Abstract:
An image forming apparatus includes: an image forming unit; and a controller. The image forming unit includes: an image bearing member; a charging member; a transfer member; and a cleaning member. The controller selectively applies first and second transfer biases to the transfer member. A first transfer current flows between the image bearing member and the transfer member upon application of the first transfer bias. A second transfer current flows therebetween upon application of the second transfer bias. The controller selectively applies first and second charging biases to the charging member. The image bearing member has a first surface potential upon application of the first charging bias, and a second surface potential upon application of the second charging bias. The controller applies the first charging bias when applying the first transfer bias, and applies the second charging bias when applying the second transfer bias.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2013-017861 filed Jan. 31, 2013. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus using an electrophotographic method. 
     BACKGROUND 
     There is known, as the image forming apparatus using an electrophotographic method, a printer provided with a photosensitive member carrying a developer image thereon and a transfer member for transferring the developer image formed on the photosensitive member onto a sheet. 
     For example, there is proposed a printer provided with a photosensitive member carrying a toner image thereon and a transfer roller for transferring the toner image formed on a surface of the photosensitive member to a recording sheet. 
     In this printer, transfer bias is made to increase in accordance with an increase in a resistance value of the transfer roller due to an increase in the number of recorded sheets. This configuration can maintain an electric field between the photosensitive member and the transfer roller, thereby preventing degradation in transfer efficiency. 
     SUMMARY 
     However, in the printer described above, a surface potential of the photosensitive member may be excessively lowered when the transfer bias is increased. 
     When the surface potential of the photosensitive member is excessively lowered during a recording operation, toner collected by an electrically-conductive brush roller may be expelled onto the surface of the photosensitive member, which causes degradation of image quality. 
     In view of the foregoing, it is an object of the present invention to provide an image forming apparatus capable of reliably cleaning an image bearing member by means of a cleaning member. 
     In order to attain the above and other objects, the present invention provides an image forming apparatus including: an image forming unit; and a controller. The image forming unit includes: an image bearing member; a charging member; a transfer member; and a cleaning member. The image bearing member has a surface and is configured to carry a developer image on the surface. The charging member is configured to apply an electric charge to the surface of the image bearing member. The transfer member is configured to transfer the developer image carried on the surface of the image bearing member to a transfer medium. The cleaning member is configured to remove residual developer that remains on the surface of the image bearing member after the developer image has been transferred onto the transfer medium from the surface of the image bearing member. The controller is configured to selectively apply a plurality of transfer biases including a first transfer bias and a second transfer bias to the transfer member. The first transfer current flows between the image bearing member and the transfer member when the first transfer bias is applied to the transfer member. The second transfer current larger than the first transfer current flows between the image bearing member and the transfer member when the second transfer bias is applied to the transfer member. The controller is further configured to selectively apply a plurality of charging biases including a first charging bias and a second charging bias to the charging member. The image bearing member has a first surface potential when the first charging bias is applied to the charging member. The image bearing member has a second surface potential larger than the first surface potential when the second charging bias is applied to the charging member. The controller applies the first charging bias to the charging member when applying the first transfer bias to the transfer member, and applies the second charging bias to the charging member when applying the second transfer bias to the transfer member. 
     According to another aspect, the present invention provides an image forming apparatus including: an image forming unit; and a controller. The image forming unit includes: an image bearing member; a charging member; and a transfer member. The image bearing member has a surface and is configured to carry a developer image on the surface. The charging member is configured to apply an electric charge to the surface of the image bearing member. The transfer member is configured to transfer the developer image carried on the surface of the image bearing member to a transfer medium. The controller is configured to selectively apply a plurality of transfer biases including a first transfer bias and a second transfer bias to the transfer member. The second transfer bias is larger than the first transfer bias. The controller is further configured to selectively apply a plurality of charging biases including a first charging bias and a second charging bias to the charging member. The image bearing member has a first surface potential when the first charging bias is applied to the charging member. The image bearing member has a second surface potential higher than the first surface potential when the second charging bias is applied to the charging member. The controller applies the first charging bias to the charging member when applying the first transfer bias to the transfer member, and applies the second charging bias to the charging member when applying the second transfer bias to the transfer member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings; 
         FIG. 1  is a cross-sectional view illustrating a printer according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating essential components of an electrical structure of the printer of  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating essential components of an electrical structure of a printer according to a second embodiment of the present invention; 
         FIG. 4  is a block diagram illustrating essential components of an electrical structure of a printer according to a third embodiment of the present invention; 
         FIG. 5  is a block diagram illustrating essential components of an electrical structure of a printer according to a fourth embodiment of the present invention; and 
         FIG. 6  is a cross-sectional view illustrating a printer according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     1. Overall Structure of Printer 
     A printer as an image forming apparatus according to a first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 . 
     As illustrated in  FIG. 1 , the printer  1  is a horizontal direct tandem-type color printer. 
     Throughout the specification, the terms “upward”, “downward”, “upper”, “lower”, “above”, “below”, “beneath”, “right”, “left”, “front”, “rear” and the like will be used assuming that the printer  1  is disposed in an orientation in which it is intended to be used. That is, directions related to the printer  1  will be given based on the state of the printer  1  when the printer  1  is resting on a level surface. More specifically, in  FIG. 1  a left side and a right side are a front side and a rear side, respectively. Further, a left side and a right side of the printer  1  will be based on the perspective of a user facing the front of the printer  1 . Hence, in  FIG. 1  a near side and a far side are a right side and a left side, respectively. Further, in  FIG. 1  a top side and a bottom side are a top side and a bottom side, respectively. 
