Abstract:
An image forming apparatus, includes: a plurality of image carriers each carrying a developer image; a primary transfer belt opposed to the plurality of image carriers and configured to carry a multicolor developer image; a secondary transfer unit opposed to the primary transfer belt and configured to transfer the multicolor developer image carried on the primary transfer belt onto a transfer medium; and a fixing unit configured to fix the multicolor developer image transferred by the secondary transfer unit on the transfer medium. The primary transfer belt is provided along a substantial vertical direction. The secondary transfer unit is provided on the opposite side of the image carrier on the primary transfer belt. The fixing unit is provided above the secondary transfer unit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a multi color image forming apparatus such as a color laser printer and an image making device used therein.  
           [0003]    2. Background Art  
           [0004]    As a color laser printer based on the electrophotography, a so-called tandem color laser printer is well known in which the photosensitive drums are provided corresponding to the toners of yellow, magenta, cyan and black color. In such tandem color laser printer, a toner image for each color is formed on each photosensitive drum at the almost same time, so that the color image is formed at the almost same speed as the monochrome printer.  
           [0005]    However, in the tandem color laser printer, the arrangement of fixing units for fixing a color toner image transferred onto the paper is needed to examine to reduce the size of apparatus.  
           [0006]    That is, for example, in a direct transfer method in which a toner image for each color formed on each photosensitive drum is directly transferred onto the paper, the fixing units are usually provided sideways on the downstream side of a paper conveying belt that is opposed to the photosensitive drums disposed in parallel. However, there is the drawback that the image forming apparatus is very long in its parallel direction, because in addition to the length of the photosensitive drums disposed in parallel, the fixing units are provided in the parallel direction.  
           [0007]    On the other hand, in an intermediate transfer method in which a toner image for each color formed on each photosensitive drum is once transferred onto an intermediate transfer belt to form a color toner image, and the color toner image formed on the intermediate transfer belt is transferred onto the paper by a secondary transfer roller, the photosensitive drums are disposed in parallel above the intermediate transfer belt in the almost horizontal direction, and the fixing units are disposed beneath the intermediate transfer belt, thereby preventing the image forming apparatus from being longer in the parallel direction of the photosensitive drums, as described in JP-A-2001-272833.  
           [0008]    By the way, when the photosensitive member is charged by discharge of a charging wire in the charger of the multicolor image forming apparatus, a discharge product (mainly ozone) is produced. When this ozone adheres to the surface of the photosensitive member, an unevenness of charge occurs on the photosensitive member, and has some adverse influence such as an image drift on the image formed in the multicolor image forming apparatus.  
           [0009]    Therefore, the multicolor image forming apparatus of this type may be provided with a duct for exhausting ozone to exhaust ozone from the side face of the multicolor image forming apparatus to the outside (e.g., refer to JP-A-2002-196635).  
         SUMMARY OF THE INVENTION  
         [0010]    However, when the fixing units are disposed beneath the intermediate transfer belt, an exhaust heat or water vapor produced from the fixing units has influence on the intermediate transfer belt and the photosensitive drums, whereby there is a risk of causing an image formation failure, as described in JP-A-2001-272833.  
           [0011]    Also, in the intermediate transfer method, if the paper and the intermediate transfer belt are closely located after the secondary transfer, or the distance from the secondary transfer roller to the fixing units is too long, the paper sticks to the intermediate transfer belt and is not peeled, after the secondary transfer, especially when the paper is very thin, resulting in a paper jam, unstable conveyance of paper, or a disordered image.  
           [0012]    On the other hand, in the tandem multicolor image forming apparatus, the photosensitive members, the chargers, and exposing unit are provided corresponding to the number of colors (e.g., each four for four colors), whereby the apparatus is likely to become larger in size. Therefore, the configuration of the components of the multicolor image forming apparatus may be reexamined to reduce the size of the apparatus.  
           [0013]    However, when the duct for exhausting ozone is provided within the apparatus as conventionally, a space for disposing this duct is required, whereby it is difficult to make the apparatus smaller.  
           [0014]    An image forming apparatus for forming the high quality image is disclosed herein, in which the apparatus is made smaller in size and the transfer medium is stably conveyed.  
           [0015]    A multicolor image forming apparatus is also disclosed herein, which is capable of exhausting ozone produced within the multicolor image forming apparatus more efficiently by providing the ventilating duct without increasing the size of the multicolor image forming apparatus.  
           [0016]    According to an aspect of the invention, an image forming apparatus, includes: a plurality of image carriers each carrying a developer image formed by developing an electrostatic latent image with a developer and each provided for each color; a primary transfer belt opposed to the plurality of image carriers and configured to carry a multicolor developer image formed by transferring the developer image for each color that is carried on the image carrier; a secondary transfer unit opposed to the primary transfer belt and configured to transfer the multicolor developer image carried on the primary transfer belt onto a transfer medium; and a fixing unit configured to fix the multicolor developer image transferred by the secondary transfer unit on the transfer medium. The primary transfer belt is provided along a substantial vertical direction. The secondary transfer unit is provided on the opposite side of the image carrier on the primary transfer belt. The fixing unit is provided above the secondary transfer unit.  
           [0017]    With such a configuration, the secondary transfer unit is disposed on the opposite side of each image carrier on the primary transfer belt, and the fixing unit is provided above the secondary transfer unit, whereby the apparatus is reduced and the installation area is saved, so that exhaust heat or water vapor produced from the fixing unit has less influence on the primary transfer belt or each image carrier. Since the secondary transfer unit and the fixing unit are disposed in close proximity, the transfer medium is stably conveyed to form the high quality image.  
           [0018]    According to another aspect of the invention, an image forming apparatus includes: a plurality of image carriers each configured to carry a developer image formed by developing an electrostatic latent image with a developer and each provided for each color; a primary transfer belt opposed to the plurality of image carriers and configured to carry a multicolor developer image formed by transferring the developer image for each color that is carried on the image carrier; and a secondary transfer unit opposed to the primary transfer belt and configured to transfer the multicolor developer image carried on the primary transfer belt onto a transfer medium. The primary transfer belt moves on a path having a concave portion disposed downstream of an opposing portion where the primary transfer belt opposes to the secondary transfer unit. The concave portion is provided with a cleaning member that cleans the primary transfer belt.  
           [0019]    With such a configuration, owing to the concave portion, the angle made between a direction in which the transfer medium after the secondary transfer goes and a direction in which the downstream portion of the primary transfer belt from the secondary transfer position goes can be increased. Therefore, the transfer medium is unlikely to twine around the primary transfer belt after the secondary transfer. As a result, a paper jam is prevent from occurring due to adherence of the transfer medium after the secondary transfer to the primary transfer belt, whereby the transfer medium is stably conveyed to form the high quality image. Also, since the cleaning member is disposed in the concave portion, the cleaning member of the primary transfer belt also serves as the member for forming the path of the primary transfer belt in the concave portion. Therefore, the number of parts is reduced, and the configuration is simplified, so that the concave portion is securely formed in the primary transfer belt.  
           [0020]    According to another aspect of the invention, a multicolor image forming apparatus includes: a plurality of image making units disposed in parallel in a substantial vertical direction, the image making units being provided for each color, each image making unit including: a photosensitive member having a surface; a charger that uniformly charges the surface of the photosensitive member; and a visible image forming unit that forms a visible image by supplying a developer of a predetermined color on an electrostatic latent image that is formed when the surface of the photosensitive member charged by the charger is exposed to light; a transfer unit that transfers a developer of the visible image formed on the surface of the photosensitive member onto a recording medium; a fixing unit that fixes on the recording medium the developer transferred onto the recording medium; and an exhaust air passage portion including an exhaust air path having an exhaust port provided on an upper face of the multicolor image forming apparatus, the exhaust air path configured to lead upwards air passing in the vicinity of the charger and to exhaust the air through the exhaust port.  
           [0021]    That is, with this configuration, since the air is passed near the charger disposed in parallel in the almost vertical direction, the exhaust air passage portion has a shaped portion extending in the almost vertical direction, so that the air through this shaped portion is exhausted through the exhaust port provided on the upper face of the apparatus.  
           [0022]    Also, the warm air heated in the inside of the apparatus ascends in this exhaust air passage portion, causing the pressure under this warm air to be lower and causing the lower air to move upwards, giving rise to an ascending current (a so-called funnel effect).  
