating unit  283 . 
     A second agitator A 2  is disposed inside the second accommodating unit  282 . The second agitator A 2  functions to convey toner accumulated in the bottom of the second accommodating unit  282  toward the first accommodating unit  281  by rotating in a direction opposite the rotational direction of the first agitator A 1  (clockwise in  FIG. 12 ). The second agitator A 2  includes a rotational shaft A 21  and a support part A 22  having substantially the same structure as the first agitator A 1  described above, and an agitating blade A 23  having a different structure from that in the first agitator A 1 . 
     As shown in  FIG. 15 , the agitating blade A 23  is configured of a pair of elastically deformable third films A 24  disposed in the left-to-right center of the agitating blade A 23 , and rectangular elastically deformable fourth films A 25  disposed two on both the left and right sides of the third films A 24 . 
     The third films A 24  are juxtaposed in the left-to-right direction and fall within the width of the opposed wall  281 C. Each third film A 24  has a leading edge A 26  for slidingly contacting the inner surface of the second accommodating unit  282 . The leading edges A 26  slope toward the rotational center of the second agitator A 2  (rotational shaft A 21 ) as the leading edges A 26  approach the adjacent third film A 24  side. Specifically, by forming the third films A 24  in a suitable shape and suitably setting the orientation mentioned-above, the third films A 24  can rotate so that the outer portions of the leading edges A 26  in the left-to-right direction are behind the inner portions of the leading edges A 26  in the left-to-right direction when sliding along the inner surface of the second accommodating unit  282 . 
     With the outer portions of the leading edges A 26  in the left-to-right direction sliding over the inner surface of the second accommodating unit  282  behind the inner portions of the same, the third films A 24  can convey toner outward in the left and right directions. Hence, the third films A 24  serve as blades that convey toner from the left-to-right center to the left and right outer sides. Accordingly, toner conveyed by the third films A 24  gravitates to the fourth film A 25  sides so as to avoid the opposed wall  281 C disposed on the rear side of the third films A 24 . Subsequently, the fourth films A 25  supply the toner into the first accommodating unit  281 . 
     As shown in  FIG. 12 , the third accommodating unit  283  is disposed adjacent to the second accommodating unit  282  in a horizontal direction when the toner cartridge  208  is mounted on the developing device  207  (and specifically when the developing unit  206  is mounted in the main casing  202 ). The third accommodating unit  283  has a substantially hollow cylindrical shape, with substantially the entire rear portion in communication with the second accommodating unit  282 . 
     A third agitator A 3  is disposed inside the third accommodating unit  283 . The third agitator A 3  conveys toner that accumulates in the bottom of the third accommodating unit  283  toward the second accommodating unit  282  by rotating in a direction opposite the rotating direction of the first agitator A 1  (clockwise in  FIG. 12 ). The third agitator A 3  includes a rotational shaft A 31  and a support part A 32  having substantially the same structure as the second agitator A 2  described above, and an agitating blade A 33  having a slightly different structure from the second agitator A 2 . 
     As shown in  FIG. 15 , the agitating blade A 33  has a single fifth film A 34  that is rectangular in shape and elastically deformable disposed in the left-to-right center of the agitating blade A 33 , and fourth films A 25  identical to those in the support part A 22  disposed two on each of the left and right sides of the fifth film A 34 . 
     As shown in  FIG. 13 , a first agitator gear AG 1 , a second agitator gear AG 2 , and a third agitator gear AG 3  are coaxially and integrally provided on one end of the respective first agitator A 1 , second agitator A 2 , and third agitator A 3 . Further, two gears G are disposed between the first agitator gear AG 1  and second agitator gear AG 2 , and a single gear G is disposed between the second agitator gear AG 2  and third agitator gear AG 3 . A coupling part CP is formed over the central shaft of the third agitator gear AG 3 . 