     The printer  1  includes a main casing  2  having substantially a box-like shape. The main casing  2  is formed with an opening  3  at a top portion thereof. A top cover  4  opening and closing the opening  3  is provided at the top portion of the main casing  2  so as to be pivotally movable about its rear end portion. The printer  1  further includes a plurality of process units  5  (four in the embodiment), a plurality of LED units  6  (four in the embodiment), a transfer unit  7 , and a fixing unit  8 . 
     All the process units  5  are detachably provided in the main casing  2 . The plurality of process units  5  corresponds respectively to yellow, magenta, cyan, and black colors, and is arranged juxtaposed with and spaced apart from one another in a front-rear direction. Specifically, from a front side to a rear side of the main casing  2 , a yellow process unit  5 Y, a magenta process unit  5 M, a cyan process unit  5 C, and a black process unit  5 K are arranged in this order. 
     The process unit  5  includes a drum unit  9  and a developing unit  10  detachably mounted in the drum unit  9 . 
     The drum unit  9  includes a photosensitive drum  11 , a scorotron charger  12 , and a drum cleaning roller  24 . 
     The photosensitive drum  11  is formed in substantially a cylindrical shape that is elongated in a left-right direction and rotatably provided at a rear end portion of the drum unit  9 . 
     Incidentally, the photosensitive drum  11  provided in the black process unit  5 K is an example of a first image bearing member. The photosensitive drums  11  provided in the yellow process unit  5 Y, the magenta process unit  5 M, and the cyan process unit  5 C, respectively, are an example of a second image bearing member. 
     Hereinafter, the photosensitive drum  11  provided in the yellow process unit  5 Y will be referred to as the yellow photosensitive drum  11 ; the photosensitive drum  11  provided in the magenta process unit  5 M will be referred to as the magenta photosensitive drum  11 ; the photosensitive drum  11  provided in the cyan process unit  5 C will be referred to as the cyan photosensitive drum  11 ; and the photosensitive drum  11  provided in the black process unit  5 K will be referred to as the black photosensitive drum  11 . 
     The scorotron charger  12  is disposed opposite to and spaced apart from the photosensitive drum  11  at an upper-rear side thereof. 
     Hereinafter, the scorotron charger  12  provided in the yellow process unit  5 Y will be referred to as the yellow scorotron charger  12 ; the scorotron charger  12  provided in the magenta process unit  5 M will be referred to as the magenta scorotron charger  12 ; the scorotron charger  12  provided in the cyan process unit  5 C will be referred to as the cyan scorotron charger  12 ; and the scorotron charger  12  provided in the black process unit  5 K will be referred to as the black scorotron charger  12 . 
     The drum cleaning roller  24  is disposed below the scorotron charger  12  at a rear side of the photosensitive drum  11 . The drum cleaning roller  24  contacts the photosensitive drum  11  from the rear side thereof. The drum cleaning roller  24  is formed in substantially a cylindrical shape that is elongated in the left-right direction. 
     Hereinafter, the drum cleaning roller  24  provided in the yellow process unit  5 Y will be referred to as the yellow drum cleaning roller  24 ; the drum cleaning roller  24  provided in the magenta process unit  5 M will be referred to as the magenta drum cleaning roller  24 ; and the drum cleaning roller  24  provided in the cyan process unit  5 C will be referred to as the cyan drum cleaning roller  24 ; and the drum cleaning roller  24  provided in the black process unit  5 K will be referred to as the black drum cleaning roller  24 . 
     The developing unit  10  includes a developing roller  13  and a supply roller  14  for supplying toner to the developing roller  13 . 
     The developing roller  13  is rotatably supported at a rear end portion of the developing unit  10  so as to be exposed in a rear side of the developing unit  10 . The developing roller  13  contacts the corresponding photosensitive drum  11  from an upper-front side thereof. The developing roller  13  is formed in substantially a cylindrical shape that is elongated in the left-right direction. 
     The supply roller  14  is rotatably supported to the developing unit  10  and disposed at an upper-front side of the developing roller  13  so as to contact the developing roller  13 . The supply roller  14  is formed in substantially a cylindrical shape that is elongated in the left-right direction. 
     The developing unit  10  further includes a layer thickness regulating blade  15  for regulating a thickness of toner supplied to the developing roller  13 . Further, the developing unit  10  accommodates toner (developer) therein above the developing roller  13  and the supply roller  14 . 
     Each of the plurality of LED units  6  is supported at the top cover  4  so as to be opposed to an upper portion of the corresponding photosensitive drum  11  of each of the plurality of process units  5 . 
     The transfer unit  7  is disposed below and opposite to the plurality of process units  5 . The transfer unit  7  includes a driving roller  16 , a driven roller  17 , a conveyor belt  18 , and a plurality of transfer rollers  19 . 
     The driving roller  16  is rotatably supported at a rear end portion of the transfer unit  7 . 
     The driven roller  17  is rotatably supported at a front end portion of the transfer unit  7 . 