           [0023]    According to another aspect of the invention, a multicolor image forming apparatus for forming a multicolor image on the recording medium, includes: a mainframe;  
           [0024]    a plurality of image making unit disposed in parallel in a substantial vertical direction to form a multicolor image on the recording medium, the image making unit being provided for each color, each image making unit including: a photosensitive member having a surface; a charger that uniformly charges the surface of the photosensitive member; and a visible image forming unit that forms a visible image by supplying a developer of a predetermined color on an electrostatic latent image that is formed when the surface of the photosensitive member charged by the charger is exposed to light; a transverse exhaust air passage portion including a transverse exhaust air path disposed in the neighborhood of the charger to circulate air around the charger for ventilation; a longitudinal exhaust air passage portion including a longitudinal exhaust air path configured to lead air passing through the transverse exhaust air path and to exhaust the air outside, the longitudinal exhaust air passage portion being connected to the transverse exhaust air passage portion; and a unit loadable in and unloadable from the mainframe. The transverse exhaust air passage portion is included in the unit. The unit supports at least one of the plurality of the image making unit.  
           [0025]    Consequently, in the multicolor image forming apparatus, a part of the transverse exhaust air passage portion is shared with the unit, the amount of material composing the apparatus is smaller than when the unit and the transverse exhaust air passage portion are provided separately, whereby the apparatus is reduced. Therefore, the interval between the transverse exhaust air passage portions disposed is narrowed, the length of the longitudinal exhaust air passage portion is shorter, and the resistance to the air flow is reduced, whereby the exhaust efficiency is improved.  
           [0026]    According to another aspect of the invention, an image making device for use in a multicolor image forming apparatus, wherein the multicolor image forming apparatus includes: a mainframe; and a plurality of image making unit disposed in parallel in a substantial vertical direction to form a multicolor image on the recording medium, the image making unit being provided for each color, each image making unit including: a photosensitive member having a surface; a charger that uniformly charges the surface of the photosensitive member; and a visible image forming unit that forms a visible image by supplying a developer of a predetermined color on an electrostatic latent image that is formed when the surface of the photosensitive member charged by the charger is exposed to light; the image making device includes: a transverse exhaust air passage portion including a transverse exhaust air path to be disposed in the neighborhood of the charger to circulate air around the charger for ventilation. The image making device supports at least one of the plurality of image making units. The image making device is loadable in and unloadable from the mainframe.  
           [0027]    Consequently, in the image making device, the transverse exhaust air passage portion does not exist on the way of mounting or demounting the image making unit, and the space for avoiding the transverse exhaust air passage portion is not needed, whereby the multicolor image forming apparatus is reduced. Also, there is no need for providing the complicated way of mounting or demounting the image making unit to avoid the transverse exhaust air passage portion, thereby facilitating the exchange work. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    The present invention may be more readily described with reference to the accompanying drawings:  
         [0029]    [0029]FIG. 1 is a side cross-sectional view of the essence showing a color laser printer as an image forming apparatus according to one embodiment of the present invention (in which a primary transfer belt is formed with a concave portion).  
         [0030]    [0030]FIG. 2 is a side cross-sectional view of the essence showing a color laser printer as an image forming apparatus according to another embodiment of the invention (in which a secondary transfer roller is disposed above a central part of a primary transfer belt in almost vertical direction).  
         [0031]    [0031]FIG. 3 is a schematic side cross-sectional view showing the overall configuration of a color laser printer according to a second embodiment of the present invention.  
         [0032]    [0032]FIG. 4 is a schematic front view showing the overall configuration of the color laser printer according to the second embodiment of the invention.  
         [0033]    [0033]FIG. 5 is a schematic side cross-sectional view showing the overall configuration of a variation of a duct according to a third embodiment of the invention.  
         [0034]    [0034]FIG. 6 is a schematic front view showing the overall configuration of the variation of the duct according to the third embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    The First Preferred Embodiment  
         [0036]    [0036]FIG. 1 is a side cross-sectional view showing the essence of a color laser printer as an image forming apparatus according to one embodiment of the present invention. In FIG. 1, this color laser printer  1  is the tandem color laser printer using an intermediate transfer method, comprising, within a main body casing  2 , a feeder portion  4  for feeding the paper  3  as the transfer medium and an image forming portion  5  for forming the image on the supplied paper  3 .  
         [0037]    The main body casing  2  is provided with an operation panel  6  for operating this color laser printer  1  sideways at an upper portion on one side thereof, and a front cover  7  beneath the operation panel  6 . This front cover  7  is borne rotatably via a hinge  8  on the main body casing  2  at its lower end, and is opened or closed from or on the main body casing  2  by rotation around the hinge  8  as the fulcrum.  
         [0038]    In the following description, it is supposed that the side where the operation panel is provided is the fore side of this color laser printer  1  and the opposite side is the rear side.  
         [0039]    An exhaust paper tray  46  is provided at an upper portion of the main body casing  2 .  
         [0040]    The feeder portion  4  includes, on the bottom portion within the main body casing  2 , a paper feed tray  9  as a storage portion that can be mounted removably, a paper feed roller  10  functioning as a separation unit disposed above one end portion of the paper feed tray  9  (on the side where a secondary transfer roller  15  is disposed), and a resist roller  11  disposed above the paper feed roller  10 .  
         [0041]    Within the paper feed tray  9 , a paper pressing plate  12  is provided the paper feed roller  10  having an end portion opposed to the paper feed roller  10  movable vertically, in which sheets of paper  3  are stacked on the paper pressing plate  12 . The paper pressing plate  12  is urged from the back face by a spring, not shown. The paper  3  at the uppermost level on the paper pressing plate  12  is pressed toward the paper feed roller  10  by the spring and fed one by one by rotation of the paper feed roller  10 .  
         [0042]    The paper  3  fed by the paper feed roller  10  is fed to the resist roller  11 , and resisted by the resist roller  11 . After resist, the paper  3  is fed to a secondary transfer position of the image forming portion  5  (a contact portion between a primary transfer belt  33  and the secondary transfer roller  15 ).  
         [0043]    The image forming portion  5  includes a process portion  13 , an intermediate transfer mechanism portion  14 , the secondary transfer roller  15  functioning as a secondary transfer unit, and a fixing unit  16  as a fixing unit.  
         [0044]    The process portion  13  is provided for each of four colors, and includes a developing cartridge  17  as the developing unit, a photosensitive drum  18  as the image carrier, a Scorotron type charger  19 , a scanner unit  20 , a primary transfer roller  21 , and a drum cleaning roller  22 . Also, the process portions  13  are disposed in parallel in the almost vertical direction with a predetermined spacing apart from each other on the fore side of the main body casing  2 .  
         [0045]    Four developing cartridges  17  are yellow developing cartridge  17 Y, magenta developing cartridge  17 M, cyan developing cartridge  17 C and black developing cartridge  17 K, in which each developing cartridge is attached removably on the process portion  13  by opening or closing the front cover  7 , and has a toner storage portion  23 , a supply roller  24 , a developing roller  25  and a film thickness regulating blade  26 . Also, the developing cartridges  17  are disposed in parallel to overlap each other in the almost vertical direction with a predetermined spacing apart on the fore side of the main body casing  2  in the cross direction.  
         [0046]    In the toner storage portion  23  for each developing cartridge  17 , the toner is filled as the positively charged developer of nonmagnetic one component having the color of yellow for the yellow developing cartridge  17 Y, magenta for the magenta developing cartridge  17 M, cyan for the cyan developing cartridge  17 C or black for the black developing cartridge  17 K.  
         [0047]    More specifically, the toner for each color is polymer toner having the shape of rough sphere produced by polymerization. The polymer toner contains, as a main component, a binding resin produced by copolymerizing styrene monomer, acrylic acid, or acrylic monomer such as alkyl (C1 to C4) acrylate or alkyl (C1 to C4) methacrylate by a well-known polymerization method of suspension polymerization, and a coloring agent, a charging control agent, and wax that are blended into the binding resin to form toner parent particles, with outside additive agents added to enhance the fluidity.  
         [0048]    The coloring agent is provided for each color of yellow, magenta, cyan and black. Also, the charging control agent is a charging control resin, for example, which is produced by copolymerization of an ionic monomer having an ionic functional group such as ammonium salt, and a monomer copolymerizable with ionic monomer, such as styrene monomer or acryl monomer. Also, the outside additive agent may be the powder of metal oxide, such as silica, aluminum oxide, titanium oxide, strontium titanate, cerium oxide, or magnesium oxide, or the inorganic powder such as powder of carbide or powder of metal salt.  
