Abstract:
An image forming apparatus uses a plurality of image forming units which are rotatable arranged in a vertical plane. The image forming units are divided into a photosensitive member unit for supporting a photosensitive member and a developing unit for supporting a developing roller and wherein the units are rockably supported about a rocking center axis. A point of application of a drive force for transmitting a drive force to the developing roller is located substantially on the rocking center axis.

Description:
This application is a divisional of application Ser. No. 09/599,595, filed Jun. 22, 2000, which application(s) are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an image forming apparatus that is applicable, for example, as a printer, a copying machine or a facsimile, and to an image forming unit used therefor. 
     BACKGROUND OF THE INVENTION 
     In a conventional image forming unit of an image forming apparatus, units having a handle for carrying it easily, has been much developed. For example, JP 2-11155B discloses an image forming unit shown in FIG. 14, in which a handle  300  is formed as one unit with an image forming unit main body  301 . 
     However, in the image forming unit having this conventional handle, since the handle is formed as one unit with the image forming unit main body, it is necessary to have a large space for containing the image forming unit in the image forming apparatus. Consequently, it is not possible to downsize the image forming apparatus. 
     Furthermore, in this kind of image forming apparatus, it is necessary to detect the amount of toner that remains in the image forming unit. As a technology for detecting the amount of remaining toner, for example, the technology disclosed in JP 6-317986A is well known. This publication discloses an image forming apparatus including a light transmission window on the developing device and an aperture on a developing cartridge so that a developing cartridge has a light path passing through a toner containing portion of the developing device on a line connecting a rotary unit (which is a holding member of a switching mechanism) and an outer position sandwiching the developing cartridge between the rotary unit and the outside position when a developing cartridge is positioned at the developing position. In this apparatus, the amount of toner remaining in the toner containing portion of the developing cartridge positioned at the developing position is detected by a light transmission method at the position between the rotary unit and the outer position. 
     Furthermore, in the image forming apparatus having this conventional means for detecting the amount of remaining toner, since the amount of remaining toner in the toner containing portion is detected by the light transmission method at a position outside of the position sandwiching the developing cartridge between the rotary unit and the outside position, the light path for detection becomes longer. As a result, the optical sensor becomes specific, which may deteriorate the accuracy. 
     Furthermore, as a conventional color image forming apparatus, in particular, an image forming unit used therefor, for example, one disclosed in JP 6-93141B is well known. 
     The following is a description of the conventional image forming unit disclosed in this publication, with reference to FIG.  15 . In FIG. 15, numeral  302  denotes a photosensitive member,  303  denotes a developing roller, and  304  denotes a charger. Numeral  305  denotes a photosensitive member unit supporting the photosensitive member  302  and the charger  304  and formed into one unit with a cleaner case, and  306  denotes a developing unit supporting a developing roller and formed into one unit with a toner case. The developing unit  306  is linked rotatably to the photosensitive member unit  305  by a supporting axis  307 . Numeral  308  denotes a compression spring, which is provided at the opposite side to the photosensitive member  302  while sandwiching the supporting axis  307  between the developing unit  306  and the photosensitive member unit  305 . Thereby, the photosensitive member  302  and the developing roller  303  are pressed to each other and in contact with each other. The developing roller  303  is driven to be rotated by using a rotation force of the photosensitive member  302 . A gear fixed to the photosensitive member  302  and a gear fixed to the developing roller  303  (both gears are not shown) are coupled with each other and driven to be rotated in the arrow directions, respectively. 
     In general, when such a image forming unit is attached to an electro-photographic apparatus main body, the axis center position of the photosensitive member  302  and a part of the photosensitive member unit  305  are supported firmly by the apparatus main body, and the photosensitive member  302  is driven to be rotated. Thus, images are formed. 
     In the formation of a color image, it is necessary to reduce unnecessary disturbance to the photosensitive member so as to improve the rotation accuracy of the photosensitive member and to bring the developing roller into light and uniform contact with the photosensitive member, thus to form a image with less unevenness in color or thickness of the image. 
     However, in the above-mentioned conventional image forming unit, the unevenness in rotation is transferred easily to the photosensitive member. Furthermore, the rotation driving force applying to the developing roller works as a pressing power with respect to the photosensitive member. As a result, the contact pressure of both rollers is uneven, and thus the unevenness in colors or thickness of the image tends to occur. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image forming unit having a handle and a compact size with a smaller containing space needed and an image forming apparatus using this image forming unit. Furthermore, it is another object of the present invention to provide an image forming unit capable of detecting the amount of toner remaining in a toner containing portion without deteriorating the detection accuracy and having a simple structure; and an image forming apparatus using this image forming unit. Furthermore, it is a further object of the present invention to provide an image forming unit and an image forming apparatus using this image forming unit, which do not tend to provide disturbance to the rotation of the photosensitive member; keep the contact pressure between the developing roller and the photosensitive member small and uniform; and are capable of forming a high quality image without unevenness in colors or thickness of the image. 
     In order to attain the above-mentioned object, a first configuration of an image forming unit according to the present invention includes an image forming unit main body having an image forming means, and a handle provided at the image forming unit main body so that it is capable of taking a holdable posture in which a user can hold the handle and a non-holdable posture in which a user cannot hold the handle, and is urged toward the non-holdable posture constantly by its elasticity. According to the first configuration of the image forming unit, it is possible to attain an image forming unit equipped with a handle and having a compact size with a smaller containing space required as compared with conventional units. 
     It is preferable in the first configuration of the image forming unit according to the present invention that the handle is made of an elastic plate having elongated holes at both ends thereof, is attached to an attachment surface of the image forming unit main body through the elongated hole by an anchoring member, and is capable of taking a non-holdable posture in which the handle is in contact with or close to the attachment surface and a holdable posture in which at least one part of the handle is separated from the attachment surface. With such a preferred configuration, it is possible to realize an image forming unit having a handle with a simple structure and inexpensively. 
     Furthermore, it is preferable in the first configuration of the image forming unit according to the present invention that the image forming unit is used for the image forming apparatus in which a plurality of image forming units are arranged rotatably in a vertical plane, and the plurality of image forming units are shifted sequentially to the image forming position so as to form images, wherein the handle is provided on the outer periphery of the rotation surface of the image forming unit main body. With such a preferred configuration, it is possible to attach/detach the image forming unit to/from the image forming apparatus easily. In this case, it is further preferable that the handle is urged toward the non-holdable posture by an elasticity that is stronger than a self-weight by which the handle is urged toward the holdable posture. With such a preferred configuration, it is possible to prevent the handle from taking the holdable posture when it comes to the lowermost place. In this case, it is still further preferable that the image forming unit includes a photosensitive member and a developing device and that the photosensitive member is exposed to the outer periphery of the rotation surface of the image forming unit main body. With such a preferred configuration, it is possible to transfer a toner image on the photosensitive member to the recording paper, easily. In this case, it is further preferable that the handle in its non-holdable posture is present inside the rotation track surface of the photosensitive member, and the handle in its holdable posture is present outside the rotation track surface of the photosensitive member. With such a preferred configuration, when the image forming unit is rotated in the image forming apparatus, the handle avoids contact with the transfer means without the need for additional complex operation. 
     It is further preferable that the image forming unit of the first configuration according to the present invention includes a photosensitive member unit that supports the photosensitive member and a developing unit that supports the developing device and that the handle is provided at the side of the developing unit. With such a preferred configuration, since the handle is provided at the side of the developing unit containing heavier parts such as a developing device, etc., it is possible to balance the image forming unit when it is suspended by holding the handle. Furthermore, by providing the handle at the side of the developing unit, it is possible to secure a sufficient space for placing the handle. 
     According to the first configuration of the image forming apparatus according to the present invention, the image forming apparatus including a unit-rotating member that supports a plurality of image forming units in a vertical plane rotatably, and forming images by shifting the plurality of image forming units sequentially to the image forming position, wherein the image forming unit includes an image forming unit main body having an image forming means, and a handle provided at the image forming unit main body so that it is capable of taking a holdable posture in which a user can hold the handle and a non-holdable posture in which a user cannot hold the handle, and is urged toward the non-holdable posture constantly by its elasticity. 