     The conveyor belt  18  is looped around the driving roller  16  and the driven roller  17  in such a manner that an upper portion of the conveyor belt  18  contacts all the photosensitive drums  11  from below. When the driving roller  16  is driven to rotate, the conveyor belt  18  circulates so that its upper portion moves rearward, and the driven roller  17  rotates along with the circulating movement of the conveyor belt  18 . 
     Each of the plurality of transfer rollers  19  is disposed below and opposite to the corresponding photosensitive drum  11 , with the upper portion of the conveyor belt  18  interposed between the top of the transfer roller  19  and the bottom of the corresponding photosensitive drum  11 . 
     The transfer roller  19  corresponding to the yellow photosensitive drum  11  will be referred to as the yellow transfer roller  19 ; the transfer roller  19  corresponding to the magenta photosensitive drum  11  will be referred to as the magenta transfer roller  19 ; the transfer roller  19  corresponding to the cyan photosensitive drum  11  will be referred to as the cyan transfer roller  19 ; and the transfer roller  19  corresponding to the black photosensitive drum  11  will be referred to as the black transfer roller  19 . 
     The fixing unit  8  is disposed rearward of the transfer unit  7  so as to be opposed thereto. The fixing unit  8  includes a heating roller  20  and a pressure roller  21  opposite to the heating roller  20 . 
     When a print job is inputted to the printer  1  from an external personal computer  51  (see  FIG. 2 ), an image forming operation is started. Then, the toner accommodated in each developing unit  10  is conveyed between the supply roller  14  and the corresponding developing roller  13  to be positively tribo-charged between the supply roller  14  and the corresponding developing roller  13 . The layer thickness regulating blade  15  regulates the thickness of toner supplied to the corresponding developing roller  13 , maintaining the toner carried on the surface of the corresponding developing roller  13  at a thin uniform thickness. 
     In the meantime, each scorotron charger  12  applies a uniform charge to a surface of the corresponding photosensitive drum  11 . Subsequently, the photosensitive drum  11  is exposed to light by the corresponding LED unit  6  based on predetermined image data, forming an electrostatic latent image on the surface of the photosensitive drum  11  based on the image data. The toner carried on the developing roller  13  is then supplied to the electrostatic latent image formed on the surface of the corresponding photosensitive drum  11 . In this way, a toner image (developer image) is carried on the surface of the photosensitive drum  11 . 
     The main casing  2  is provided with a sheet supply tray  22  at a bottom portion thereof. Each sheet P (transfer medium) accommodated in the sheet supply tray  22  is conveyed upward and rearward by various rollers while passing through a U-shaped path, and conveyed to a position between the photosensitive drum  11  and the conveyor belt  18  at a prescribed timing. Subsequently, the sheet P is conveyed by the conveyor belt  18  to pass between each photosensitive drum  11  and the corresponding transfer roller  19  from front to rear. At this time, toner images are transferred onto the sheet P by transfer bias applied to the transfer rollers  19 . 
     Next, the sheet P is subjected to heat and pressure while passing between the heating roller  20  and the pressure roller  21  of the fixing unit  8 , thereby thermally fixing the toner images to the sheet P. 
     Thereafter, the sheet P is conveyed upward and frontward along a U-shaped path to be discharged to a discharge tray  23  provided on the top cover  4 . 
     2. Detailed Description of Process Unit 
     (1) Photosensitive Drum 
     As illustrated in  FIG. 2 , each photosensitive drum  11  includes a drum body  31  and a shaft  32 . 
     The drum body  31  is made of metal and is formed in substantially a cylindrical shape that is elongated in the left-right direction. A photosensitive layer is formed on a peripheral surface of the drum body  31 . While not shown in  FIG. 2 , flange members are provided one on each of left and right end portions of the drum body  31 . Each flange member is non-rotatably fitted to the left-right end portion of the drum body  31 . 
     The shaft  32  is made of metal and is formed in substantially a cylindrical shape that extends along an axis of the drum body  31 . The shaft  32  is non-rotatably supported to the flange members so as to penetrate a center in a radial direction of each of the flange members. Further, the shaft  32  is electrically connected to an inner surface of the drum body  31  through an electrically-conductive metal member (not illustrated). The shaft  32  is grounded to the main casing  2 . 
     (2) Scorotron Charger 
     Each scorotron charger  12  includes a grid  37  and a charging wire  38 . 
     The grid  37  is made of metal and is formed in a frame-like shape that is substantially U-shaped in cross-section with an opening at an upper-rear side thereof and that extends in the left-right direction. 
     The charging wire  38  is made of metal and is substantially linearly formed. The charging wire  38  is stretched in the left-right direction in the grid  37 . 
     (3) Transfer Roller 
     Each transfer roller  19  includes a transfer roller shaft  33  and a transfer roller body  34 . 
     The transfer roller shaft  33  is made of metal and is formed in substantially a cylindrical shape that extends in the left-right direction. 
     The transfer roller body  34  is made of a resin material having electrically-conductive properties and is formed in substantially a cylindrical shape that is elongated in the left-right direction. The transfer roller body  34  covers the transfer roller shaft  33  so as to expose left and right end portions of the transfer roller shaft  33  to the outside. 
     (4) Drum Cleaning Roller 
     Each drum cleaning roller  24  includes a drum cleaning roller shaft  35  and a drum cleaning roller body  36 . 
     The drum cleaning roller shaft  35  is made of metal and is formed in substantially a cylindrical shape that extends in the left-right direction. 