         [0049]    Such polymer toner, which is produced by polymerization, has uniform particle diameter in the shape of sphere, with quite excellent fluidity. Also, such polymer toner has an average particle diameter of 8 to 10 μm, and a glass transition point (Tg) from 60 to 65° C., in which the charging amount and polarity are controlled by the charging control agent.  
         [0050]    And the toner within the toner storage portion  23  is discharged to the supply roller  24  through a toner supply port opened sideways of the tone storage portion  23  by agitation with an agitator  27  provided within the toner storage portion  23 .  
         [0051]    The supply roller  24  is rotatably provided sideways of the toner supply port, and the developing roller  25  is rotatably provided opposite to the supply roller  24 . And the supply roller  24  and the developing roller  25  are contacted with each other in a state where the supply roller  24  is compressed to some extent.  
         [0052]    The supply roller  24  has the roller of conductive sponge member covered around a metallic roller shaft.  
         [0053]    The developing roller  25  has the roller of conductive elastic member of rubber material covered around a metallic roller shaft. More specifically, the developing roller  25  has a two layer structure composed of a roller portion of elastic body made of conductive urethane rubber, silicon rubber or EPDM rubber containing carbon particles, and a coat layer mainly composed of urethane rubber, urethane resin or polyimide resin that is covered on the surface of the roller portion.  
         [0054]    Also, a development bias is applied to this developing roller  25  from a power source, not shown, at the time of development.  
         [0055]    Also, the layer thickness regulating blade  26  is provided in the vicinity of the developing roller  25 . This layer thickness regulating blade  26  has a pressing portion made of insulating silicon rubber in the shape of semi-circular section at the top end portion of a blade main body made from a metallic leaf spring, one end portion of the blade main body being borne by the developing cartridge  17  in the vicinity of the developing roller  25 , in which the pressing portion is provided to exert a pressure on the developing roller  25  due to an elastic force of the blade main body.  
         [0056]    And the toner discharged through the toner supply port is supplied to the developing roller  25  by rotation of the supply roller  24 , and frictionally charged in positive polarity between the supply roller  24  and the developing roller  25 . Furthermore, the toner supplied onto the developing roller  25  enters between the pressing portion of the layer thickness regulating blade  26  and the developing roller  25  along with the rotation of the developing roller  25 , and carried on the developing roller  25  as a thin film of fixed thickness.  
         [0057]    The photosensitive drum  18  is rotatably provided on the rear and obliquely lower side of the developing roller  25  in contact with the developing roller  25 . This photosensitive drum  18  has a drum main body grounded, with its surface being formed of a photosensitive layer of an organic photosensitive member mainly composed of polycarbonate. Also, the photosensitive drums  18  are disposed to overlap each other in almost vertical direction, with a certain spacing apart from each other, at the almost central part of the main body casing  2  in the cross direction.  
         [0058]    The Scorotron type charger  19  is provided beneath the photosensitive drum  18  to be out of contact with the photosensitive drum  18 . This Scorotron type charger  19  is one for positive charging to produce a corona discharge from a charging wire made of tungsten or the like, the surface of photosensitive drum  18  being uniformly charged in positive polarity.  
         [0059]    The scanner unit  20  is provided along the longitudinal direction of the developing cartridge  17  beneath the developing cartridge  17 . Thereby, each developing unit  17  and each scanner unit  20  are disposed alternately in the almost vertical direction. This scanner unit  20  includes a laser radiating portion (not shown), a polygon mirror  28  driven for rotation, the reflecting mirrors  28   a  and  28   b , and the lenses  28   c  and  28   d . A laser beam radiated from the laser radiating portion and based on the image data is transmitted or reflected in the order of the polygon mirror  28 , the lenses  28   c  and  28   d , and the reflecting mirrors  28   a  and  28   b , as indicated by the arrow, and applied on the surface of the photosensitive drum  18  by fast scanning.  
         [0060]    And the toner for each color is exposed and developed in the following way. That is, first of all, the surface of the photosensitive drum  18  is uniformly charged positively by the Scorotron charger  19 , along with the rotation of the photosensitive drum  18 , and then exposed to light emitted from the scanner unit  20 , so that an electrostatic latent image based on image data is formed. Then, when the positively charged toner carried on the developing roller  25  is opposed to and contacted with the photosensitive drum  18 , along with the rotation of the developing roller  25 , the toner is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  18 , namely, a part of the uniformly positively charged surface of the photosensitive drum  18  where there is a lower potential due to exposure from the scanner unit  20 , and selectively carried and developed, thereby achieving the reverse development.  
         [0061]    The primary transfer roller  21  is provided roughly opposite the photosensitive drum  18 , with the primary transfer belt  33  sandwiched, on the downstream side of the developing roller  25  in the rotational direction of the photosensitive drum  18 . This primary transfer roller  21  has the roller made of conductive rubber material covered around a metallic roller shaft, and is rotated following the driving of the photosensitive drum  18 , in which a transfer bias is applied from a power source, not shown, at the time of primary transfer.  
         [0062]    And the monochrome toner image carried on each photosensitive drum  18  is transferred onto the primary transfer belt  33 , while the primary transfer belt  33  passes between the photosensitive drum  18  and the primary transfer roller  21 .  
         [0063]    A drum cleaner  22  is disposed between the primary transfer roller  21  and the Scorotron charger  19  in the rotational direction of the photosensitive drum  18  to withdraw the toner remaining after transfer.  
         [0064]    The intermediate transfer mechanism portion  14  is disposed on the opposite side of each developing cartridge  17  in each photosensitive drum  18  within the main body casing  2 , namely, on the rear side of each photosensitive drum  18 , and includes a first roller  29 , a second roller  30 , a third roller  31 , a cleaner  32  as the cleaning member, and the primary transfer belt  33 .  
         [0065]    The first roller  29  is disposed above the uppermost photosensitive drum  18  (photosensitive drum  18  corresponding to the yellow developing cartridge  17 Y) and obliquely opposed to the uppermost photosensitive drum  18 , with the primary transfer belt  33  sandwiched between them. Also, the second roller  30  is provided under the first roller  29  in the almost vertical direction, and disposed beneath the lowermost photosensitive drum  18  (photosensitive drum  18  corresponding to the black developing cartridge  17 K) and obliquely opposed to the lowermost photosensitive drum  18 , with the primary transfer belt  33  sandwiched between them. The third roller  31  is provided on the rear and obliquely lower side the first roller  29 , and on the rear and obliquely upper side of the second roller  30 , and disposed oppositely between the lowermost photosensitive drum  18  and its adjacent upper (second lowest) photosensitive drum  18  (photosensitive drum  18  corresponding to the cyan developing cartridge  17 C) in the almost horizontal direction.  
         [0066]    Thereby, the first roller  29 , the second roller  30  and the third roller  31  are disposed in the shape of rough triangle projecting backward in a state where the third roller  31  is disposed under a central part of the line segment connecting the first roller  29  and the second roller  30  in the almost vertical direction.  
         [0067]    The cleaner  32  is provided on the rear and obliquely lower side of the first roller  29 , and on the fore and obliquely upper side of the third roller  31  on the line segment connecting the first roller  29  and the third roller  31 , and disposed oppositely between the second lowest photosensitive drum  18  (photosensitive drum  18  corresponding to the cyan developing cartridge  17 C) and its adjacent upper (third lowest) photosensitive drum  18  (photosensitive drum  18  corresponding to the magenta developing cartridge  17 M) in the almost horizontal direction.  
         [0068]    This cleaner  32  includes, within a cleaner casing  34 , a cleaning roller  35 , a withdrawal roller  36 , a withdrawal box  37  and a scraper  38 .  
         [0069]    The cleaner casing  34  has the shape of a box having almost rectangular section that is opened on one side, and is disposed longitudinally along the almost vertical direction, with its opening portion directed downwards, at a position where the cleaner  32  is disposed. Thereby, an opening portion of the cleaner casing  34  on which the cleaning roller  35  is supported is disposed inside a rough triangle formed by the first roller  29 , the second roller  30  and the third roller  31 .  
         [0070]    The cleaning roller  35  has the roller of elastic member made of conductive rubber material covered around a metallic roller shaft, and is disposed to face the path of the primary transfer belt  33  while being partially exposed out of the opening portion of the cleaner casing  34 . This cleaning roller  35  is provided rotatably in the cleaner casing  34 . At the time of cleaning, a cleaning bias is applied from a power source, not shown, to produce a potential difference between the primary transfer belt  33  and it.  