     It is preferable that the first configuration of the image forming apparatus according to the present invention includes an opening/closing door for allowing attaching/detaching of the image forming unit to/from the upper part of the unit-rotating member, wherein the opening/closing door is capable of pressing, in its holdable posture, the handle of the image forming unit installed to the unit-rotating member. With such a preferred configuration, even if a user forgets to put the handle in the non-holdable posture, when the image forming unit is replaced with a new unit, by only closing the opening/closing door, the handle can be put in the non-holdable posture. Furthermore, in this case, it is preferable that the opening/closing door is provided with a protruding portion capable of being brought into contact with the handle of the image forming unit in its holdable posture. 
     Furthermore, according to the second configuration of the image forming apparatus of the present invention, the image forming apparatus includes a unit-rotating member that supports a plurality of image forming units in a vertical plane rotatably, and forming images by shifting the plurality of image forming units sequentially to the image forming position, wherein the image forming unit comprises an image forming unit main body, a photosensitive member exposed to the outer periphery of the rotation surface of the image forming unit main body, a handle provided at the image forming unit main body, and the handle of the image forming unit at the position where the image forming unit is replaced with a new unit in the unit-rotating member, is arranged farther from a front side of the image forming apparatus than the photosensitive member. According to the second configuration of the image forming apparatus, when the image forming unit is replaced with a new unit, it is possible reliably to prevent the operator&#39;s finger from touching the exposed photosensitive member. 
     Furthermore, according to the third configuration of the image forming apparatus of the preset invention, the image forming apparatus includes a unit-rotating member that supports a plurality of image forming units in a vertical plane rotatably, and forming images by shifting the plurality of image forming units sequentially to the image forming position, wherein the image forming unit comprises a photosensitive member unit that supports the photosensitive member, a developing unit that supports the developing device, and a handle; the photosensitive member is exposed to the outer periphery of the rotation surface of a photosensitive member unit main body; in the image forming unit at the position where the image forming unit is replaced with a new unit in the unit-rotating member, the photosensitive member unit and the developing unit are arranged in this order from the front side of the image forming apparatus; and the handle is provided at the side of the developing unit. According to the third configuration of the image forming apparatus, it is possible to locate the handle of the image forming unit in the unit-rotating member at a replacement position where the image forming unit is replaced with a new unit farther from the front side of the image forming apparatus than the photosensitive member. As a result, when the image forming unit is replaced with a new unit, it is possible reliably to prevent the operator&#39;s finger from touching the exposed photosensitive member. 
     Furthermore, it is preferable in the second or third configuration of the image forming apparatus according to the present invention that the replacement of the image forming unit from the rear side of the image forming apparatus is suppressed. With such a preferred configuration, it is possible reliably to prevent the operator&#39;s finger from touching the photosensitive member. In this case, it is still further preferable that the image forming apparatus includes an opening/closing door for attaching/detaching the image forming unit to/from the upper part of the unit-rotating member, thereby regulating the replacement of the image forming unit from the rear side of the image forming apparatus. 
     According to the second configuration of the image forming unit of the present invention, an image forming unit is used for the image forming apparatus in which a plurality of image forming units are arranged rotatably in a vertical plane, and the plurality of image forming units are shifted sequentially to the image forming position, wherein a translucent detection window for detecting the amount of remaining toner is provided on the periphery of the rotation surface of the image forming unit main body that communicates to a toner hopper. According to the second configuration of the image forming unit, a translucent detection window for detecting the remaining toner is provided on the outer periphery of the rotation surface of the image forming unit main body that communicates to the toner hopper. Therefore, by arranging a detection member such as a light emitting element, a light receiving element, or the like, in the vicinity of the detection window, it is possible to detect the amount of remaining toner, thus to shorten the light path for detection. As a result, it is possible to enhance the accuracy in detecting the amount of remaining toner. Furthermore, the translucent detection window is exposed to the outside of the image forming unit main body, it is possible to check visibly the amount of remaining toner after taking out the image forming unit from the image forming apparatus main body. 
     Furthermore, it is preferable in the second configuration of the image forming unit according to the present invention that the detection window is located at the corner on the periphery of the rotation surface of the image forming unit main body. With such a preferred configuration, since the detection window is located at the bottom surface of the toner hopper, it is possible to detect only a small amount of toner easily. 
     Furthermore, it is preferable in the second configuration of the image forming unit according to the present invention that the detection window is provided in a cavity formed in the toner hopper, and has a pair of opposed side faces, further comprising reflection planes refracting light for detecting the amount of remaining toner, the reflection planes being provided in the vicinity of the pair of side faces. With such a preferred configuration, it is not necessary to arrange a detection member such as a light emitting element, a light-receiving element, etc., and it is possible to make the light path for detection relatively short. 
     Furthermore, it is preferable that the second configuration of the image forming unit according to the present invention includes a cleaning member for cleaning the detection window. With such a preferred configuration, it is possible to enhance the accuracy in detecting the amount of remaining toner by appropriately cleaning the detection window. 
     According to the fourth configuration of the image forming apparatus according to present invention, an image forming apparatus includes a unit rotating member that supports a plurality of image forming units in a vertical plane rotatably, and forming images by shifting the plurality of image forming units sequentially to the image forming position; wherein a translucent detection window for detecting the amount of remaining toner is provided on the periphery of the rotation surface of the image forming unit main body that communicates with a toner hopper. 
     It is preferable in the fourth configuration of the image forming apparatus according to the present invention that the detection window is located at the corner on the periphery of the rotation surface of the image forming unit main body. 
     Furthermore, it is preferable in the fourth configuration of the image forming apparatus according to the present invention that the detection window is provided in a cavity formed in the toner hopper, and has a pair of opposed side faces, further comprising reflection planes refracting light for detecting the amount of remaining toner, the reflection planes being provided in the vicinity of the pair of side faces. Furthermore, in this case, it is preferable that a detection means that emits/receives light for detecting the amount of remaining toner is provided facing the periphery of the rotation surface of the image forming unit main body. 
     Furthermore, it is preferable in the fourth configuration of the image forming apparatus according to the present invention that the detection of the amount of remaining toner is performed at a different position from the image forming position. With such a preferred configuration, it is possible to arrange the detection means that emits/receives light for detection of the amount of remaining toner. Furthermore, in this case, it is preferable that the detection of the amount of remaining toner is performed when the detection window is located at a lower place than the horizontal line including the rotation center of the unit-rotating member. With such a preferred configuration, it is possible to detect only a small amount toner. Furthermore, in this case, it is preferable that the detection of the amount of remaining toner is performed when the image forming unit is located at the opposite side to the image forming position. Furthermore, in this case, it is preferable that a cleaning member for cleaning the detection window is provided, and the cleaning of the detection window is performed only at the image forming position. With such a preferred configuration, it is not necessary to provide a mechanism for driving the cleaning member at several positions, thus simplifying the structure of the image forming apparatus. 
     Furthermore, it is preferable in the second configuration of the image forming apparatus according to the present invention that the amount of remaining toner of the image forming unit for one color is detected several times while the unit-rotating member rotates several times thus to obtain the detected results, and the amount of the remaining toner is determined based on the detected results. With such a preferred configuration, it is possible to reduce the detection error and to detect the amount of the remaining toner precisely. 
     Furthermore, according to the third configuration of the image forming unit according to the present invention, an image forming unit includes a photosensitive member unit having a photosensitive member on the surface of which a electrostatic latent image is formed, and a developing unit which has a developing agent, a developing roller supporting the developing agent at the developing region, and a driving transmitting means for rotating the developing roller, and which develops the electrostatic latent image to be a manifest image, wherein the developing unit is supported by the photosensitive member unit rockably, the point of application, which transmits the rotation force to the developing unit, is provided in the vicinity of the rocking center axis of the photosensitive member unit and the developing unit. With the third configuration of the image forming unit, the developing unit is driven to be rotated by the force that transmits a rotation force to the developing unit, and the developing roller is not pressed onto the photosensitive member and the pressing power that is set when the developing roller is not rotated can be maintained. Therefore, it is possible to bring the developing roller into contact with the photosensitive member constantly and stably. 