     With this construction, when a drive force from a drive source provided in the casing  202  (not shown) is transmitted to the coupling part CP formed on the third agitator gear AG 3 , the third agitator gear AG 3  rotates in the same direction as the second agitator gear AG 2  (counterclockwise in  FIG. 13 ) and in the opposite direction as the first agitator gear AG 1  (clockwise in  FIG. 13 ). All of the gears are covered with a gear case  285 . The gear case  285  is detachably mounted on the right side wall  284  of the toner cartridge  208 . 
     The gear case  285  has a hole  285 A formed therein for inserting the coupling part CP described above, and a shaft part  285 B formed thereon and protruding outward (rightward). The gear case  285  functions to pivotably support an operating member  286  that serves to open and close the toner shutter S 1  described above and a process shutter S 2  described later (see  FIG. 18 ) in association with each other. 
     The operating member  286  is disposed on the inner side of a cover member  287  that covers the outer side of the gear case  285 . The operating member  286  primarily includes a base part  286 A, an arm part  286 B, and an operation engaging part  286 C. The base part  286 A has a circular plate shape with a hole  286 D formed in the center thereof for rotatably engaging with the shaft part  285 B. A plurality of protrusions  286 E are formed along the rear edge of the base part  286 A for engaging with a plurality of depressions S 26  formed in a second engaging plate S 25  described later (see  FIG. 18 ). 
     The arm part  286 B extends radially outward from the base part  286 A (in approximately the forward direction. The operation engaging part  286 C is disposed on the distal end of the arm part  286 B and protrudes rightward therefrom. The operation engaging part  286 C protrudes (is exposed) externally through an arc-shaped elongated hole  287 A formed in the cover member  287 . 
     In addition to the elongated hole  287 A, a hole  287 B for receiving the coupling part CP described above and a hole  287 C for receiving the shaft part  285 B are formed in the cover member  287 . Further, the shaft part  285 B, operating member  286 , and cover member  287  having a construction similar to those described above are disposed on the left side of the toner cartridge  208 , as shown in  FIG. 17 . 
     11. Structure of the Developing Device 
     As shown in  FIG. 12 , the developing device  207  further includes a developer case  276  in which the developing roller  271  is provided, a support case  277  that supports the process cartridge  205  from the underside thereof, a developing chamber  274  accommodating the developing roller  271  and the like described above, and an auger  275  disposed above the supply roller  273 . 
     The front wall of the developing chamber  274  is a second arc wall W 2  that opposes the first arc wall W 1  of the toner cartridge  208  and curves in an arc-shape conforming with the first arc wall W 1 . A first through-hole W 21  is formed in the upper half of the second arc wall W 2  at a position in the left-to-right center thereof opposing the first supply hole W 11  of the first arc wall W 1 . Second through-holes W 22  are formed in the lower half of the second arc wall W 2 , with one in either left and right end thereof, at positions opposing the second supply holes W 12  formed in the first arc wall W 1 . 
     Hence, the first through-hole W 21  is disposed higher than the second through-holes W 22  and provided communication in a direction diagonally downward from the developer cartridge  208  to the developing device  207 . The second through-holes W 22  provide communication in a direction diagonally upward from the toner cartridge  208  to the developing device  207 . 
     A process shutter S 2  is provided on the front side of the second arc wall W 2  form opening and closing the first through-hole W 21  and second through-holes W 22  by moving along the circumference of the second arc wall W 2 . As shown in  FIGS. 18 ,  19 A, and  19 B, the process shutter S 2  includes a metal plate S 21 , and a pair of engaging plates S 22  disposed one on either left and right end of the metal plate S 21 . 
     The metal plate S 21  is formed in the same shape as the metal plate S 11  of the toner shutter S 1  described earlier. By moving similarly to the metal plate S 11 , as shown in  FIGS. 14A and 14B , the metal plate S 21  opens and closes the first through-hole W 21  and second through-holes W 22  described above. 