         [0071]    The withdrawal roller  36  is composed of a metallic roller, and provided above the cleaning roller  35  within the cleaner casing  34  to be rotatable in the cleaner casing  34  in a state where it is opposed in contact to the cleaning roller  35 . A withdrawal bias is applied to this withdrawal roller  36  from a power source, not shown, to produce a potential difference between the cleaning roller  35  and it at the time of cleaning.  
         [0072]    The withdrawal box  37  is provided above the withdrawal roller  36  within the cleaner casing  34 , and opened in a portion opposed to the withdrawal roller  36 .  
         [0073]    The scraper  38  is provided in an opening portion of the withdrawal box  37  in a state where it is pressed onto the withdrawal roller  36 .  
         [0074]    The primary transfer belt  33  is a conductive endless belt and wound around the outer circumference of the first roller  29 , the second roller  30  and the third roller  31  with its inner side face being contact with the first roller  29  and the second roller  30  and the third roller  31 , and its outer side face being contact with the cleaning roller  35 .  
         [0075]    Thereby, the path of the primary transfer belt  33  extends between the first roller  29  and the second roller  30  in the almost vertical direction to be oppositely contact with each photosensitive drum  18 , extends between the second roller  30  and the third roller  31  to be rearwards and obliquely upwards from the second roller  30  to the third roller  31 , and extends between the third roller  31  and the first roller  29  upwards from the third roller  31  to the first roller  29  to pass around the outer circumference of the cleaning roller  35  in a state where a concave portion  39  is inwardly depressed in the shape of rough V-character, because the cleaning roller  35  is disposed inside a rough triangle formed by the first roller  29 , the second roller  30  and the third roller  31 .  
         [0076]    And in this intermediate transfer mechanism portion  14 , the second roller  30  is driven, and followed by the first roller  29 , the third roller  31  and the cleaning roller  35 , so that the primary transfer belt  33  is moved around in the arrow direction (clockwise direction), in which the primary transfer belt  33  is moved from upper to lower in the region between the first roller  29  and the second roller  30  opposed to each photosensitive drum  18 .  
         [0077]    In this way, the outer side face of the primary transfer belt  33  is opposed to the photosensitive drums  18  in succession by the movement of the primary transfer belt  33  between the first roller  29  and the second roller  30 , so that the toner image for each color formed on the photosensitive drum  18  is superposed successively on the primary transfer belt  33  to form a color toner image on the primary transfer belt  33 .  
         [0078]    That is, a yellow toner image formed on the photosensitive drum  18  by the yellow toner filled in the yellow developing cartridge  17 Y is primarily transferred onto the primary transfer belt  33 . Then, a magenta toner image formed on the photosensitive drum  18  by the magenta toner filled in the magenta developing cartridge  17 M is primarily transferred onto the primary transfer belt  33  onto which the yellow toner image has been already transferred. And a cyan toner image formed by the cyan toner and a black toner image formed by the black toner are primarily transferred thereon in the same way. Thereby, a color toner image is formed on the primary transfer belt  33 .  
         [0079]    The secondary transfer roller  15  is rotatably provided at a position opposite the third roller  31  of the intermediate transfer mechanism portion  14  in the almost horizontal direction, with the paper  3  sandwiched, on the opposite side (i.e., rear side) of each photosensitive drum  18  on the primary transfer belt  33 . The secondary transfer roller  15  is disposed to overlap the paper feed roller  10  and the resist roller  11  in the almost vertical direction. Therefore, the conveyance distance of the paper  3  laid on the paper feed tray  9  up to the secondary transfer roller  15  is short and roughly linear, whereby a paper jam is less likely to occur. Also, the secondary transfer roller  15  has the roller made of conductive rubber material covered around a metallic roller shaft, in which a transfer bias is applied from a power source, not shown, to the secondary transfer roller  15  at the time of transfer.  
         [0080]    And the color toner image formed on the primary transfer belt  33  is transferred onto the paper  3  by applying a transfer bias to the secondary transfer roller  15 , while the paper  3  is passed between the primary transfer belt  33  and the secondary transfer roller  15  at the secondary transfer position.  
         [0081]    Also, the residual toner remaining on the primary transfer belt  33  after being transferred onto the paper  3  adheres to the cleaning roller  35  by a cleaning bias applied to the cleaning roller  35 , when the residual toner is opposed to the cleaning roller  35 . Thereafter, the residual toner adhering onto the cleaning roller  35  adheres to the withdrawal roller  36  by a withdrawal bias applied to the withdrawal roller  36 , when the residual toner is opposed to the withdrawal roller  36 , and is scraped by the scraper  38  and withdrawn into the withdrawal box  37 .  
         [0082]    In this color laser printer  1 , the cleaner  32  is disposed on the path of the primary transfer belt  33 . The concave portion  39  is formed in a downstream portion Din the movement direction of the primary transfer belt  33  from a contact portion with the secondary transfer roller  15  on the primary transfer belt  33 , so that the angle made between a direction X1 where the paper  3  passed between the secondary transfer roller  15  and the primary transfer roller  33  goes and a direction X2 where the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion of the primary transfer belt  33  with the secondary transfer roller  15  goes is set at 45° or greater (75° in the embodiment of FIG. 1). The direction X2 where the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion of the primary transfer belt  33  with the secondary transfer roller  15  goes indicates the direction where the primary transfer belt  33  is moved between the third roller  31  and the cleaning roller  35 .  
         [0083]    The fixing unit  16  is disposed above the secondary transfer roller  15  to overlap the secondary transfer roller  15  and the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion of the primary transfer belt  33  with the secondary transfer roller  15  in the almost vertical direction, and to overlap the cleaner  32  between the uppermost level (i.e., the top portion of the first roller  29 ) and the lowermost level (i.e., the bottom portion of the second roller  30 ) of the primary transfer belt  33  in the almost vertical direction.  
         [0084]    This fixing unit  16  includes a first heating roller  40 , a second heating roller  41  opposed to the first heating roller  40  in the almost horizontal direction, with the paper  3  sandwiched, and a pair of conveying rollers  42  provided above the first heating roller  40  and the second heating roller  41 .  
         [0085]    The first heating roller  40  has a tungsten halogen lamp for heating within a cylindrical elementary tube made of metal such as aluminum, and is provided with an elastic layer on the outer circumferential face of the elementary tube. The second heating roller  41 , like the primary heating roller  40 , has a tungsten halogen lamp for heating within a cylindrical elementary tube made of metal such as aluminum, and is provided with an elastic layer on the outer circumferential face of the elementary tube to press the first heating roller  40 .  
         [0086]    And the paper  3  fed from the secondary transfer position to the fixing unit  16  has a color toner image thermally fixed by the first heating roller  40  and the second heating roller  41  while passing between them. Then, the paper  3  is conveyed to an exhaust paper path  43  by the pair of conveying rollers  42 .  
         [0087]    The exhaust paper path  43  is provided along the almost vertical direction in the main body casing  2 , in which one pair of conveying rollers  44  are provided to face the exhaust paper path  43 . A pair of paper exhaust rollers  45  are provided in a paper exhaust opening on the exhaust paper path  43 .  
         [0088]    This exhaust paper path  43  and the conveying path of the paper  3  from the paper feed roller  10  through the secondary transfer position to the fixing unit  16  are formed as a roughly straight path along the almost vertical direction.  
         [0089]    And the paper  3  fed onto the exhaust paper path  43  by the conveying rollers  42  in the fixing unit  16  is conveyed between the paper exhaust rollers  45  by the conveying rollers  44 , and then exhausted onto the exhaust paper tray  46  by the paper exhaust rollers  45 .  
         [0090]    In this color laser printer  1 , a fan  47  is provided opposite to the exhaust paper path  43  at an upper portion of the main body casing  2 . Heat and water vapor produced in the fixing unit  16  are expelled outside by the fan  47 , thereby preventing occurrence of a faulty condition such as a great rise in temperature of the apparatus or dewing due to water vapor.  
         [0091]    Since this color laser printer  1  is the so-called tandem type color laser printer in which the photosensitive drum  18  is provided for each color, the color image is formed by transferring the toner image formed for each color successively for each color at the roughly same speed as when the monochrome image is formed.  