     Furthermore, it is preferable in the third configuration of the image forming unit according to the present invention that a driving force for rotating the developing roller is applied from the outside of the image forming unit. With such a preferred configuration, the photosensitive member is not subjected to a load fluctuation as the photosensitive member drives the developing roller. Moreover, since the contact pressure of the developing roller to the photosensitive member can be set to be small, the photosensitive member is not susceptible to the disturbance from the developing roller, whereby the stable rotation can be secured. Furthermore, in this case, it is preferable that the developing roller is in contact with the photosensitive member, thereby developing an electrostatic latent image to be a manifest image. Furthermore, in this case, it is preferable that the developing unit is supported by the photosensitive member unit rockably at the driving side of the developing unit, and the relative position between the photosensitive member of the photosensitive member unit and the developing roller is determined by a sliding guide provided at the photosensitive member unit at the opposite side to the driving side of the developing unit. With such a preferred configuration, it is possible to maintain the state in which the photosensitive member is in contact with the developing roller evenly over the entire surface regardless of the torsion, etc., of the box. Furthermore, in this case, it is preferable that the driven gear that is driven directly from the outside of the image forming unit among the driving force transmitting means for rotating the developing roller is supported rockably with the rotating axis of the developing roller as a center. With such a preferred configuration, it is possible automatically to couple a carriage, to which the image forming unit is attached, at the side of the main body to a developing driving main body gear at the side of the main body by only rotating the carriage. Furthermore, in this case, it is preferable that the image forming unit further includes a rocking member that supports the driven gear rockably, wherein the rocking member engages an anchoring portion provided at the image forming unit when the rotation force is transmitted from the outside of the image forming unit to the developing roller. With such a preferred configuration, since the rotation moment around the engagement portion is not applied from the outside of the developing unit, it is possible to bring the photosensitive member into contact with the developing roller in a stable state. Furthermore, in this case, it is preferable that the image forming unit includes anchoring portions provided at both ends of the photosensitive member and supported by the apparatus main body, and a rotation anchoring portion provided at the driving force transmitting side of the developing unit and positioning the rotation position around the rotation axis of the photosensitive member. With such a preferred configuration, since the image forming unit is supported by the apparatus main body at three points, i.e. at the engagement portion at both ends of the rotation axis of the photosensitive member and at the rotation anchoring portion, it is possible to position the photosensitive member with respect to the apparatus main body reliably. At the same time, it is possible to solve the problem that it is difficult to bring the photosensitive member into contact with the developing roller reliably because the image forming unit is subjected to a developing driving force and the unit is twisted. Furthermore, in this case, it is preferable that the rotation anchoring portion is provided in the photosensitive unit. With such a preferred configuration, since the rotation force is supported only by the photosensitive member unit, the state in which the photosensitive member is in contact with the developing roller is not affected. Furthermore, in this case, it is preferable that the rotation anchoring portion is a supporting axis that supports the developing unit rockably or a receiving portion of the supporting axis. With such a preferred configuration, at the place where the outside power is applied to the image forming unit, the carriage at the side of the main body supports the image forming unit, the excess twisting power does not occur in the image forming unit. 
     Furthermore, according to the fifth configuration of the image forming apparatus, an image forming apparatus includes a plurality of image forming units combining a developing device and a photosensitive member for each color; at image forming unit transmitting means for switching the image forming units by sequentially shifting the image forming unit for each color between an image forming position and other waiting positions; a positioning means for positioning the photosensitive member at the image forming position; an exposing means for exposing the photosensitive member located at the image forming position; a transfer means for transferring sequentially the toner images of many colors, which are formed on the photosensitive member at the image forming position by the developing device, to form a toner image in which toner images of many colors are overlapped on the photosensitive member; and a rotation driving means for rotating the photosensitive member and the transfer means, wherein an image forming unit according to the present invention is used as the image forming unit. According to the fifth configuration of the image forming apparatus, it is possible to position the photosensitive member reliably by a simple structure and further to form a high quality and stable image capable of securing the uniform development by pressing lightly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing an image forming apparatus in one embodiment according to the present invention. 
     FIG. 2 is an outside perspective view showing an image forming unit in one embodiment according to the present invention. 
     FIG. 3 is a cross-sectional view taken on line A—A of FIG.  2 . 
     FIG. 4 is a cross-sectional view taken on line B—B of FIG.  2 . 
     FIG. 5 is a cross-sectional view taken on line C—C of FIG.  2 . 
     FIG. 6 is a left side view of FIG. 2 (a gear cover is not shown). 
     FIG. 7 is an outside perspective view showing an image forming unit when a handle is in a holdable posture in one embodiment according to the present invention. 
     FIG. 8 is a cross-sectional view showing a state in which an image forming unit is replaced with a new one by an operator. 
     FIG. 9 is an exploded perspective view showing a positioning mechanism of a carriage and a photosensitive member and a driving mechanism in one embodiment according to the present invention. 
     FIG. 10 is a cross-sectional view showing a carriage taken on a line passing the image forming position of the image forming apparatus in one embodiment of the present invention. 
     FIG. 11 is a cross-sectional view showing a rocking lever provided in a developing unit in one embodiment of the present invention. 
     FIG. 12 is a right side view of FIG.  2 . 
     FIG. 13 is a view to explain how a photosensitive member and a developing roller are driven in one embodiment according to the present inventions 
     FIG. 14 is an outside perspective view showing a conventional image forming unit. 
     FIG. 15 is an outside perspective view showing another conventional image forming unit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described by way of embodiments with reference to drawings. 
     FIG. 1 is a cross-sectional view showing a configuration of an image forming apparatus in one embodiment according to the present invention; FIG. 2 is an outside perspective view showing a configuration of an image forming unit in one embodiment according to the present invention; FIG. 3 is a cross-sectional view taken on line A—A of FIG. 2; FIG. 4 is a cross-sectional view taken on line B—B of FIG. 2; FIG. 5 is a cross-sectional view taken on line C—C of FIG. 2; FIG. 6 is a left side view of FIG. 2 (a gear cover is not shown); and FIG. 7 is an outside perspective view showing the state in which a handle of the image forming unit is in a holdable posture in one embodiment according to the present invention. 
     In FIG. 1, the right-hand face is the front face of the apparatus. The apparatus main body  1  includes a carriage  2  at approximately the center thereof, a front alligator  1 A on the front face and a top door  17  on a top face. 
     In FIGS. 1 to  3 , numeral  30  denotes a photosensitive member,  34  denotes a corona charger for homogeneously charging the photosensitive member  30  negatively, and  35  denotes a developing unit including a developing roller  31 , a supply roller  217  for supplying toner, a doctor blade  24  for forming a toner layer, and a toner hopper  39  for containing toner  32 . Numeral  36  denotes a rubber cleaning blade for cleaning off the toner that remains on the surface of the photosensitive member  30  after transfer. Numeral  38  is a waste toner case for containing the toner scraped off by the cleaning blade  36 . A corona charger  34  and a photosensitive member  30  are attached to the waste toner case  38 , thus forming a photosensitive member unit  37 . Furthermore, the photosensitive member unit  37  and the developing unit  35  are linked rockably as one unit by a supporting pin  90  and thus an image forming unit  3  is formed. The photosensitive member  30 , the developing roller  31  and the supply roller  217  are supported rotatably and rotate in the arrow direction, respectively. The toner  32  is made of polyester resin in which a pigment is dispersed and charged negatively. The toner  32  is supplied from the supply roller  217  to the developing roller  31  through a supply port  25 , made into a thin film with the doctor blade  24 , and then supplied to the photosensitive member  30  so as to develop an electrostatic latent image on the photosensitive member  30 . 
     A carriage  2  as a unit-rotating member contains four image forming units  3 Y,  3 M,  3 C and  3 Bk for four colors (yellow, magenta, cyan, and black). The carriage  2  is supported on a cylinder drum  21  rotatably, thereby switching the image forming units  3  by sequentially shifting the photosensitive member  30  of the image forming unit  3  for each color between the image forming position  10  and the other waiting positions. 