     The drum cleaning roller body  36  is made of a foamed silicon resin or a formed urethane resin, having semi-electrically-conductive properties. The drum cleaning roller body  36  is formed in substantially a cylindrical shape that is elongated in the left-right direction and covers the drum cleaning roller shaft  35  so as to expose left and right end portions of the drum cleaning roller shaft  35  to the outside. 
     3. Electrical Structure of Printer 
     The printer  1  includes, within the main casing  2 , a controller  40  and a humidity sensor  43 . 
     The controller  40  includes a power supply substrate  41  and a control substrate  42 . 
     The power supply substrate  41  includes a transfer circuit  45  for supplying electric power to the transfer rollers  19 , a drum cleaning circuit  46  for supplying electric power to the drum cleaning rollers  24 , and a charging circuit  47  for supplying electric power to the scorotron chargers  12 . 
     The transfer circuit  45  is electrically connected to the transfer roller shaft  33  of each of the plurality of transfer rollers  19  through a wiring. The transfer circuit  45  applies transfer bias individually to the plurality of transfer rollers  19  under control of the control substrate  42 . 
     The drum cleaning circuit  46  is electrically connected to the drum cleaning roller shaft  35  of each of the plurality of drum cleaning rollers  24  through a wiring. The drum cleaning circuit  46  applies the same drum cleaning bias to all the drum cleaning rollers  24  under control of the control substrate  42 . 
     The charging circuit  47  is electrically connected to the charging wire  38  of each of the plurality of scorotron chargers  12  through a wiring. Under control of the control substrate  42 , the charging circuit  47  applies the same charging bias collectively to the yellow, magenta, and cyan scorotron chargers  12  and applies charging bias independently to the black scorotron charger  12 . 
     The control substrate  42  includes a CPU and a memory. The control substrate  42  includes, as configurations to be realized in a software manner by program processing under control of the CPU, a transfer bias controller  48  for controlling the transfer circuit  45 , a drum cleaning bias controller  49  for controlling the drum cleaning circuit  46 , and a charging bias controller  50  for controlling the charging circuit  47 . 
     The humidity sensor  43  is a sensor for measuring a relative humidity inside the main casing  2  and is electrically connected to the control substrate  42  through a signal wiring. 
     4. Image Forming Operation 
     (1) Setting of Transfer Current, Charging Bias, and Drum Cleaning Bias 
     When the above-described image forming operation is performed, the charging bias controller  50  sets the charging bias, the transfer bias controller  48  sets the transfer current, and the drum cleaning bias controller  49  sets the drum cleaning bias. 
     (1-1) Normal Humidity Environment 
     First, settings of the charging bias, the transfer current, and the drum cleaning bias when the main casing  2  is in a normal humidity environment will be described. Note that, when the main casing  2  is in the normal humidity environment, the relative humidity inside the main casing  2  measured by the humidity sensor  43  is higher than 30% and lower than 60%. 
     When the main casing  2  is in the normal humidity environment, the charging bias controller  50  sets the charging bias to be applied to all the scorotron chargers  12  to, for example, +700 V. This charging bias is an example of a first charging bias. 
     The transfer bias controller  48  sets the transfer current to be made to flow between the yellow photosensitive drum  11  and the yellow transfer roller  19  to, for example, −8 μA. This transfer current is an example of a first transfer current in the yellow transfer roller  19 . 
     The transfer bias controller  48  also sets the transfer current to be made to flow between the magenta photosensitive drum  11  and the magenta transfer roller  19  to, for example, −9 μA. This transfer current is an example of a first transfer current in the magenta transfer roller  19 . 
     The transfer bias controller  48  also sets the transfer current to be made to flow between the cyan photosensitive drum  11  and the cyan transfer roller  19  to, for example, −9 μA. This transfer current is an example of a first transfer current in the cyan transfer roller  19 . 
     The transfer bias controller  48  also sets the transfer current to be made to flow between the black photosensitive drum  11  and the black transfer roller  19  to, for example, −9 μA. This transfer current is an example of a first transfer current in the black transfer roller  19 . 
     The drum cleaning bias controller  49  sets the drum cleaning bias to be applied to all the drum cleaning rollers  24  to, for example, −300 V. 
     (1-2) Low Humidity Environment 
     Next, settings of the charging bias, the transfer current, and the drum cleaning bias when the main casing  2  is in a low humidity environment will be described. Note that, when the main casing  2  is in the low humidity environment, the relative humidity inside the main casing  2  measured by the humidity sensor  43  is equal to or lower than 30%. 
     When the main casing  2  is in the low humidity environment, the charging bias controller  50  sets the charging bias to be applied to the yellow, magenta, and cyan scorotron chargers  12  to, for example, +750 V. This charging bias is an example of a second charging bias. 
     Further, the charging bias controller  50  sets the charging bias to be applied to the black scorotron charger  12  to, for example, +800 V. This charging bias is an example of a third charging bias. 
     The transfer bias controller  48  sets the transfer current to be made to flow between the yellow photosensitive drum  11  and the yellow transfer roller  19  to, for example, −9 μA. This transfer current is an example of a second transfer current in the yellow transfer roller  19 . 
     The transfer bias controller  48  also sets the transfer current to be made to flow between the magenta photosensitive drum  11  and the magenta transfer roller  19  to, for example, −10 μA. This transfer current is an example of a second transfer current in the magenta transfer roller  19 . 