         [0092]    In this color laser printer  1 , the secondary transfer roller  15  is disposed opposite the third roller  31 , with the paper  3  sandwiched, on the opposite side of each photosensitive drum  18  on the primary transfer belt  33  in the almost horizontal direction, and the fixing unit  16  is disposed above the secondary transfer roller  15  to overlap the secondary transfer roller  15  and the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion of the primary transfer roller  33  with the secondary transfer roller  15  in the almost vertical direction. Therefore, exhaust heat or water vapor produced from the fixing unit  16  has less influence on the primary transfer belt  33  or each photosensitive drum  18 , and owing to overlapping of each portion in the almost vertical direction, the apparatus is reduced in the almost horizontal direction and the installation area is saved. Also, the secondary transfer roller  15  and the fixing unit  16  are disposed in proximity, whereby the paper  3  is stably conveyed to form the high quality image.  
         [0093]    Also, this color laser printer  1 , the fixing unit  16  is disposed between the uppermost and lowermost levels of the primary transfer belt  33  in the almost vertical direction to overlap the cleaner  32 , whereby the apparatus has a smaller size in the almost vertical direction.  
         [0094]    Also, in this color laser printer  1 , the cleaner  32  is disposed on the path of the primary transfer belt  33 , and the concave portion  39  is formed in the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion of the primary transfer belt  33  with the secondary transfer roller  15 , so that the angle made between direction X1 in which the paper  3  passed between the secondary transfer roller  15  and the primary transfer belt  33  goes and direction X2 in which the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion of the primary transfer belt  33  with the secondary transfer roller  15  is moved is set at 45° or greater. Therefore, after the secondary transfer, the paper  3  is unlikely to twine around the primary transfer belt  33 , preventing a paper jam from occurring due to adherence of the paper  3  to the primary transfer belt  33  after the secondary transfer, whereby the paper  3  is stably conveyed to form the high quality image.  
         [0095]    In this arrangement, the roller diameter of the third roller  31  is preferably set in a range from 20 mm to 30 mm, for example. If the roller diameter falls in this range, it is possible to effectively prevent the paper  3  from twining around the primary transfer belt  33 .  
         [0096]    In this way, the cleaner  32  is disposed by forming the concave portion  39  in the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion with the secondary transfer roller  15  on the path of the primary transfer belt  33 , so that the angle between direction X1 in which the paper  3  goes after the secondary transfer and direction X2 in which the downstream portion D from the secondary transfer position on the primary transfer belt  33  goes is securely kept by the concave portion  39 . Therefore, the paper  3  is conveyed more stably while preventing a paper jam from occurring. Also, since the cleaner  32  is disposed in a depressed portion above this concave portion  39 , the apparatus is made smaller in the almost horizontal direction, making it possible to save more space of the installation area.  
         [0097]    And the cleaning roller  35  of the cleaner  32  can serve as a cleaning member of the primary transfer belt  33  and a member for forming the path of the primary transfer belt  33  in the concave portion  39 . Therefore, the number of parts is reduced, and the constitution is simplified, whereby the concave portion  39  is securely formed in the primary transfer belt  33 .  
         [0098]    Also, in this color laser printer  1 , the photosensitive drum  18  and the developing cartridge  17  corresponding to it are provided to overlap each other in the almost vertical direction, whereby the apparatus is made smaller in the almost horizontal direction, making it possible to save more space of the installation area.  
         [0099]    Also, in the embodiment as shown in FIG. 1, the cleaner  32  is disposed by forming the concave portion  39  in the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion with the secondary transfer roller  15  on the path of the primary transfer belt  33 , so that the angle made between direction X1 in which the paper  3  goes after the secondary transfer and direction X2 in which the downstream portion D from the secondary transfer position on the primary transfer belt  33  goes is set at 45° or greater. However, in the embodiment as shown in FIG. 2, for example, the secondary transfer roller  15  may be provided above the central part of the primary transfer belt  33  in the almost vertical direction, so that the angle made between direction X1 in which paper  3  goes after the secondary transfer and direction X2 in which the downstream portion D from the secondary transfer position on the primary transfer belt  33  goes may be set at 45° or greater.  
         [0100]    That is, in FIG. 2, the third roller  31  and the cleaner  32  of the intermediate transfer mechanism portion  14 , the secondary transfer roller  15 , and the fixing unit  16  are disposed in the following way. In FIG. 2, the apparatus constitution is almost the same as in the embodiment of FIG. 1, except for the constitution described below, and the duplicate description of the constitution is omitted.  
         [0101]    In FIG. 2, the third roller  31  is disposed on the rear and obliquely lower side of the first roller  29 , and on the rear and obliquely upper side of the second roller  30 , and opposed between the uppermost photosensitive drum  18  (photosensitive drum  18  corresponding to the yellow developing cartridge  17 Y) and its adjacent lower (second uppermost) photosensitive drum  18  (photosensitive drum  18  corresponding to the magenta developing cartridge  17 M) in the almost horizontal direction. Thereby, the first roller  29 , the second roller  30  and the third roller  31  is disposed in the shape of a rough triangle projecting backwards in a state where the third roller  31  is disposed above the central part of the line segment connecting the first roller  29  and the second roller  30  in the almost vertical direction.  
         [0102]    The cleaner  32  is provided at an upper portion between the first roller  29  and the third roller  31 , and opposed to the uppermost photosensitive drum  18  in the almost horizontal direction. Also, the cleaner casing  34  is disposed around the position of the cleaner  32  to have its opening portion outside the rough triangle formed by the first roller  29 , the second roller  30  and the third roller  31  and opposed to the first roller  29 , with the longitudinal direction extending along the line segment connecting between the first roller  29  and the third roller  31 . Also, this cleaner  32  is provided with a cleaning brush  48  within the cleaner casing  34 , instead of the cleaning roller  35 .  
         [0103]    The cleaning brush  48  is formed radially with conductive brushes around a cylindrical main body, which are supported rotatably in the opening portion of the cleaner casing  34  in a state of being partially exposed downwards, and opposed to the primary roller  29 , with the primary transfer belt  33  sandwiched. Also, a cleaning bias is applied from a power source, not shown to this cleaning brush  48  to have a potential difference between the primary transfer belt  33  and it at the time of cleaning in the same way as described above.  
         [0104]    In the embodiment as shown in FIG. 2, the cleaner  32  is disposed outside the rough triangle formed by the first roller  29 , the second roller  30  and the third roller  31 , whereby the primary transfer belt  33  has its inner side face contact with the first roller  29 , the second roller  30  and the third roller  31 , and is wound in the shape of rough triangle without forming the concave portion  39 .  
         [0105]    Also, the secondary transfer roller  15  is disposed opposite to the third roller  31  of the intermediate transfer mechanism portion  14  in the almost horizontal direction, with the paper  3  sandwiched, and disposed above the central part of the line segment connecting the first roller  20  and the second roller  30  in the almost vertical direction.  
         [0106]    Also, the fixing unit  16  is disposed above the secondary transfer roller  15  and the cleaner  32  to overlap the secondary transfer roller  15 , the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion with the secondary transfer roller  15  on the primary transfer belt  33  and the cleaner  32  in the almost vertical direction, and disposed above the uppermost level of the primary transfer belt  33  in the almost horizontal direction.  
         [0107]    In the embodiment as shown in FIG. 2, the secondary transfer roller  15  is disposed above the central part of the main body casing  2  in the vertical direction, whereby a pair of conveying rollers  49  for feeding the paper  3  from the paper feed roller  10  to the resist roller  11  are provided on the conveyance path leading from the paper feed roller  10  to the resist roller  11 , though no conveying roller  44  is provided on the exhaust paper path  43 .  
         [0108]    And in the color laser printer  1  of the embodiment as shown in FIG. 2, the secondary transfer roller  15  is disposed opposite to the third roller  31 , with the paper  3  sandwiched, on the opposite side of each photosensitive drum  18  on the primary transfer belt  33  in the almost horizontal direction, and the fixing unit  16  is disposed above the secondary transfer roller  15  to overlap the secondary transfer roller  15  and the downstream portion D in the movement direction of the primary transfer belt  33  from the contact portion with the secondary transfer roller  15  on the primary transfer belt  33  in the almost vertical direction. Therefore, exhaust heat or water vapor produced from the fixing unit  16  has less influence on the primary transfer belt  33  or the photosensitive drum  16 , and the apparatus is made smaller in the almost horizontal direction and the installation area is saved by overlapping of each portion in the almost vertical direction. Also, the secondary transfer roller  15  and the fixing unit  16  are disposed in proximity, whereby the paper  3  is stably conveyed to form the high quality image.  