     Inside the carriage  2 , the image forming unit  3  can form images only at an image forming position  10  where the photosensitive member  30  is irradiated with a pixel laser signal light  8  and a transfer belt unit  5  is in contact with the photosensitive member  30 . The image forming unit  3  is connected to a driving source or a power source of the apparatus main body  1  at the image forming position  10 , thereby performing the image forming operation. The other positions are waiting positions in which the image forming units  3  are not operated. 
     As shown in FIGS. 2 to  4 , the image forming unit  3  is provided with a handle  200  on the outer periphery of its rotating surface (a surface along the outer periphery of the carriage  2  in FIG.  1 ). Thus, a user can, for example, carry the image forming unit  3  by holding a handle  200  by the hand. The handle  200  is provided at the side of the developing unit  35  where heavy parts such as the developing roller  31 , etc. are included. Thus, when the image forming unit  3  hangs by holding the handle  200  by the hand, balance of the unit can be maintained. Furthermore, by providing the handle  200  at the developing unit  35 , it is possible to secure a sufficient space for placing the handle  200 . Furthermore, since the handle  200  is provided on the outer periphery of the rotating surface of the image forming unit, it is possible to attach/detach the image forming unit  3  to/from the carriage  2  of the apparatus main body  1  easily. 
     The outer periphery of the rotation surface of the developing unit  35  is provided with a rectangular-shaped concave portion  201  along the longitudinal direction thereof, so that the handle  200  made of an elastic rectangular-shaped plate can be contained in the concave portion  201 . With such a configuration in which the handle  200  is contained in the concave portion  201 , it is possible to obtain an image forming unit  3  having a compact configuration as compared with conventional units. At both ends of the handle  200 , elongated holes  202  extending along the longitudinal direction of the handle  200  are provided, and the elongated holes  202  engage anchoring members  216  provided in the concave portion  201 . When the handle  200  is contained in the concave portion  201  by bringing the handle  200  into contact with the bottom surface of the concave portion  201 , the anchoring member  216  is in contact with the inner end of the elongated hole  202 . Moreover, the handle  200  is urged to a position in which it is contained in the concave portion  201  by its own elastic force (non-holdable posture). In this non-holdable posture, the handle  200  is located at the inner portion as compared with the rotation track surface of the photosensitive member  30 . Thus, when the image forming unit  3  is rotated inside the image forming apparatus, the handle  200  is kept out of contact with the transfer belt unit  5  without requiring an additional complex operation. Moreover, the image forming unit  3  is rotated in a vertical plane while it is contained in the carriage  2  (FIG.  1 ). In this case, it is desirable that the elasticity of the handle  200  is set to be larger than the force by which the handle  200  takes a holdable posture, for preventing the handle  200  of the image forming unit  3  at the lowermost position from assuming a holdable posture by its self-weight. 
     Furthermore, on the outer periphery of the rotation surface of the developing unit  35 , an entrance groove  203  is provided on the central portion in the longitudinal direction of the developing unit  35 . The entrance groove  203  extends below the handle  200  with a step with respect to the concave portion  201 , thereby securing a sufficient space into which a finger is inserted below the handle  200 . 
     Since the handle  200  portion is formed as mentioned above, when putting the finger into the introducing groove  203  and holding the handle  200 , the outer end portions of the elongaged holes  202  formed at both ends of the handle  200  are in contact with the anchoring member  216  by the weight of the image forming unit  3 . Consequently, the state in which the handle  200  is out of the concave portion  201  (holdable posture) is realized (a state illustrated by a two-dot chain line in FIG.  4  and in FIG.  7 ). In this holdable posture, the handle  200  is located at an outer position from the rotation track surface of the photosensitive member  30 . 
     It is desirable that as materials of the handle  200 , ABS, polycarbonate, or the like, is used. A specific size of the handle  200  is, for example, 170 mm in length, 18 mm in width and 1.2 mm in thickness. 
     The image forming unit  3  is attached to the apparatus main body  1  detachably. When one of the image forming units  3  needs to be replaced with a new unit, it can be done after rotating the carriage  2  so that the image forming unit  3  of the color to be replaced is located underneath the top door  17 , and opening the top door  17 . Since the replacement of the image forming unit  3  is carried out by holding the handle  200  provided on the outer periphery of the rotating surface of the developing unit  35 , it is easy to attach/detach the image forming unit  3  to/from the apparatus main body  1 . Furthermore, the replacement of the image forming unit  3  generally is carried out from the front side of the apparatus (the right side of the image forming apparatus shown in FIG. 1) (see, FIGS.  1  and  8 ). Herein, the image forming unit  3 , inside the carriage  2  at the replacement position, includes the photosensitive member unit  37  and the developing unit  35  in this order from the front side of the apparatus. The handle  200  is provided at the side of the developing unit  35 . Namely, at the replacement position of the image forming unit  3 , the handle  200  is arranged farther from the apparatus front side than the photosensitive member  30 . In this way, by providing the handle  200  farther from the apparatus front side (i.e., an operators&#39; position) than the photosensitive member  30 , when the image forming unit  3  is replaced with a new unit, it is possible to prevent the operator&#39;s finger from touching the photosensitive member  30  exposed to the outside. Moreover, when the image forming unit  3  is replaced with a new unit, the top door  17  is open so as to prevent the operation of the image forming unit  3  from the rear side of the apparatus, thus inhibiting the replacement of the image forming unit  3  from the apparatus rear side. Thereby, when the image forming unit  3  is replaced with a new unit, it is possible reliably to prevent the operator&#39;s finger from touching the photosensitive member  30 . 
     The inner surface of the top door  17  is provided with a protruding portion  204 . The shape of the lower surface of the protruding portion  204  when the top door  17  is closed is a shape so that it is along the outer periphery of the carriage  2 : The handle  200  can be contained in the concave portion  201  to have a non-holdable posture by its elasticity only by pushing it lightly by the finger tip. Therefore, by providing the protruding portion  204  on the inner surface of the top door  17 , even if a user forgets to keep the handle in a non-holdable posture, as long as the top door  17  is closed after the image forming unit  3  is replaced with a new unit, the lower surface of the protruding portion  204  can be brought into contact with the handle  200  so as to urge the handle  200  toward the non-holdable posture (a state illustrated by a two-dot chain line in FIG. 4 and a state in FIG.  7 ). 
     Furthermore, as shown in FIGS. 2,  5  and  6 , a toner detection case  205  as a detection window having an arc-shaped cross section is provided at the end in the longitudinal direction of the developing unit  35  positioned at the corner of the outer periphery of the rotation surface of the developing unit  35 . This toner detection case  205  is formed in the cavity  213  formed in a toner hopper  39 . The toner detection case  205  is formed continuously with the toner hopper  39 . The toner detection case  205  is made of a translucent member. On both sides of the toner detection case  205 , reflection planes  206  are provided at an angle of 45° with respect to the side faces of the toner detection case  205 , respectively. As shown in FIG. 5, a light beam emitted from a light emitting element  207  arranged facing the toner detection case  205  on the outer periphery of the rotation surface of the developing unit  35  is reflected from one of the reflection planes  206 , is incident on the toner detection case  205  from one side face (a detection window) of the toner detection case  205 , and is released from the other side face (a detection window) and the light reflected from the other reflection plane  206  is received by a light receiving element  208  arrange facing the toner detection case  205  on the outer periphery of the rotation surface of the developing unit  35 . In the above-mentioned process, the amount of toner that remains in the toner hopper  39  is detected. That is, since the amount of light beam received at a light-receiving element  208  differs depending upon the amount of toner remaining in the toner hopper  39  (the amount of toner in the toner detection case  205 ), the amount of toner in the toner hopper  39  can be detected. By detecting the amount of toner remaining in the toner hopper  39  with the above-mentioned configuration, it is not necessary to arrange the detection members such as a light emitting element  207 , a light-receiving element  208 , etc. in the cavity  213  provided in the toner hopper  39 . Also, it is necessary to put/take the detection members into/out of the cavity  213 . Furthermore, it is possible to make-the detecting light path as short as possible. In addition, since the toner detection case  205  is made of a translucent member, it is possible to check the remaining amount of toner visibly by taking out the image forming unit  3  from the apparatus main body  1  It is desirable that the material constituting the toner detection case  205  includes acrylic resins, polystyrene resin, or the like. 