     The transfer bias controller  48  also sets the transfer current to be made to flow between the cyan photosensitive drum  11  and the cyan transfer roller  19  to, for example, −10 μA. This transfer current is an example of a second transfer current in the cyan transfer roller  19 . 
     The transfer bias controller  48  also sets the transfer current to be made to flow between the black photosensitive drum  11  and the black transfer roller  19  to, for example, −11 μA. This transfer current is an example of a second transfer current in the black transfer roller  19 . 
     The drum cleaning bias controller  49  sets the drum cleaning bias to be applied to all the drum cleaning rollers  24  to, for example, −300 V. 
     (2) Transfer and Cleaning Operations 
     (2-1) Transfer and Cleaning Operations in Normal Humidity Environment 
     When the main casing  2  is in the normal humidity environment at the time of execution of the above-described image forming operation, the charging bias controller  50  controls the charging circuit  47  to apply the above charging bias to the charging wires  38  of all the scorotron chargers  12 . 
     Then, surfaces of all the photosensitive drums  11  are each charged at a charging potential of, for example, +700 V in a state before being exposed by the corresponding LED units  6 . This charging potential is a first surface potential. 
     The transfer bias controller  48  controls the transfer circuit  45  to apply the transfer bias to the respective transfer rollers  19  so as to make the transfer current constantly flow between the transfer rollers  19  and their corresponding photosensitive drums  11 . 
     Specifically, the transfer bias controller  48  applies a transfer bias of, for example, −1200 V to the yellow transfer roller  19 . This transfer bias is an example of a first transfer bias in the yellow transfer roller  19 . 
     The transfer bias controller  48  also applies a transfer bias of, for example, −1350 V to the magenta transfer roller  19 . This transfer bias is an example of a first transfer bias in the magenta transfer roller  19 . 
     The transfer bias controller  48  also applies a transfer bias of, for example, −1350 V to the cyan transfer roller  19 . This transfer bias is an example of a first transfer bias in the cyan transfer roller  19 . 
     The transfer bias controller  48  also applies a transfer bias of, for example, −1350 V to the black transfer roller  19 . This transfer bias is an example of a first transfer bias in the black transfer roller  19 . 
     The drum cleaning bias controller  49  controls the drum cleaning circuit  46  to apply the above-described drum cleaning bias to all the drum cleaning rollers  24 . 
     Then, while the sheet P is passing through positions where the photosensitive drums  11  oppose to their corresponding transfer rollers  19 , the toner image carried on each photosensitive drum  11  is transferred onto the sheet P. 
     At this time, the transfer current flows from the yellow transfer roller  19  to the yellow photosensitive drum  11 , with the result that the surface potential of the yellow photosensitive drum  11  is lowered from +700 V to about +300 V. 
     Further, the transfer current flows from the magenta transfer roller  19  to the magenta photosensitive drum  11 , with the result that the surface potential of the magenta photosensitive drum  11  is lowered from +700 V to about +250 V. 
     Further, the transfer current flows from the cyan transfer roller  19  to the cyan photosensitive drum  11 , with the result that the surface potential of the cyan photosensitive drum  11  is lowered from +700 V to about +250 V. 
     Further, the transfer current flows from the black transfer roller  19  to the black photosensitive drum  11 , with the result that the surface potential of the black photosensitive drum  11  is lowered from +700 V to about +250 V. 
     Thereafter, in each of the plurality of photosensitive drums  11 , residual toner that has not been transferred onto the sheet P and remains on the peripheral surface of the photosensitive drum  11  is opposed to the corresponding drum cleaning roller  24  in association with rotation of the photosensitive drum  11  and is then electrostatically held on a peripheral surface of the corresponding drum cleaning roller  24  by the drum cleaning bias. 
     (2-2) Transfer and Cleaning Operations in Low Humidity Environment 
     When the main casing  2  is in the low humidity environment at the time of execution of the above-described image forming operation, the charging bias controller  50  controls the charging circuit  47  to apply the above charging bias to the charging wires  38  of each of the plurality of scorotron chargers  12 . 
     Then, the surfaces of the yellow, magenta, and cyan photosensitive drums  11  are each charged at a charging potential of, for example, +750 V in a state before being exposed by the corresponding LED units  6 . This charging potential is a second surface potential. 
     The surface of the black photosensitive drum  11  is charged at a charging potential of, for example, +800 V in a state before being exposed by the corresponding LED unit  6 . This charging potential is a third surface potential. 
     The transfer bias controller  48  controls the transfer circuit  45  to apply the transfer bias to the respective transfer rollers  19  so as to make the transfer current constantly flow between the transfer rollers  19  and their corresponding photosensitive drums  11 . 
     Specifically, the transfer bias controller  48  applies a transfer bias of, for example, −2250 V to the yellow transfer roller  19 . This transfer bias is an example of a second transfer bias in the yellow transfer roller  19 . 
     The transfer bias controller  48  applies a transfer bias of, for example, −2500 V to the magenta transfer roller  19 . This transfer bias is an example of a second transfer bias in the magenta transfer roller  19 . 
     The transfer bias controller  48  also applies a transfer bias of, for example, −2500 V to the cyan transfer roller  19 . This transfer bias is an example of a second transfer bias in the cyan transfer roller  19 . 
     The transfer bias controller  48  also applies a transfer bias of, for example, −2750 V to the black transfer roller  19 . This transfer bias is an example of a second transfer bias in the black transfer roller  19 . 