         [0109]    Also, in the color laser printer  1  of the embodiment as shown in FIG. 2, the secondary transfer roller  15  is disposed above the central part of the line segment connecting the first roller  29  and the second roller  30  in the almost vertical direction, namely, above the central part of the primary transfer belt  33  in the almost vertical direction, whereby the secondary transfer roller  15  and the fixing unit  16  are disposed in closer proximity. Also, the angle made between direction X1 in which the paper  3  goes after the secondary transfer and direction X2 in which the downstream portion D goes from the secondary transfer position on the primary transfer belt  33  is increased (this angle is from 45° to 60° in this embodiment). Therefore, it is possible to prevent a paper jam from occurring due to adherence of the paper  3  to the primary transfer belt  33  after the secondary transfer, whereby the paper  3  is stably conveyed to form the higher quality image.  
         [0110]    As described above, according to one aspect of the invention, the apparatus is reduced and the installation area is saved, so that exhaust heat or water vapor produced from the fixing unit has less influence on the primary transfer belt or each image carrier. Also, since the secondary transfer unit and the fixing unit are disposed in close proximity, the transfer medium is stably conveyed to form the high quality image.  
         [0111]    According to another aspect of the invention, since the secondary transfer unit and the fixing unit are provided to overlap each other in the almost vertical direction, the installation area is further saved.  
         [0112]    According to another aspect of the invention, since the primary transfer belt and the fixing unit are provided to overlap each other in the almost vertical direction, the installation area is further saved.  
         [0113]    According to another aspect of the invention, a paper jam is prevented from occurring due to adherence of the transfer medium after the secondary transfer to the primary transfer belt, whereby the transfer medium is stably conveyed to form the high quality image.  
         [0114]    According to another aspect of the invention, a paper jam is further prevented from occurring due to adherence of the transfer medium after the secondary transfer to the primary transfer belt, whereby the transfer medium is stably conveyed to form the higher quality image.  
         [0115]    According to another aspect of the invention, a paper jam is prevented from occurring, and the transfer medium is further stably conveyed. Also, the apparatus is reduced in the almost horizontal direction, and the installation area is further saved.  
         [0116]    According to another aspect of the invention, the apparatus is reduced in the almost vertical direction.  
         [0117]    According to another aspect of the invention, the number of parts is reduced, and the configuration is simplified, so that the concave portion is securely formed in the primary transfer belt.  
         [0118]    According to another aspect of the invention, a paper jam is prevented from occurring.  
         [0119]    According to another aspect of the invention, a paper jam is prevented from occurring due to adherence of the transfer medium after the secondary transfer to the primary transfer belt, whereby the transfer medium is stably conveyed to form the high quality image. Also, the number of parts is reduced, and the constitution is simplified, so that the concave portion is securely formed in the primary transfer belt.  
         [0120]    According to another aspect of the invention, the apparatus is reduced in the almost horizontal direction, and the installation area is saved.  
         [0121]    The Second Preferred Embodiment  
         [0122]    A second embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 3 is a schematic cross-sectional view showing the internal configuration of a color laser printer  101  as a multicolor image forming apparatus according to the second embodiment of the invention, as seen from the side. Also, FIG. 4 is a schematic cross-sectional view thereof, as seen from the front.  
         [0123]    The color laser printer  101  includes a visible image forming portion  104 , a paper conveying belt  105 , a fixing unit  108 , a paper feed unit  109 , and an exhaust paper tray  110   a , in which a four color image corresponding to image data input externally is formed on the paper P as the recording medium, as shown in FIG. 3.  
         [0124]    And the visible image forming portion  104  includes, for each visible image forming process with the toner for each color of magenta (M), cyan (C), yellow (Y) and black (Bk), a photosensitive drum  103 M,  103 C,  103 Y or  103 Bk as the photosensitive member, a charger  171 M,  171 C,  171 Y or  171 Bk, an exposing unit  141 M,  141 C,  141 Y or  141 Bk, and a developing unit  151 M,  151 C,  151 Y or  151 Bk. The exposing units  141 M,  141 C,  141 Y and  141 Bk and the developing units  151 M,  151 C,  151 Y and  151 Bk function as a visible image forming unit.  
         [0125]    Each of the above components will be described in detail.  
         [0126]    First of all, the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk is composed of a roughly cylindrical member, and rotatably disposed at almost regular interval in the vertical direction. The roughly cylindrical member of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk has a positively chargeable photosensitive layer formed on an aluminum substrate, for example. And the aluminum substrate is grounded at the ground line in the color laser printer  101 .  
         [0127]    Also, the charger  171 M,  171 C,  171 Y or  171 Bk includes a charging wire  172 M,  172 C,  172 Y or  172 Bk opposite to the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk and extending in its width direction, and a shield case  173 M,  173 C,  173 Y or  173 Bk for storing the charging wire  172 M,  172 C,  172 Y or  172 Bk, the shield case being opened on the side of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk and on its opposite side. The charger positively charges the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk by applying a high voltage to the charging wire  172 M,  172 C,  172 Y or  172 Bk. Also, the shield case  173 M,  173 C,  173 Y or  173 Bk has a structure in which a grid is provided in an opening portion on the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, These chargers are the so-called Scorotron type charger in which a bias voltage is applied to this grid so that the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk may be charged in the nearly same potential as the bias voltage.  
         [0128]    Also, the exposing unit  141 M,  141 C,  141 Y or  141 Bk is disposed in the horizontal transverse direction to the rotation axis of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, in which a laser beam corresponding to one color of image data input externally is emitted from a light source, and applied through an optical system onto the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk by scanning over the specular surface of a polygon mirror driven and rotated by a polygon motor.  
         [0129]    When the exposing unit  141 M,  141 C,  141 Y or  141 Bk applies a laser beam corresponding to the image data onto the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, an electrostatic latent image for each color is formed on the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk.  
         [0130]    The developing unit  151 M,  151 C,  151 Y or  151 Bk includes a developing unit case  155 M,  155 C,  155 Y or  155 Bk for storing the toner and a developing roller  152 M,  152 C,  152 Y or  152 Bk. And the developing unit  151 M,  151 C,  151 Y or  151 Bk charges the toner positively, supplies the charged toner as a uniform thin film to the developing roller  152 M,  152 C,  152 Y or  152 Bk, and develops an electrostatic latent image of positive polarity (positively charged) formed on the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk with the positively charged toner at a contact part between the developing roller  152 M,  152 C,  152 Y or  152 Bk and the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk to form the image by the reversal method.  
         [0131]    The developing roller  152 M,  152 C,  152 Y or  152 Bk is formed in the cylindrical shape from conductive silicone rubber as a base substance, with a coat layer of resin or rubber material containing fluorine formed on the surface.  
         [0132]    Also, the toner stored in the developing unit case  155 M,  155 C,  155 Y or  155 Bk is a positively charged, non-magnetic one component developer, in which the tone of magenta, cyan, yellow or black is stored in the developing unit case  155 M,  155 C,  155 Y or  155 Bk.  
         [0133]    Also, the paper feed unit  109  is provided on the lowermost part of the apparatus, and includes a storage tray  191  for storing the sheets of paper P, and a pickup roller  192  for feeding out the paper P. And the pickup roller  192  picks up the paper P fed out of the paper feed unit  109  and feeds it between a conveying roller  199  and the paper conveying belt  105 .  
         [0134]    Also, the paper conveying belt  105  is wider than the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk and made of a material having a viscosity not to cause the paper P to be slid, and looped between a drive roller  160  and a follower roller  162 , as shown in FIG. 3. The transfer roller  161 M,  161 C,  161 Y or  161 Bk is provided near the position opposite the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk. And the paper conveying belt  105  is moved, along with the rotation of the drive roller  160  and the follower roller  162 , in such a way that the surface opposed to the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk is passed from the lower to upper side in the vertical direction, as shown in FIG. 3. The paper P fed from the conveying roller  199  is conveyed in succession between the paper conveying belt  105  and the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, and passed to the fixing unit  108 .  
         [0135]    Also, the transfer roller  161 M,  161 C,  161 Y or  161 Bk, to which a predetermined voltage is applied, transfers the toner image formed on the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk onto the paper P conveyed by the paper conveying belt  105 .  