     Each of the engaging plates S 22  is disc-shaped and has a plurality of depressions S 23  formed in the rear edge thereof for engaging with the plurality of protrusions S 14  formed on the sliding piece S 12  of the toner shutter S 1 . The left and right engaging plates S 22  are also coupled together by two beam-like plates S 24 . The beam-shaped plates S 24  ensure that the rigidity of the process shutter S 2  is maintained. 
     A disc-shaped second engaging plate S 25  is coaxially disposed on the outer surface of each engaging plate S 22 . A plurality of depressions S 26  is formed in the rear edge of each second engaging plate S 25  for engaging with the plurality of protrusions  286 E on the operating member  286  described above. 
     An engaging hole S 27  is formed in the center portions of each engaging plate S 22  and second engaging plate S 25 . The engaging holes S 27  rotatably engage with rotational shafts  276 A protruding outward from the developer case  276  supporting the developing roller  271 . Through this construction, the process shutter S 2  can rotate relative to the developer case  276 . 
     The support case  277  has support holes  277 A formed one in either widthwise side wall thereof. The rotational shafts  276 A of the developer case  276  penetrate and are supported in the support holes  277 A. The support case  277  also has columnar-shaped protrusions  277 B formed one on each widthwise side wall thereof so as to protrude outward. The protrusions  277 B formed on the support case  277  and the rotational shafts  276 A inserted through the support holes  277 A are loosely supported in the elongated holes  205 A of the process cartridge  205  so as to be capable of shifting slightly in the front-to-rear direction. 
     As shown in  FIGS. 19A and 19B , the process shutter S 2  having the above construction is configured to open and close together with the toner shutter S 1  in association with the pivoting of the operating member  286 . For convenience, the developing device  207  and toner cartridge  208  are shown separated in  FIGS. 19A and 19B . 
     In  FIG. 19A , the toner shutter S 1  and the process shutter S 2  is in a closed position. Specifically, the first supply hole W 11  and second supply holes W 12  are closed by the toner shutter S 1  and the first through-hole W 21  and the second through-holes W 22  are closed by the process shutter S 2 , at which time the operation member  286  is positioned downward. When the operation engaging part  286 C of the operating member  286  is pivoted upward from the closed position, the protrusions  286 E formed on the operating member  286  push the depressions S 26  of the second engaging plate S 25  downward, causing the process shutter S 2  to rotate clockwise as shown in  FIG. 19B . When the process shutter S 2  is rotated in this way, the depressions S 23  formed in the engaging plates S 22  push the protrusions S 14  formed on the toner shutter S 1  in the clockwise direction of  FIG. 19 . Accordingly, the toner shutter S 1  rotates clockwise together with the process shutter S 2 . 
     Consequently, the first supply hole W 11  and second supply holes W 12  are opened (see  FIGS. 14A and 14B ). To close the first supply hole W 11  and second supply holes W 12 , the operation engaging part  286 C of the operating member  286  is pivoted downward, i.e., in the direction opposite that described above. Accordingly, the toner shutter S 1  and process shutter S 2  rotate counterclockwise in  FIG. 19  and close the first supply hole W 11  and second supply holes W 12 . 
     As shown in  FIG. 15 , the auger  275  is configured to convey toner supplied through the first through-hole W 21  toward the left and right second through-holes W 22 . Specifically, the auger  275  includes a rotational shaft  275 A, and a first helical blade  275 B and a second helical blade  275 C wound in spiral shapes about the rotational shaft  275 A. 
     The first helical blade  275 B and second helical blade  275 C are respectively disposed on right and left sides of the rotational shaft  275 A, separated by the axial center thereof (a point near the first through-hole W 21 . The spiraling directions of the first helical blade  275 B and second helical blade  275 C are opposite each other, so that the first helical blade  275 B conveys toner rightward and the second helical blade  275 C conveys toner leftward. 