     The drum cleaning bias controller  49  controls the drum cleaning circuit  46  to apply the above-described drum cleaning bias to all the drum cleaning rollers  24 . 
     Then, while the sheet P is passing through positions where the photosensitive drums  11  oppose to their corresponding transfer rollers  19 , the toner image carried on each photosensitive drum  11  is transferred onto the sheet P. 
     At this time, the transfer current flows from the yellow transfer roller  19  to the yellow photosensitive drum  11 , with the result that the surface potential of the yellow photosensitive drum  11  is lowered from +750 V to about +300 V. 
     Further, the transfer current flows from the magenta transfer roller  19  to the magenta photosensitive drum  11 , with the result that the surface potential of the magenta photosensitive drum  11  is lowered from +750 V to about +250 V. 
     Further, the transfer current flows from the cyan transfer roller  19  to the cyan photosensitive drum  11 , with the result that the surface potential of the cyan photosensitive drum  11  is lowered from +750 V to about +250 V. 
     Further, the transfer current flows from the black transfer roller  19  to the black photosensitive drum  11 , with the result that the surface potential of the black photosensitive drum  11  is lowered from +800 V to about +250 V. 
     Thereafter, in each of the plurality of photosensitive drums  11 , residual toner that has not been transferred onto the sheet P and remains on the peripheral surface of the photosensitive drum  11  is electrostatically held on the peripheral surface of the corresponding drum cleaning roller  24  as in the case where the main casing  2  is in the normal humidity environment. 
     4. Operational Advantages 
     (1) According to the printer  1 , the setting values of both the transfer current and the charging bias are made to increase when the relative humidity inside the main casing  2  measured by the humidity sensor  43  is equal to or lower than 30%. 
     Specifically, the transfer current to be made to flow between the yellow photosensitive drum  11  and the yellow transfer roller  19  is changed from −8 μA to −9 μA, and the charging bias to be applied to the charging wire  38  of the yellow scorotron charger  12  is changed from +700 V to +750 V. 
     The transfer current to be made to flow between the magenta photosensitive drum  11  and the magenta transfer roller  19  is changed from −9 μA to −10 μA, and the charging bias to be applied to the charging wire  38  of the magenta scorotron charger  12  is changed from +700 V to +750 V. 
     The transfer current to be made to flow between the cyan photosensitive drum  11  and the cyan transfer roller  19  is changed from −9 μA to −10 μA, and the charging bias to be applied to the charging wire  38  of the cyan scorotron charger  12  is changed from +700 V to +750 V. 
     The transfer current to be made to flow between the black photosensitive drum  11  and the black transfer roller  19  is changed from −9 μA to −11 μA, and the charging bias to be applied to the charging wire  38  of the black scorotron charger  12  is changed from +700 V to +800 V. 
     As a result, when larger transfer current flows between the photosensitive drum  11  and the corresponding transfer roller  19 , the surface potential of the photosensitive drum  11  is made larger. 
     Thus, when the relative humidity inside the main casing  2  measured by the humidity sensor  43  is equal to or lower than 30%, that is, when electric resistance of the sheet P is expected to increase, the surface potential of each photosensitive drum  11  can be prevented from being excessively lowered due to an increase in the value of the transfer current while the toner image can reliably be transferred onto the sheet P, thereby preventing the residual toner collected by each drum cleaning roller  24  from being adhered once again onto the corresponding photosensitive drum  11 . 
     As a result, each photosensitive drum  11  can reliably be cleaned by the corresponding drum cleaning roller  24 . 
     (2) Further, according to the printer  1 , when the relative humidity inside the main casing  2  measured by the humidity sensor  43  is equal to or lower than 30%, the charging bias applied to the black scorotron charger  12  provided in the black process unit  5 K which is the most downstream side process unit in a conveying direction of the sheet P, i.e., the rearmost process unit, is set larger than the charging bias applied to the scorotron chargers  12  of the process units  5  disposed frontward of the black process unit  5 K. 
     As a result, the surface potential of the black photosensitive drum  11  can be prevented from being excessively lowered in the rearmost black process unit  5 K having larger transfer current. 
     Thus, the residual toner collected by the black drum cleaning roller  24  can be prevented from being adhered once again onto the black photosensitive drum  11 . 
     As a result, the black photosensitive drum  11  can reliably be cleaned by the black drum cleaning roller  24 . 
     5. Second Embodiment 
     A printer  101  as an image forming apparatus according to a second embodiment of the present invention will be described while referring to  FIG. 3 , wherein like parts and components are designated with the same reference numerals to avoid duplicating description. In the following description, only parts differing from those of the embodiment will be described in detail. 
     (1) Overview of Second Embodiment 
     In the first embodiment described above, the relative humidity inside the main casing  2  is detected using the humidity sensor  43 , and the setting values of both the transfer current and the charging bias are made to increase based on the detected relative humidity. 
     On the other hand, in the second embodiment, electric resistance between the transfer roller  19  and the corresponding photosensitive drum  11  is measured, and the setting values of both the transfer current and the charging bias are made to increase based on the measured electric resistance. 
     Further, in the first embodiment described above, the charging circuit  47  applies the same charging bias collectively to the yellow, magenta, and cyan scorotron chargers  12  and applies the charging bias independently to the black scorotron charger  12 . 