     The light emitting element  207  and the light-receiving element  208  are arranged near the carriage  2  so that they can detect the amount of toner remaining in the toner hopper  39  of the image forming unit  3  (a cyan image forming unit  3 C in FIG. 1) positioned at the opposite side of the image position  10  shown in FIG.  1 . In other words, the position for detecting the amount of remaining toner is positioned at a lower part with respect to the horizontal line including the rotation center of the carriage  2 . At this detection position, since the toner detection case  205  is located at the bottom portion of the toner hopper  39 , it is possible to detect the amount of toner even if it is only a small amount. Moreover, as shown in FIG. 1, at the image forming position  10 , toner in the toner hopper  39  is collected to the inner periphery of the carriage  2 . Therefore, if the amount of remaining toner is detected at the image forming position  10 , it is necessary to provide the light emitting element  207 , the light receiving element  208 , and the like, at the inner periphery of the carriage  2 . Thus, it is made to be difficult to place the light emitting element  207 , the light-receiving element  208 , and the like. However, as mentioned above, by detecting the amount of remaining toner at the different position and posture from those of the image forming position  10 , it is possible easily to place the light emitting element  207 , the light-receiving element  208 , and the like, for detecting the amount of remaining toner. 
     As shown in FIGS. 5 and 6, inside the toner detection case  205  is a cleaning member  209  for cleaning the inner wall of the toner detection case  205 . The cleaning member  209  includes a cleaning member main body  211  supported rotatably by the central axis  210  and a cleaning blade  212  protruding from the both ends of the cleaning member main body  211 . Then, the arc-shaped surface of the toner detection case  205  is cleaned with the both ends of the cleaning member main body  211  and both side walls (detection window) of the toner detection case  205  are cleaned with the blades  212 . By providing the cleaning member  209  for cleaning off the inner surface of the toner detection case  205  inside the toner detection case  205 , it is possible to enhance the accuracy in detecting the amount of remaining toner appropriately by cleaning the inner wall of the toner detection case  205 . The driving method of the cleaning member  209  is described later. 
     In FIGS. 2 and 7, numeral  221  denotes a gear cover. A transfer belt unit  5  receives a toner image formed on the photosensitive member  30  of the image forming unit  3  at the image forming position  10  and transfers again the received toner image onto the recording paper. The transfer belt unit  5  includes an intermediate transfer belt  50 , a group of pulleys (a driving pulley  55 A, a back-up pulley  55 B, a guide pulley  55 C and a tension pulley  55 D) for suspending the intermediate transfer belt  50 , a cleaner  51  and a waste toner case  57  for containing the waste toner after cleaning. These members are formed into one unit, and the unit is attached to the apparatus main body  1  detachably. 
     The intermediate transfer belt  50  is an endless belt having a thickness of about 100 μm, and is made of a film of semiconductive (middle resistivity) urethane coated with a fluororesin such as PFA, PTFE, or the like. The total thickness of this films is set in the range from 100 to 300 μm. The peripheral length of the intermediate transfer belt  50  is set to be a little bit more than the length corresponding to the maximum length of the A4 recording paper size (297 mm) accepted, so that A4 size or letter size recording paper can be used for full color printing. 
     Numeral  51  denotes a cleaner, which cleans the toner remaining on the intermediate transfer belt  50 . This cleaner  51  includes a cleaning blade  53  made of rubber, and a screw  52  for carrying scraped toner into the waste toner case  57 . This cleaner  51  is placed so that it can rotate with a supporting point  58  as a center and is separated from the transfer belt  50  in order not to scrape off a toner image on the intermediate transfer belt unit  50  when forming a color image on the intermediate transfer belt  50 . 
     Among the group of pulleys suspending the intermediate transfer belt  50 , the pulley  55 A is a driving pulley for driving the intermediate transfer belt  50 , which also has a function of backing up the cleaning blade  53 . The pulley  55 B is a back-up pulley backing up a secondary transfer roller  9  that transfers a toner image on the intermediate transfer belt  50  onto the recording paper. The pulley  55 C is a guide pulley that also functions as a roller for applying a primary transfer bias for transferring the toner image onto the intermediate transfer belt  50  from the photosensitive member  30 . The pulley  55 D is a tension pulley providing the intermediate transfer belt  50  with a tension. These pulleys  55 A,  55 B,  55 C, and  55 D suspend the intermediate transfer belt  50 . The intermediate transfer belt  50  can be rotated by the rotation of the driving pulley  55 A. Numeral  56  denotes a cover for protecting the intermediate transfer belt  50 . 
     A transfer belt unit  5  is positioned reliably at a predetermined position when it is attached to the apparatus main body  1 , and the portion facing the image forming position  10  is in contact with the photosensitive member  30  of the image forming unit  3 . Furthermore, at the same time, each portion of the transfer belt unit  5  is connected electrically to the side of the apparatus main body and the driving pulley  55 A is connected to the driving means at the side of the apparatus main body. Thereby, intermediate transfer belt  50  can be rotated. 
     Furthermore, numeral  7  denotes an electricity-removing needle, which prevents a toner image from being disturbed when the recording paper is separated from the intermediate transfer belt  50 . 
     Numeral  6  denotes a laser exposure device, which is placed underneath the transfer belt unit  5 . The laser exposure device  6  includes a semiconductor laser (not shown), a polygon mirror  6 A, a lens system  6 B, a first mirror  6 C, and the like. The pixel laser signal light  8  corresponding to a transient serial electrical pixel signal for image information passes through an optical path  22  formed between a photosensitive member unit  37  of a yellow image forming unit  3 Y and the developing unit  35  of a black image forming unit  3 Bk. The pixel laser signal light  8  passes through an exposure window  97  of the cylinder drum  21 ; is incident on the second mirror  98  of the cylinder drum  21  (the second mirror  98  is fixed in place regardless of the movement of the cylinder drum  21 ), which is fixed to the apparatus main body  1 ; is reflected from the second mirror  98  and incident into an exposure portion of the left side face of the photosensitive member  30  located at the image forming position  10  while passing through the gap provided between the photosensitive member unit  37  and the development unit  35  of the yellow image forming unit  3 Y, so as to scan and expose the photosensitive member  30  in the direction of the main line. 
     Numeral  12  denotes a paper feed unit,  14  denotes a paper feed roller,  16  denotes a resist roller,  18  denotes a paper ejecting roller, and  13   a ,  13   b ,  13   c , and  13   d  denote a paper guide, respectively, which link between the above-mentioned rollers as well as between the contact point of the intermediate transfer belt  50  and the secondary transfer roller  9  and a fixing device  15 . 
     The front alligator  1 A is hinged to the apparatus main body  1  with a hinge axis  1 B, and can be lowered and opened toward the front. The front alligator  1 A is provided with the fixing device  15 , a secondary transfer roller  9 , the electricity-removing needle  7 , a front side of the paper guide  13   a ,  13   b ,  13   c  and  13   d , and the front side of the resist roller  16 . When the front alligator  1 A is lowered toward the front, these components also are lowered toward the front together. Therefore, it is possible to open the front surface of the it apparatus main body  1  widely and to attach/detach the transfer belt unit  5  into/from this opened part. At the same time, it is possible to remove paper easily at the time of paper jamming. 
     The following is a detailed description of a positioning mechanism and a driving mechanism for performing precise color alignment of all colors in the image forming position  10 , with reference to FIGS. 9 and 10. 
     FIG. 9 is an exploded perspective view showing a positioning mechanism of a carriage and a photosensitive member and a driving mechanism in one embodiment according to the present invention. FIG. 10 is a cross-sectional view showing a carriage taken on a line passing the image forming position of the image forming apparatus in one embodiment according to the present invention. 