     12. The Flow of Toner 
     Next, the flow of toner in the developing unit  206  will be described. The third agitator A 3  in the third accommodating unit  283  supplies toner from the third accommodating unit  283  into the second accommodating unit  282 . The third films A 24  of the second agitator A 2  convey toner in the second accommodating unit  282  outward in the left and right directions, after which the fourth films A 25  supply this toner into the first accommodating unit  281 . As shown in  FIG. 12 , the agitating blade A 13  of the first agitator A 1  rotates and slidingly contacts the opposed wall  281 C in the first accommodating unit  281 , pushing toner within the first accommodating unit  281  upward. As the agitating blade A 13  continues to rotate, the toner supported on the agitating blade A 13  is conveyed along the top wall  281 D and over the top of the first supply hole W 11 . 
     Through this conveyance, the toner falls by its own weight in a downward slope through the open first supply hole W 11  and first through-hole W 21  and is supplied into the developing device  207 . The auger  275  then conveys the toner supplied into the developing device  207  outward in the left and right directions, as shown in  FIG. 15 . 
     Through this process, the toner is spread over substantially the entire width of the supply roller  273 , enabling the supply roller  273  to supply toner to the developing roller  271  effectively. Further, when toner agitated by the auger  275  in the developing device  207  migrates over the second through-holes W 22 , the toner drops by its own weight in downward slope through the second through-holes W 22  and the second supply holes W 12  and returns to the first accommodating unit  281 . 
     When toner is returned to the first accommodating unit  281 , the second films A 15  of the first agitator A 1  convey the toner toward the left-to-right center thereof, while the first film A 14  supplies the toner back through the first supply hole W 11 , as shown in  FIG. 15 . 
     13. Operations and Effects of the Second Embodiment 
     The following effects can be obtained by the laser printer  201  according to the second embodiment described above. As the first agitator A 1  (first film A 14 ) slides over the first arc wall W 1  in a direction from top to bottom, i.e. the first supply hole W 11  is formed on the upstream side of the first arc wall W 1  with respect to the rotational direction of the first agitator A 1 , the first agitator A 1  can push toner through the first supply hole W 11  and effectively supply toner to the developing device  207 . Further, since the first agitator A 1  (second films A 15 ) rotate in a manner for pushing toner away from the second supply holes W 12 , the structure of the first agitator A 1  inhibits toner that was returned into the first accommodating unit  281  through the second supply holes W 12  from being pushed back into the developing device  207 . Hence, this construction improves circulation of the toner. 
     Since the second agitator A 2  rotates in a direction opposite the rotating direction of the first agitator A 1 , the second agitator A 2  can supply toner from the bottom of the second accommodating unit  282  into the first accommodating unit  281 , thereby effectively conveying toner from the second accommodating unit  282  to the first accommodating unit  281 . Further, since the third agitator A 3  rotates in the same direction as the second agitator A 2 , the third agitator A 3  can effectively convey toner from the third accommodating unit  283  into the second accommodating unit  282 . 
     By disposing the opposed wall  281 C between the first accommodating unit  281  and second accommodating unit  282 , toner can be retained between the first agitator A 1  and opposed wall  281 C, enabling the first agitator A 1  to convey toner effectively to the first supply hole W 11  disposed in the top of the first arc wall W 1 . Further, since the opposed wall  281 C is formed with an arc-shaped cross section, the first agitator A 1  can convey toner smoothly upward. 
     By providing the second films A 15  on the first agitator A 1  for conveying toner from the second supply holes W 12  toward the first supply hole W 11 , toner returned into the first accommodating unit  281  can be conveyed back toward the first supply hole W 11 . Hence, this construction further improves circulation of the toner. 
     By forming the first supply hole W 11  as an opening that slopes downward into the developing device  207  and the second supply holes W 12  as openings that slope upward toward the developing device  207 , the weight of the toner can be used to supply toner into the developing device  207  and return toner from the developing device  207 , thereby improving the circulation of toner between the developing device  207  and developer cartridge  208 . 
     By forming the first supply hole W 11  higher than the second supply holes W 12 , the second embodiment inhibits toner accumulated in the developing device  207  from flowing back into the first accommodating unit  281  through the first supply hole W 11 , while effectively encouraging the return of toner through the second supply holes W 12  disposed lower than the first supply hole W 11 . 