         [0136]    Also, the fixing unit  108  is composed of a first heating roller  181  and a second heating roller  182 , and fixes the toner images on the paper P by applying a heat and pressure on the paper P carrying the toner images of four colors, while the paper P is being sandwiched and conveyed by the first heating roller  181  and the second heating roller  182 . Also, an exhaust heat duct  139  as an exhaust heat passage portion for releasing heat when the paper P is heated is provided.  
         [0137]    Also, an upper face cover  110  is provided at the uppermost part of the apparatus, its part constituting an exhaust paper tray  11   a . The exhaust paper tray  11   a  is provided on the exit side of the fixing unit  108  and receives the paper P expelled from the fixing unit  108 .  
         [0138]    Also, an operation portion  111  disposed with the buttons or indicators for setting the image forming conditions and indicating various states is provided on the front face of the upper face cover  110 . Here, it is assumed that the operation portion  111  is disposed on the front face of the color laser printer  101 .  
         [0139]    In the color laser printer  101  with the above constitution according to this embodiment, the operation of forming the image on the paper P is as follows.  
         [0140]    First of all, one paper P is supplied from the paper feed unit  109  by the pickup roller  192 , and fed between the conveying roller  199  and the paper conveying belt  105 .  
         [0141]    Then, the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk uniformly charged by the charger  171 M,  171 C,  171 Y or  171 Bk is exposed by the exposing unit  141 M,  141 C,  141 Y or  141 Bk, corresponding to the image data of magenta, cyan, yellow or black color externally input, so that the electrostatic latent image is formed. Then, the developing unit  151 M,  151 C,  151 Y or  151 Bk develops the electrostatic latent image by depositing the toner of magenta, cyan, yellow or black on the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk. And the transfer roller  161 M,  161 C,  161 Y or  161 Bk transfers the toner image formed in this way onto the surface of the paper P conveyed by the paper conveying belt  105 .  
         [0142]    The toner image for each color is formed with a slight time difference according to the moving speed of the paper conveying belt  105  and the position of each photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, and transferred and superposed onto the paper P conveyed by the paper conveying belt  105 .  
         [0143]    Then, the fixing unit  108  fixes the toner image for each of four colors formed on the paper P, and the paper P is expelled onto the exhaust paper tray  110   a.    
         [0144]    By the way, the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, the charger  171 M,  171 C,  171 Y or  171 Bk, the developing unit  151 M,  151 C,  151 Y or  151 Bk, and a cleaning roller  170 M,  170 Y,  170 Y or  170 Bk are made up as an image making unit  150 M,  150 C,  150 Y or  150 Bk held integrally and detachably on the apparatus for each color.  
         [0145]    And a back panel  128  is inclined, together with the paper conveying belt  105  and the transfer roller  161 M,  161 C,  161 Y or  161 Bk, around a lower edge part of the back panel  128  as the fulcrum, in which the image making unit  150 M,  150 C,  150 Y or  150 Bk can be demounted from the plane where the back panel  128  is inclined, and exchanged.  
         [0146]    On the other hand, when the charger  171 M,  171 C,  171 Y or  171 Bk charges the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk in this embodiment, ozone is produced around the charger by discharge of the charging wire  172 M,  172 C,  172 Y or  172 Bk.  
         [0147]    If ozone adheres to the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, the surface of the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk is unevenly charged, having adverse influence on the electrostatic latent image formed thereon.  
         [0148]    Therefore, to exhaust ozone outside, the color laser printer  101  is provided with a horizontal duct  131 M,  131 C,  131 Y or  131 Bk as the transverse exhaust air passage portion disposed over the charger  171 M,  171 C,  171 Y or  171 Bk and a vertical duct  133  as the longitudinal exhaust air passage portion connected to the horizontal ducts and extending in the almost vertical direction, as shown in FIG. 4.  
         [0149]    The horizontal duct  131 M,  131 C,  131 Y or  131 Bk is composed of a member having the shape of roughly square barrel with an opening on one side, and disposed along the shield case  173 M,  173 C,  173 Y or  173 Bk so that its opening may be opposed to an opening portion on the upper side of the shield case  173 M,  173 C,  173 Y or  173 Bk. And the horizontal duct  131 M,  131 C,  131 Y or  131 Bk has its one end connected to a side opening portion  120 M,  120 C,  120 Y or  120 Bk provided on the side face of the apparatus and leads to the outside.  
         [0150]    The shield case  173 M,  173 C,  173 Y or  173 Bk is formed with a large hole leading to the inside on the face opposed to the horizontal duct  131 M,  131 C,  131 Y or  131 Bk over the entire length.  
         [0151]    Also, the vertical duct  133  is composed of a barrel member of rough square, and has one end connected to an upper face opening portion  110   b  provided on the upper face cover  110  to lead to the outside, with the other end sealed off. And a catching hole for the horizontal duct  131 M,  131 C,  131 Y or  131 Bk is formed on the side face of the vertical duct  133 , and connected with one end of the horizontal duct  131 M,  131 C,  131 Y or  131 Bk that is not connected to the side opening portion  120 M,  120 C,  120 Y or  120 Bk.  
         [0152]    Also, a catching hole for connecting one end of the exhaust heat duct  139  in the fixing unit  108  is formed on the side face of the vertical duct  133 . The exhaust heat duct  139  has one end connected to the vertical duct  133 , with the other end connected to a side opening portion  121  to lead to the outside.  
         [0153]    Also, the upper face opening portion  10   b  is provided at the uppermost position of the vertical duct  133  extending straightly upwards, namely, rearward at the right edge part of the upper face cover  110 , as seen from the front face.  
         [0154]    Also, a fan  135  and an ozone filter  137  are provided at the end portion of the vertical duct  133  connected to the upper face opening portion  10   b.    
         [0155]    The fan  135  is attached to exhaust the air out of the vertical duct  133 .  
         [0156]    Also, the ozone filter  137  contains a material adsorbing ozone such as activated carbon, in which the air within the vertical duct  133  is passed through this material.  
         [0157]    Herein, the air flows through the horizontal duct  131 M,  131 C,  131 Y or  131 Bk and the vertical duct  133  in the following way as indicated by the arrow in FIG. 4.  
         [0158]    First of all, the fan  135  is rotated to exhaust the air out of the vertical duct  133 , so that the air within the vertical duct  133  flows to the fan  135  to make the inside of the vertical duct  133  at negative pressure. Therefore, the air within the horizontal duct  131 M,  131 C,  131 Y or  131 Bk and the exhaust heat duct  139  is sucked to flow into the vertical duct  133 , so that the internal pressures of the horizontal duct  131 M,  131 C,  131 Y or  131 Bk and the exhaust heat duct  139  are decreased. Thereby, the air is flowed in from the side opening portion  120 M,  120 C,  120 Y or  120 Bk. In this way, the air flow from the side opening portion  120 M,  120 C,  120 Y or  120 Bk to the upper face opening portion  110   b  is formed.  
         [0159]    Moreover, the air flowing through each duct absorbs heat within the color laser printer  101  and is warmed. Particularly, the air flowing through the exhaust heat duct  139  is warmed by the fixing unit  108 . Because the warmed air is lighter, the air ascends within the vertical duct  133  after entering into the vertical duct  133 . In this way, the pressure under the ascending air is decreased within the vertical duct  133 , producing an ascending current to cause the unwarmed air to ascend together (a so-called funnel effect). And the air is sucked from the horizontal duct  131 M,  131 C,  131 Y or  131 Bk and the exhaust heat duct  139  due to a suction force of the fan  135  and a force of the ascending current.  
         [0160]    At this time, due to the air flow through the horizontal duct  131 M,  131 C,  131 Y or  131 Bk, the air entering through the side opening  120 M,  120 C,  120 Y or  120 Bk flows into the shield case  173 M,  173 C,  173 Y or  173 Bk leading to the horizontal duct  131 M,  131 C,  131 Y or  131 Bl, so that the air inside the shield case  173 M,  173 C,  173 Y or  173 Bk flows into the vertical duct  133 .  
         [0161]    As described above, the color laser printer  101  according to the second embodiment includes the horizontal duct  131 M,  131 C,  131 Y or  131 Bk disposed near the charger  171 M,  171 C,  171 Y or  171 Bk, and the vertical duct  133 , connecting to the horizontal duct  131 M,  131 C,  131 Y or  131 Bk, for conducting the air through the horizontal duct  131 M,  131 C,  131 Y or  131 Bk upwards and exhausting the air through the upper face opening portion  110   b . Thereby, the air is flowed into the shield case  173 M,  173 C,  173 Y or  173 Bk due to the air flow into the horizontal duct  131 M,  131 C,  131 Y or  131 Bk, conducting ozone produced in the charging wire  172 M,  172 C,  172 Y or  172 Bk for the charger  171 M,  171 C,  171 Y or  171 Bk into an ozone filter  137  to adsorb ozone and exhaust the air outside.  