     As shown in FIGS. 9 and 10, flanges  41 R and  41 L having taper holes  48 R and  48 L for positioning the image forming unit  3  are adhered to the both ends of the photosensitive member  30 . The outer peripheries of the flanges  41 R and  41 L are attached rotatably to photosensitive member bearings  43 R and  43 L fixed to the side wall of the photosensitive member unit  37 . The tip of the flange  41 R is provided with a coupling tongue  47  for rotating the photosensitive member  30 . The coupling tongue  47  can engage a coupling plate  61  of the main body side. 
     The carriage  2  has a right wall  20 R and a left wall  20 L, which are fixed to the cylindrical drum  21  provided in the center of the carriage  2 . Partition ribs  23  for partitioning the carriage  2  into four sections are fixed to the right and left walls  20 R and  20 L. An image forming unit  3  for each color is installed in each space in the carriage  2 , which is partitioned with the partition ribs  23 . The cylindrical drum  21  has a total of four exposure windows  97  at positions corresponding to the positions where the pixel laser signal light  8  for exposing the photosensitive member passes through. The carriage  2  is supported rotatably by right and left side walls  1 R and  1 L of the apparatus main body via bearings  46 R and  46 L. 
     Numeral  28  denotes a carriage gear, which is integrally formed on the outside of the left wall  20 L, and connected to a carriage driving mechanism  86  provided at the side of the main body. The carriage driving mechanism  86  includes a worm  89  connected to a driving source (not shown), a worm wheel  88 , and a gear  87  that is formed into one piece with the worm wheel  88  and meshes with the carriage gear  28 . The carriage  2  is positioned at the rotation position freely by means of the carriage driving mechanism  86 . 
     Numerals  45 R and  45 L denote drop prevention guides for preventing the image forming unit  3  provided at about the lower half along the outer periphery of the carriage  2  from dropping out of the carriage  2 . The drop prevention guides  45 R and  45 L are fixed to the side walls  1 R and  1 L of the main body, respectively. 
     Moreover, the second mirror  98  is attached firmly to the side walls  1 R and  1 L of the main body with a fixing member (not shown) around the center of the cylindrical drum  21  and is fixed in position regardless of the rotation of the carriage  2 . 
     The side walls  20 R and  20 L are provided with cutouts  26 R and  26 L at the portion of the image forming unit  3  into which flange  41 R and  41 L are inserted. The cutouts  26 R and  26 L and the partition ribs  23  serve as a guide when the image forming unit  3  is attached to the inside of the carriage  2 . At the side part or the lower part of the carriage  2 , the image forming unit  3  is guided by the drop prevention guides  45 R and  45 L, so that it is not detached from the carriage  2 . 
     The sizes of the cutouts  26 R and  26 L set to be larger than those of the outer diameter of the flanges  41 R and  41 L, so that there is a play, in all directions, between the photosensitive member  30  and the carriage  2  of regular position, when the photosensitive member  30  is positioned at the image forming position  10  to be in the standard position. In this embodiment, about 1 mm of space is secured. Thereby, even if the positioning of the carriage  2  is not so precise, the positioning operation of the photosensitive member  30  is not affected. 
     A photosensitive member driving mechanism  60  and a detent mechanism  80  are provided at the side walls  1 R and  1 L of the apparatus main body  1  in order to position the photosensitive member  30  precisely at the image forming position  10 . 
     The photosensitive member driving mechanism  60 , which is attached to the right side wall  1 R of the main body, includes an output axis  70 , a coupling plate  61  that is fixed to the output axis  70  and rotates together as one piece with the output axis  70 , an output axis driving gear  71 , and a driving mechanism for driving these elements mentioned above. The output axis  70  is supported, movably in the thrust direction and rotatably, by the bearings  77  fixed to the side right wall  1 R of the main body and a base plate  67 . 
     One end of the output axis  70  has a tip-tapered portion  75 , which has a convex tapered surface corresponding to the tapered hole  48 R of the photosensitive member  30 . The other end of the output axis  70  has a spherical shape so as to be in contact with a thrust bearing  69 R with small area. The output axis driving gear  71  is a helical gear that is fixed to the output axis  70  and meshes with a motor-side gear  72 . Numeral  74  denotes a compression spring, which is inserted between the bearing  77  and the output axis driving gear  71 . This compression spring  74  constantly urges the output axis  70  and the coupling plate  61  toward the position that is separated from the flange  41 R of the photosensitive member  30  (FIG. 10 shows a state in which the coupling plate  61  engages the flange  41 R). 
     The output axis  70  can move against the spring force by a driving means (not shown) that moves the thrust bearing  69 R, between a position separated from the flange  41 R and a position where the taper hole  48 R shown FIG. 10 engages the tip-tapered portion  75 . The motor-side gear  72  has a sufficient tooth width so that the output axis driving gear  71  meshes with the motor-side gear  72  in any position. When the output axis  70  moves in the thrust direction, the output axis driving gear  71  and the motor-side gear  72  slide against each other on the tooth surfaces. 
     Numeral  61  is a coupling plate, which meshes with the coupling tongue  47  of the flange  41 R so as to transmit the power, and has eight coupling tongues  65  on its tip end. 
     Next, the following is a description of the detent mechanism  80 , which is attached to the left side wall IL of the main body. 
     The detent mechanism  80  is supported between a left side wall  1 L of the main body and the base plate  68  via the bearing  78 . The detent mechanism  80  includes a detent axis  81  capable of moving in the thrust direction and rotating by a driving mechanism (not shown) moving a thrust bearing  69 L and a compression spring  85 . One end of the detent axis  81  has a convex tapered surface  84  corresponding to the tapered hole  48 L of the flange  41 L. The other end of the detent axis  81  has a spherical shape, similar to the output axis  70 , and pressed onto the thrust bearing  69 L. Numeral  85  denotes a compression spring, which is inserted between the left side wall  1 L of the main body and a spring stopper  82  fixed to the detent axis  81 . The compression spring  85  keeps the detent axis  81  separated from the flange  4 L. 
     With the above-mentioned configuration, when the image forming unit  3  supported by the carriage  2  is carried to the image forming position  10  and stops while the output axis  70  and the detent axis  81  are separated from the flanges  41 R and  41 L of the photosensitive member  30 , the thrust bearings  69 R and  69 L are pushed inwardly by the driving mechanism, respectively. Consequently, the tip-tapered portion  75  of the output axis  70  engages the taper hole  48 R of the flange  41 R, and a taper surface  84  of the detent axis  81  engages the taper hole  48 L of the flange  41 L, respectively. Thus, the photosensitive member  30  is positioned precisely at the image forming position  10 . When the output axis driving gear  71  is rotated with the output axis  70  and the detent axis  81  pressed inwardly, since the coupling tongue  65  of the coupling plate  61  engages the coupling tongue  47  of the flange  41 R, the rotation of the output axis  70  is transmitted to the photosensitive member  30 , whereby the photosensitive member  30 .is rotated at the image forming position  10 . At this time, all of the photosensitive member  30  supported by the output axis  70  and the detent axis  81 , the second mirror  98  and the laser exposure device  6  are positioned in the apparatus main body  1 . Therefore, the photosensitive member  30  can be positioned precisely regardless of the position of the carriage  2 . As a result, even if the image forming units  3  are switched, the photosensitive member  30  can be positioned precisely, inhibiting an occurrence of the color misalignment. 
     The following is a description of a configuration of the image forming unit  3  and a driving mechanism of the developing roller  31 , with reference to FIGS. 6 and 10 to  12 . 
     FIG. 11 is a cross-sectional view showing a rocking lever provided in a developing unit in one embodiment of the present invention. FIG. 12 is a right side view of FIG.  2 . 
     First, the following is a description of how the photosensitive member unit  37  is connected to the developing unit  35  in the image forming unit  3 . 
     As shown in FIGS. 6,  10  and  12 , the right side wall  37 R of the photosensitive member unit  37  is provided with a protruding portion  91  to which a supporting pin  90  is press-fitted. Furthermore, a guide pin  108  is press-fitted to the left side wall  37 L of the photosensitive member unit  37 . Numeral  115  denotes guide grooves, which are formed on the left side wall  37 L of the photosensitive member unit  37 . These guide grooves  115  guide the developing roller  31  in the direction of the photosensitive member  30 . The developing unit  35  is placed inside of the both side walls  37 R and  37 L of the photosensitive member unit  37 . At the right side wall  37 R of the photosensitive member unit  37 , the supporting pin  90  is inserted into the positioning hole  116  so as to be supported rotatably. 