     By forming the adjoining wall  281 A of the first accommodating unit  281  as the first arc wall W 1  having an arc-shaped cross section that is concave on the side facing the interior of the first accommodating unit  281 , the first supply hole W 11  and second supply holes W 12  can be formed with a desired slope simply by forming holes in the upper half and lower half of the first arc wall W 1 . Therefore, circulation of toner can be improved through a simple construction. 
     By providing the toner shutter S 1  for opening and closing the first supply hole W 11  and second supply holes W 12 , the second embodiment can prevent toner from leaking when the toner cartridge  208  is not mounted in the developing device  207 . Further, the construction for moving the toner shutter S 1  along the circumference of the first arc wall W 1  is simpler than a construction for moving the shutter in the axial direction of the agitator. Further, having a toner shutter S 1  with an arc-shaped cross section rotating about its center axis (the reference line BL), as described in the second embodiment, ensures stable opening and closing operations. Specifically, the shutter does not tilt during movement, as can occur when using a flat plate-shaped shutter that is moved linearly. 
     Since two second supply holes W 12  are provided in the second embodiment, toner in the developing device  207  can be more easily returned to the first accommodating unit  281 . 
     The second embodiment further improves the circulation of toner by providing one first supply hole W 11  in the left-to-right center of the first supply hole W 11  and one second supply hole W 12  on each of the left and right ends of the first arc wall W 1 , and by disposing the auger  275  in the developing device  207  for conveying toner from the center toward both left and right ends of the developing device  207 . 
     Providing the second supply holes W 12  outside of the printing area PA inhibits the accumulation of toner outside of the printing area PA more so than within the printing area PA. Accordingly, this construction can suppress the occurrence of light image densities caused when only a small amount of toner has accumulated within the printing area PA. 
     The auger  275  provided for conveying toner received through the first through-hole W 21  toward the second through-holes W 22  further improves the circulation of toner. 
     By providing the cover member  287  for covering the operating member  286  and exposing only a portion of the operating member  286  (the operation engaging part  286 C) through the elongated hole  287 A formed in the cover member  287 , the construction of the second embodiment inhibits foreign matter from entering between the operating member  286  and the toner cartridge  208  more than a structure that does not include such a cover member, for example. Hence, this construction improves operability. 
     While the invention has been described in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     In the first embodiment described above, the intermediate transfer type color printer  1  shown in  FIG. 1  temporarily transfers a toner image in each color from the respective photosensitive drum  19  to the intermediate transfer belt  27 , and subsequently transfers the entire color toner image onto a sheet of paper P. However, the present invention is not limited to an intermediate transfer type color printer. For example, the invention may be applied to a direction transfer type color printer in which toner images are transferred directly from each photosensitive drum  19  onto the sheet of paper. 
     Further, the curved wall  103  is depressed inward the circular center of the circumferential wall  101  to form an arc-shaped curve. However, the curved wall may not be a good “arc-shaped curve” if the toner shutter  112  can slide along the circumferential direction of the curved wall  103  between the open position and the close position. 
     Further, while the photosensitive drums  19  are exposed by laser beams emitted from the scanning unit  10  in the color printer  1  of the first embodiment, the photosensitive drums  19  may be exposed using LEDs (Light-Emitting Diode) instead of the scanning unit  10 . 
     In the second embodiment described above, the first supply hole W 11  is disposed above the second supply holes W 12 , but the vertical arrangement of the first supply hole W 11  and second supply holes W 12  may be reversed or, alternatively, the first supply hole W 11  and second supply holes W 12  may be arranged at the same vertical position. Further, the left-to-right positions and the numbers of the first and second supply holes are not limited to the description in the second embodiment, but may be set as desired. 
     While the present invention is applied to the laser printer  201  in the second embodiment described above, the present invention may be applied to other image-forming devices such as photocopiers or multifunction peripherals.