     On the other hand, in the second embodiment, the charging circuit  47  applies the charging bias individually to the plurality of scorotron chargers  12 , as illustrated in  FIG. 3 . 
     (2) Structure of Second Embodiment 
     In the second embodiment, the printer  101  includes a plurality of ammeters  61  and a plurality of voltmeters  62 , as illustrated in  FIG. 3 . 
     Each ammeter  61  is interposed between the transfer circuit  45  and the transfer roller shaft  33  of the transfer roller  19 . The ammeter  61  measures a value of current flowing from the transfer circuit  45  to the transfer roller shaft  33 . Further, the ammeter  61  is electrically connected to the control substrate  42  through a signal wiring (not illustrated). The ammeter  61  transmits the measured current value to the control substrate  42 . 
     Each voltmeter  62  is electrically connected to the transfer roller shaft  33  of the transfer roller  19  and the shaft  32  of the corresponding photosensitive drum  11 . The voltmeter  62  measures voltage developed between the transfer roller shaft  33  and the shaft  32 . The voltmeter  62  is electrically connected also to the control substrate  42  through a signal wiring (not illustrated). The voltmeter  62  transmits the measured voltage to the control substrate  42 . 
     The control substrate  42  calculates an electric resistance value between each transfer roller  19  and its corresponding photosensitive drum  11  based on the current value measured by its corresponding ammeter  61  and the voltage value measured by its corresponding voltmeter  62 . That is, the ammeter  61  and the voltmeter  62  constitute a resistance detector. 
     (3) Setting of Transfer Current and Charging Bias 
     In the second embodiment, when the electric resistance of the sheet P is equal to or higher than, for example, 500 MΩ, the charging bias controller  50  sets the charging bias to be applied to the scorotron charger  12  to be the same value as in the case of the low humidity environment described in the first embodiment. 
     Further, the transfer bias controller  48  sets the transfer current to be made to flow between the photosensitive drum  11  and its corresponding transfer roller  19  to be the same value as in the case of the low humidity environment described in the first embodiment. 
     Incidentally, a method of measuring the electric resistance of the sheet P is not especially limited. For example, current may be made to flow when the sheet P enters into a position between the photosensitive drum  11  and the corresponding transfer roller  19  to measure the electric resistance of the sheet P. 
     When the electric resistance between the transfer roller  19  and its corresponding photosensitive drum  11  is less than 500 MΩ, the charging bias controller  50  sets the charging bias to be applied to the scorotron charger  12  to be the same value as in the case of the normal humidity environment described in the first embodiment. 
     Similarly, the transfer bias controller  48  sets the transfer current to be made to flow between the photosensitive drum  11  and its corresponding transfer roller  19  to be the same value as in the case of the normal humidity environment described in the first embodiment. 
     That is, in the second embodiment, the charging bias to be applied to each of the scorotron chargers  12  is individually set, based on the electric resistance detected by the corresponding ammeter  61  and voltmeter  62 , so as to be either the same value as in the case of the low humidity environment or the same value as in the case of the normal humidity environment. Hence, the charging bias to be applied to one of the scorotron chargers  12  may have the same value as in the case of the low humidity environment, while the charging bias to be applied to the remainder may have the same value as in the case of the normal humidity environment. 
     Similarly, the transfer current to be made to flow between each of the photosensitive drums  11  and its corresponding transfer roller  19  is also individually set, based on the electric resistance detected by the corresponding ammeter  61  and voltmeter  62 , so as to be either the same value as in the case of the low humidity environment or the same value as in the case of the normal humidity environment. Hence, the transfer current to be made to flow between one of the photosensitive drums  11  and its corresponding transfer roller  19  may have the same value as in the case of the low humidity environment, while the transfer current to be made to flow between each of the remaining photosensitive drums  11  and its corresponding transfer roller  19  may have the same value as in the case of the normal humidity environment. 
     (4) Operational Advantages of Second Embodiment 
     (4-1) According to the second embodiment, when an actual measurement value of the electric resistance of the sheet P (electric resistance between the transfer roller  19  and its corresponding photosensitive drum  11 ) is high, the setting values of both the transfer current and the charging bias are made to increase. 
     As a result, when the actual electric resistance of the sheet P is high, the surface potential of each photosensitive drum  11  can be prevented from being excessively lowered due to an increase in the value of the transfer current, thereby preventing the residual toner collected by each drum cleaning roller  24  from being adhered once again onto the corresponding photosensitive drum  11 . 
     As a result, each photosensitive drum  11  can reliably be cleaned by the corresponding drum cleaning roller  24 . 
     (4-2) Besides, the printer  101  according to the second embodiment can obtain the same operational advantages described for the printer  1  according to the first embodiment. 
     (5) Variation of Second Embodiment 
     Incidentally, only one of the process units  5  and its corresponding transfer roller  19  may be provided with a set of the ammeter  61  and the voltmeter  62 , and the remaining three sets of the ammeters  61  and the voltmeter  62  may be dispensed with. For example, the ammeter  61  may only be interposed between the transfer circuit  45  and the transfer roller shaft  33  of the yellow transfer roller  19 , and the voltmeter  62  may only be electrically connected to the transfer roller shaft  33  of the yellow transfer roller  19  and the shaft  32  of the yellow photosensitive drum  11 . 