     As shown in FIGS. 6 and 10 to  12 , the developing roller  31  is supported rotatably by the bearings  105  and  106 , which are fixed respectively to the right side wall  35 R and the left side wall  35 L of the developing unit  35 . The bearing  106  is fitted into the guide groove  115  at the side of the left side wall  37 L of the photosensitive member unit  37 . The left side of the developing unit  35  can move along the guide groove  115  together as one piece with the developing unit  35  with respect to the photosensitive member unit  37 . The guide pin  108  is inserted into a hole  107  provided on the left side wall  35 L of the developing unit  35 . However, the guide pin  108  is arranged with about 1 mm of play with respect to the hole  107 , and so the positioning is not performed by this insertion. 
     The photosensitive member unit  37  and the developing unit  35  are combined in the above-mentioned configuration. The right side wall  37 R of the photosensitive member unit  37  and the right side wall  35 R of the developing unit  35  are rotated, with the supporting pin  90  as a center, by means of the compression spring  102  suspended between them, and thereby the developing roller  31  stops with the developing roller  31  pressed by the photosensitive member  30 . Furthermore, the left side wall  37 L of the photosensitive member unit  37  and the left side wall  35 L of the developing unit  35  stop in a state in which the developing roller  31  is attracted to the side of the photosensitive member  30  with a tension coil spring  110  suspended between the bearing  106  of the developing roller  31  and a pin  111  provided at the left side wall  37 L of the photosensitive unit  37 , and thus the developing roller  31  is pressed onto the photosensitive member  30 . 
     As mentioned above, in this embodiment, since the photosensitive member  30  of the image forming unit  3  is in contact with the developing roller  31  at three points, i.e. at the supporting pin  90 , at the right side of the developing roller  31 , and at the left side of the developing roller  31 , it is possible to bring the photosensitive member  30  into contact with the developing roller  31  uniformly and at a certain pressure without being affected by the size accuracy of the photosensitive unit  37  or the developing unit  35 . 
     The following is a description of the driving mechanism of the developing roller  31 . 
     As shown in FIGS. 6 and 10 to  11 , the developing roller  31  is driven to be rotated via a developing driving main body gear  62 , a rocking gear  94 , and a developing roller gear  96  fixed to a developing roller  31 . The developing driving main body gear  62  is fixed to a developing driving axis  63  attached rotatably to the right side  1 R of the main body and a base plate  67  via a bearing  66 . The developing driving axis  63  is rotated from the side of the main body via the developing driving pulley  64  fixed to the developing driving axis  63 . Numeral  99  is a gear fixed to the feeding roller  21 . This gear  99  is driven to be rotated via a developing roller gear  96  and an idler gear  214 . 
     As mentioned above, since the driving force for rotating the developing roller  31  is applied from the outside (main body side) of the image forming unit  3 , the photosensitive member  30  is not subjected to a load fluctuation as the photosensitive member  30  drives the developing roller  31 . Moreover, since a contact pressure of the developing roller  31  to the photosensitive member  30  can be set to be small, the photosensitive member  30  is not susceptible to the disturbance from the developing roller  31 , whereby the stable rotation can be secured. 
     As shown in FIGS. 6 and 11, the rocking gear  94  is attached rotatably to the rocking lever  92  via a pin  93 . The rocking lever  92  is attached rockably to the bearing  105  of the right side wall  35 L of the developing unit  35  and the developing roller axis  215  with the developing roller axis  215  as a center. The tension spring  104  is suspended to the rocking lever  92 , whereby the rocking gear  94  is energized to the side of the developing driving main body gear  62 . However, the bottom face of the rocking lever  92  is in contact with the circular protruding potion  91  around the supporting pin  90  of the right side wall  37 R of the photosensitive member unit  37 . Moreover, the rocking lever  92  may be directly in contact with the supporting pin  90 . With such a configuration in which the rocking lever  92  is received by the supporting pin  90  or a circular protruding portion  91 , the carriage  2  at the main body side supports the image forming unit  3  at the portion where an external force is applied to the developing unit  35 . Therefore, an excess torsional power is not applied to the image forming unit  3 . 
     When the carriage  2  to which the image forming unit  3  is installed is driven to be rotated, the tooth tips of the developing driving main body gear  62  and the rocking gear  94  are hitting each other, the rocking lever  92  is rotated against the force of the tension spring  104 , and thus the rocking gear  94  is separated away from the developing driving main gear  62 . When the image forming unit  3  reaches the image forming position  10 , the rocking gear  94  reaches the developing driving main body gear  62 . However, the rocking lever  92  is in contact with the circular protruding portion  91  to be stopped. Therefore, the length between the center of the rocking gear  94  and that of the developing driving main body gear  62  is secured correctly, whereby the developing roller  31  is rotated in contact with the photosensitive member  30 . 
     Furthermore, as shown in FIG. 6, the cleaning member main body  211  of the cleaning member  209  for cleaning the inner wall of the toner detection case  205  is rotated via a developing driving main body gear  62 , the rocking gear  94 , the developing roller gear  96  and the idler gear  214 . That is, when the photosensitive member  30  of the image forming unit  3  is positioned at the image forming position  10 , and the rocking gear  94  meshes with the developing driving main body gear  62 , the cleaning member main body  211  is driven to be rotated. Therefore, the cleaning of the toner detection case  205  is performed only at the image forming position  10 . The cleaning of the toner detection case  205  is not performed at the position for detecting the amount of remaining toner. In this way, at the position for detecting the amount of remaining toner, the cleaning member main body  211  is not driven to be rotated, and the detecting light is not shielded by the cleaning member main body  211 . Therefore, it is possible to detect the amount of remaining toner precisely. Furthermore, by performing the cleaning of the toner detection case  205  only at the image forming position  10 , it is not necessary to provide the mechanism for rotating the cleaning member main body  211  in addition to the image forming position  10 , thus simplifying the structure of the apparatus. Moreover, a member for stirring toner inside the toner hopper  39  is not provided in the image forming unit of this embodiment, there may arise a case where the amount of remaining toner cannot be detected precisely. However, during the several times of rotation of the carriage  2 , the amount of remaining toner in the image forming unit  3  of one color is detected several times and the detected results are obtained, and the amount of the remaining toner is determined based on the detected results. Thus, it is possible to reduce the detection error and to detect the amount of remaining toner accurately. 
     The following is a description of a driving principle of stably driving the photosensitive member  30  and the developing roller  31  with reference to FIG.  13 . 
     FIG. 13 is a view to explain how the photosensitive member  30  and the developing roller  31  are driven in one embodiment according to the present invention. 
     In FIG. 13, numeral  96  denotes a developing roller gear,  94  denotes a rocking gear,  62  denotes a developing driving main body gear, and  20 R denotes a portion protruding towards the inside of the right side wall of the carriage  2 . Numeral  90  denotes a supporting pin connecting the developing unit  35  and the photosensitive member unit  37 , and  91  denotes a circular protruding portion provided at the right side wall  37 R of the photosensitive member unit  37 . The circular protruding portion  91  and the rocking gear  94  enter a cutout  29  of the right side wall  20 R of the carriage  2 . The rocking gear  94  meshes with the developing driving main body gear  62 . Therefore, when the image forming unit  3  swings with the photosensitive member  30  as a center, the circular protruding portion  91  is in contact with the side face  27  of the cutout  29 . 