         [0162]    Therefore, ozone produced in the charging wire  171 M,  171 C,  171 Y or  171 Bk is prevented from adhering to the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk, causing an unevenness of charging and having influence on the formed image.  
         [0163]    Also, owing to the funnel effect of the vertical duct  133 , the exhaust efficiency is made better than only by suction of the fan  135 .  
         [0164]    Also, the exhaust heat duct  139  is connected to the vertical duct  133 , the air warmed in the fixing unit  108  flows into the vertical duct  133 , so that the ascending current owing to the funnel effect is increased to increased the exhaust efficiency.  
         [0165]    Also, the upper face opening portion  110   b  is located closer to the rear at the right end part of the upper face cover  110  that overlaps substantially the end portion of the horizontal duct  131 M,  131 C,  131 Y or  131 Bk, whereby the vertical duct  133  is made almost straight to reduce the flow resistance, improving the exhaust efficiency.  
         [0166]    Since the exhaust efficiency is enhanced in the above way, the horizontal duct  131 M,  131 C,  131 Y or  131 Bk is reduced, and the apparatus is made smaller. Also, by reducing the output of the fan  135 , the energy saving and the silent operation are achieved.  
         [0167]    Also, since the upper face opening portion  110   b  serving as the exhaust port is formed on the upper face cover  110  at the upper part of the color laser printer  101 , the exhaust air is prevented from blowing off to the surrounding people. Also, since the upper face opening portion  110   b  on the upper face cover  110  is located away from the operation portion  111 , the exhaust air is away from the person approaching the apparatus to make the operation, thereby preventing ozone odor of ozone contained in the exhaust air from giving a feeling of displeasure to the surrounding people of he apparatus. Particularly, the Scorotron system that is the charging system for the charger in the second embodiment has a relatively large amount of ozone, resulting in a significant effect of prevention.  
         [0168]    Also, since the charging method of positive polarity in which ozone is difficult to occur is employed for the charger to charge electric charges, and the ozone filter  137  is provided on the upper face opening portion  110   b , there is a smaller amount of ozone exhausted outside. Therefore, there is a great effect of preventing the odor of ozone from giving a feeling of displeasure.  
         [0169]    The Third Preferred Embodiment  
         [0170]    A third embodiment of the invention will be described below with reference to the drawings. FIG. 5 is a schematic cross-sectional view showing the internal constitution of the color laser printer  201  according to the third embodiment of the invention, as seen from the side. FIG. 6 is a schematic cross-sectional view, as seen from the front.  
         [0171]    The color laser printer  201  employs the image making units  180 C,  180 Y and  180 Bk comprising the structure of horizontal ducts  131 M,  131 C and  131 Y, instead of the image making units  150 C,  150 Y and  150 Bk in the color laser printer  101  of the second embodiment. And other constitutions and functions are the same as those of the color laser printer  101  of the second embodiment.  
         [0172]    That is, the image making unit  180 C,  180 Y or  180 Bk is formed with a first plate plane  155   a C,  155   a Y or  155   a Bk and a second plate plane  155   b C,  155   b Y or  155   b Bk, which is a plate member of rough rectangle in the vertical direction, on the bottom face of the developing unit case  155 C,  155 Y or  155 Bk, so as to form three plate planes having the shape of rough U-character serving as the horizontal duct  131 M,  131 C or  131 Y for the charger  171 M,  171 C or  171 Y that is located directly below and opposed, when mounted on the color laser printer  201 , in addition to the constitution of the image making units  150 C,  150 Y and  150 Bk, as shown in FIG. 5. The plate plane in the horizontal direction is directly the bottom face of the developing unit case  155 M,  155 C or  155 Bk. Also, the image making unit  180 M has no charger directly below, and may be directly the constitution of the image making unit  150 M, but is formed with the first plate plane  155   a M and the second plate plane  155   b M in consideration of the productivity in the same way as the other image making units.  
         [0173]    Also, the color laser printer  201  is formed with a first coupling duct  131   a M,  131   a C and  131   a Y and a second coupling duct  131   b M,  131   b C and  131   b Y in accordance with the first plate plane  155   a C,  155   a Y and  155   a Bk and the second plate plane  155   b C,  155   b Y and  155   b Bk formed on the image making unit  180 C,  180 Y and  180 Bk, and the sectional shape of the bottom face for the developing unit case  155 C,  155 Y and  155 Bk, as shown in FIG. 6. The first coupling duct  131   a M,  131   a C or  131   a Y is provided between the side opening portion  120 M,  120 C or  120 Y and the image making unit  180 C,  180 Y or  180 Bk. Also, the second coupling duct  131   b M,  131   b C or  131   b Y is provided between the image making unit  180 C,  180 Y or  180 Bk and the vertical duct  133 . And a packing (not shown) is provided at the junction between the first and second coupling ducts and the image making unit  180 C,  180 Y or  180 Bk to have no interstice in the color laser printer  201 . The horizontal duct  131 Bk is installed within the apparatus in the same way as in the second embodiment.  
         [0174]    In the color laser printer  201  with the above constitution according to the third embodiment, the image making unit  180 M,  180 C,  180 Y and  180 Bk is mounted at closer interval, whereby the color laser printer  201  can be lower in height than the color laser printer  101 .  
         [0175]    That is, in the color laser printer  101  of the second embodiment, to allow the image making unit  150 M,  150 C,  150 Y or  150 Bk to be mounted or demounted across the horizontal duct  131 M,  131 C,  131 Y or  131 Bk, the horizontal duct  131 M,  131 C,  131 Y or  131 Bk is disposed at wider interval than the thickness of the image making unit  150 M,  150 C,  150 Y or  150 Bk, and accordingly the image making unit  150 M,  150 C,  150 Y or  150 Bk is disposed at wider interval than the thickness of the image making unit  150 M,  150 C,  150 Y or  150 Bk.  
         [0176]    However, in the color laser printer  201  of the third embodiment, there is no interference problem in mounting or demounting the image making unit  180 M,  180 C,  180 Y or  180 Bk, because the horizontal ducts  131 M,  131 C and  131 Y do not exist on the path for mounting or demounting, whereby the image making unit  180 M,  180 C,  180 Y or  180 Bk is disposed at closer interval. Therefore, the total height of the apparatus is decreased.  
         [0177]    Also, the length of the vertical duct  133  is made shorter by decreasing the total height of the apparatus, making it possible to improve the exhaust efficiency.  
         [0178]    Though the second and third embodiments have been described above, this invention is not limited to the specific embodiment as above, but maybe practiced in various other forms.  
         [0179]    For example, in the second and third embodiments, the fan  135  is employed for compulsory ventilation, but the natural ventilation may be made without using the fan  135 . Also, the exhaust heat duct  139  passing through the fixing unit  108  may not be connected to the vertical duct  133 , or the ozone filter  137  may not be mounted. In this case, the ventilation occurs upwards owing to the funnel effect with the air warmed by the fixing unit  108 . Also, the charger  171 M,  171 C,  171 Y or  171 Bk may be other than the Scorotron type, or may charge the photosensitive drum  103 M,  103 C,  103 Y or  103 Bk in negative polarity.  
         [0180]    Also, though in the third embodiment, the air is exhausted upwards through the upper face opening portion  110   b  of the upper face cover  110  from the vertical duct  133  to validate the funnel effect, the air maybe exhausted from the side face by providing the opening portion on the side face without expecting the funnel effect.  
         [0181]    Also, in the second and third embodiments, the image is formed on the paper P conveyed from the paper conveying belt  105  by directly transferring the toner image from the photosensitive drum  103 M,  103 C,  103 Y and  103 Bk onto the paper P. However, the intermediate transfer belt may be provided for once transferring the toner image for each of four colors formed on the photosensitive drum  103 M,  103 C,  103 Y and  103 Bk onto the intermediate transfer belt, and then transferring the toner images transferred on the intermediate transfer belt onto the paper P at a pressure contact position between the secondary transfer roller and the intermediate transfer belt by which the paper P is passed.