     In the configuration mentioned above, when the photosensitive member  30  is coupled to the output axis  70  and the detent axis  81  to be positioned at the regular position and the developing driving main body gear  62  is rotated, the image forming unit  3  is supported at the both ends of central axis of the photosensitive member  30  from the apparatus main body  1 . In this state, when the photosensitive member  30  and developing driving main body gear  62  are rotated, the image forming unit  3  is subjected to counterclockwise rotation moment with a central axis of the supported photosensitive member  30  as a center. This rotation moment can be stopped when the circular protruding portion  91  is in contact with the side face  27  of the cutout  29  of the carriage side wall  20 R. That is, when the image forming unit  3  performs an image forming operation at the image forming position  10 , the image forming unit  3  is supported at three points, i.e. at both ends of the central axis of the photosensitive member  30 , and at the circular protruding portion  91  of the right side wall  37 R of the photosensitive member unit  37 , from the apparatus main body  1 . Therefore, it is possible to perform a secure positioning of the photosensitive member  30  with respect to the apparatus main body  1 . And at the same time, the image forming unit  3  is twisted due to the developing driving force, thus solving the problem that it is difficult to bring the photosensitive member  30  into contact with the developing roller  31  reliably. Furthermore, since the circular protruding portion  91  is provided at the right side wall  37 R of the photosensitive member unit  37  and the rotation power of the entire image forming unit  3  is supported only by the photosensitive member unit  37 , the contact condition between the photosensitive member  30  and the developing roller  31  is not affected. 
     Furthermore, the rocking gear  94  and the developing main body gear  62  are meshed with each other at approximately the center of the supporting pin  90  that is a rotation supporting point of the developing unit  35  with respect to a photosensitive member unit  37 . Therefore, the developing unit  35  is provided with a force by the developing driving main body gear  62  in the direction illustrated by the arrow P, which is a direction of the pressure angle of both gears. However, this force does not generate the rotation moment, with respect to the photosensitive member unit  37 , with the supporting pin  90  of the developing unit  35  as a center. Further, the pressing power of the developing roller  31  and the photosensitive member  30  is not generated due to the driving force by which the developing roller  31  is driven from the apparatus main body  1 . 
     According to the above-mentioned configuration, the pressing force of the developing roller  31  to the photosensitive member  30  is only the initially setting spring force Q (pressing force by the compression spring  102 ) and a tension force of the tension coil spring  110  provided at the left side wall  37 L of the photosensitive member unit  37 , even when the developing roller  31  is rotating. Therefore, it is possible to bring the photosensitive member  30  into contact with the developing roller  31  with a small force stably. Moreover, even if the direction of force applied from the developing driving main body gear  62  to the developing unit  35  is somewhat changed because the meshing between the rocking gear  94  and the developing driving main body gear  62  is changed, the pressing force between the developing roller  31  and the photosensitive member  30  is not changed. In particular, like in a one-component developing process, if it is necessary to bring the developing roller into contact with photosensitive member uniformly and all over the surface of the photosensitive member with weakest possible force, the present invention is effective in realizing an apparatus with a simple structure and inexpensive and high performance apparatus. 
     Furthermore, the photosensitive member unit  37  receives the rotation moment around the photosensitive member  30  (the rotation moment of the photosensitive member  30  and the moment due to the force, illustrated by the arrow P, applied from the developing driving main body gear  62 ) applied to the image forming unit  3  from the apparatus main body  1  at the side face  27  of the cutout  29  of the carriage  2 . Therefore, there is no problem that the developing unit  35  is pressed by the carriage  2  to change the pressing power of the developing roller  31  with respect to the photosensitive member  30 . Furthermore, similarly, since the rotation moment around the photosensitive member  30 , which is applied from the outside, is applied at substantially the same position as the position in which the force is applied from the outside (at the outer periphery of the circular protruding portion  91 ), the repulsive force is not applied to the surface of the right and left taper holes  48 R and  48 L of the photosensitive member  30 , which are positioned at the predetermined position. Accordingly, the precise positioning of the photosensitive member  30  is not inhibited. 
     Next, the following is a description of the image forming process by using the image forming apparatus having the above-mentioned structure. 
     When the transfer belt unit  5  and image forming units  3  for all colors are installed in their predetermined locations, the power for the apparatus main body  1  is turned on, and the fixing device  15  is heated up, while the polygon mirror  6 A of the laser exposing device  6  starts to be rotated, thus completing the preparations. Moreover, right after the power is turned on, the initialization mode is operated for adjusting the state of the photosensitive member  30  and the intermediate transfer belt  50 . 
     After these preparations are completed, first, an image formation by the image forming unit  3 Y for yellow is positioned at the image forming position  10 . Then, the photosensitive member  30  for yellow starts to be rotated at the image forming position  10 , and at the same time, the developing roller  31 , a corona charger  34  and an intermediate transfer belt  50  start to move. The driving pulley  55 A is driven from the apparatus main body  1 , and friction forces rotate the intermediate transfer belt  50  in the arrow direction. Herein, a peripheral speed of the photosensitive member  30  and that of the intermediate transfer belt  50  are set to be substantially the same. Furthermore, at this time, the secondary transfer roller  9  and the cleaner  51  are separated from the intermediate transfer belt  50 . 
     In accordance with the timing with which a portion that is negatively charged homogeneously by the corona charger  34  on the surface of the photosensitive member  30 , a detection means (not shown) detects the top position of the intermediate transfer belt  5 . In synchronization with this detected signal, the photosensitive member  30  is irradiated with a pixel laser signal beam  8  from the laser exposing device  6 , forming the electrostatic latent image on the photosensitive member  30 . The thus formed latent images sequentially are developed by the developing unit  35  so as to form into toner images. Next, the toner images formed on the photosensitive member  30  are moved toward the primary transfer position while being in contact with the intermediate transfer belt  50 , and transcribed sequentially on the intermediate transfer belt  50 . The yellow image forming operation is completed after the end of the image is transferred onto the intermediate transfer belt  50 , and the photosensitive member  30  and the intermediate transfer belt  50  stop at the initial position. 
     Moreover, at the time of image formation, the photosensitive member  30  is charged to −450V by the corona charger  34 . The exposure potential of the photosensitive member  30  becomes −50V. Furthermore, a DC voltage of −250V is applied to the developing roller  31 . Furthermore, a DC voltage of −1.0 kV is applied to the guide pulley  55 C and the tension pulley  55 D of the intermediate transfer belt  50 . 
     Yellow image forming is completed and the operation of the photosensitive member  30  and the intermediate transfer belt  50  stops, the engagement between the yellow photosensitive member  30  and the driving source of the apparatus main body  1  is released, and then the carriage  2  rotates 90° in the arrow direction. This moves the yellow image forming unit  3 Y away from the image forming position  10 , and the next, magenta image forming unit  3 M is positioned and stops in the image forming position  10 . When the magenta image forming unit  3 M stops in the image forming position  10 , the driving source of the apparatus main body  1  engages the photosensitive member  30 , and the image forming unit  3 M and the transfer belt unit  5  start to operate, and an image forming operation is performed, similarly as for yellow. Thus, a magenta toner image is formed overlapping a yellow toner image on the intermediate transfer belt  50 . 
     Thus, sequential switching operations and image forming operations are repeated for cyan and black, so that four toner images are formed on the intermediate transfer belt  50 . 
     When the top of the black toner image, transferred by primary transfer, comes to the position of the secondary transfer roller  9 , the secondary transfer roller  9  is moved. Then, recording paper, which is fed from the paper feed unit  12 , is sandwiched and conveyed between the secondary transfer roller  9  and the intermediate transfer belt  50 , and the four-color toner image is transferred in one batch onto the recording paper. During this time, a voltage of +800V is applied to the secondary transfer roller  9 . The toner image transferred onto the recording paper is fixed on the recording paper by passing a fixing device  15 , and is ejected out of the apparatus with the paper eject rollers  18 . 
     Any toner remaining on the intermediate transfer belt  50  is scraped off by the cleaning blade  53  that is brought into contact with the intermediate transfer belt  50 . A screw  52  collects the scraped-off toner into the waste toner case  57 . 
     When the secondary transfer is finished, the intermediate transfer belt  50  and the image forming unit  3  stop again, and the carriage  2  rotates 90°. Then, the yellow image forming unit  3 Y reaches the image forming position  10 , thus completing the preparations for the color image forming operation of the next color. 
     In the above-mentioned embodiment, the driving force of the developing unit  35  is applied directly from the apparatus main body  1 , but there is no necessary limitation to this configuration. For example, driving force may be applied from the photosensitive member  30 . 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.