     In this case, if the electric resistance of the sheet P between the yellow transfer roller  19  and the yellow photosensitive drum  11  is equal to or higher than, for example, 500 MΩ, the charging bias controller  50  sets the charging bias to be applied to each of the scorotron chargers  12  collectively to be the same value as in the case of the low humidity environment. Similarly, the transfer bias controller  48  sets the transfer current to be made to flow between each of the photosensitive drums  11  and its corresponding transfer roller  19  collectively to be the same value as in the case of the low humidity environment. 
     On the other hand, if the electric resistance between the yellow transfer roller  19  and the yellow photosensitive drum  11  is less than 500 MΩ, the charging bias controller  50  sets the charging bias to be applied to each of the scorotron chargers  12  collectively to be the same value as in the case of the normal humidity environment. Similarly, the transfer bias controller  48  sets the transfer current to be made to flow between each of the photosensitive drums  11  and its corresponding transfer roller  19  collectively to be the same value as in the case of the normal humidity environment. 
     6. Third Embodiment 
     A printer  201  as an image forming apparatus according to a third embodiment of the present invention will be described while referring to  FIG. 4 , wherein like parts and components are designated with the same reference numerals to avoid duplicating description. In the following description, only parts differing from those of the embodiment will be described in detail. 
     (1) Overview of Third Embodiment 
     In the first embodiment described above, the transfer bias controller  48  increases the setting value of the transfer current and the charging bias controller  50  increases the setting value of the charging bias, when the main casing  2  is in the low humidity environment. 
     On the other hand, in the third embodiment, the transfer bias controller  48  increases the setting value of the transfer current and the charging bias controller  50  increases the setting value of the charging bias, when a print job is inputted to the printer  201  from the external personal computer  51 , depending on a type of the sheet P specified in the print job. 
     (2) Setting of Transfer Current and Charging Bias 
     In the third embodiment, the control substrate  42  further includes a determination unit  44  for determining the type and size of the sheet P specified in the print job. When the type of the sheet P specified in the print job is determined by the determination unit  44  as, for example, a sheet whose ratio of left-right length relative to a left-right length of a maximum image formation area of the photosensitive drum  11  is equal to or less than a predetermined value, such as a postcard, or a sheet P having comparatively high electric resistance, such as a glossy paper or a heavy paper, the setting values of both the transfer current and the charging bias are made to increase. 
     Specifically, when the left-right length of the sheet P specified in the print job is equal to or less than, for example, 90%, preferably, 80% relative to the left-right length of the maximum image formation area of the photosensitive drum  11 , the charging bias controller  50  sets the charging bias to be applied to the individual scorotron chargers  12  to be the same values as in the case of the low humidity environment described in the first embodiment. 
     The transfer bias controller  48  sets the transfer current to be made to flow between each of the plurality of photosensitive drums  11  and its corresponding transfer roller  19  to be the same value as in the case of the low humidity environment described in the first embodiment. 
     When the ratio of the left-right length of the sheet P specified in the print job relative to the left-right length of the maximum image formation area of the photosensitive drum  11  is more than, for example, 90%, the charging bias controller  50  sets the charging bias to be applied to the individual scorotron chargers  12  to be the same values as in the case of the normal humidity environment described in the first embodiment. 
     Similarly, the transfer bias controller  48  sets the transfer current to be made to flow between each of the plurality of photosensitive drums  11  and its corresponding transfer roller  19  to be the same value as in the case of the normal humidity environment described in the first embodiment. 
     (3) Operational Advantages of Third Embodiment 
     The printer  201  according to the third embodiment can obtain the same operational advantages described for the printer  1  according to the first embodiment. 
     More in detail, in the third embodiment, when a printing operation is performed for a sheet P requiring larger transfer current, such as one whose ratio of left-right length relative to the left-right length of the maximum image formation area of the photosensitive drum  11  is equal to or less than the above value or one having comparatively high electric resistance, such as a glossy paper or a heavy paper, the setting values of both the transfer current and the charging bias are made to increase. 
     As a result, when a printing operation is performed for a sheet P requiring larger transfer current, a toner image can reliably be transferred onto the sheet P, and the residual toner collected by each drum cleaning roller  24  can be prevented from being adhered once again onto the corresponding photosensitive drum  11 . 
     As a result, each photosensitive drum  11  can reliably be cleaned by the corresponding drum cleaning roller  24 . 
     7. Fourth Embodiment 
     A printer  301  as an image forming apparatus according to a third embodiment of the present invention will be described while referring to  FIG. 5 , wherein like parts and components are designated with the same reference numerals to avoid duplicating description. In the following description, only parts differing from those of the embodiment will be described in detail. 
     (1) Overview of Fourth Embodiment 
     In the first embodiment described above, the charging circuit  47  applies the same charging bias collectively to the yellow, magenta, and cyan scorotron chargers  12  and applies the charging bias independently to the black scorotron charger  12 . 
     On the other hand, in the fourth embodiment, the charging circuit  47  applies the charging bias individually to the plurality of scorotron chargers  12 , as illustrated in  FIG. 5 . 
     (2) Setting of Transfer Current and Charging Bias 
     In the fourth embodiment, when the main casing  2  is in the normal humidity environment, the charging bias controller  50  sets the charging bias to be applied to the yellow scorotron charger  12  to, for example, +650 V. 
     The charging bias controller  50  sets the charging bias to be applied to the magenta scorotron charger  12  to, for example, +700 V. 
     The charging bias controller  50  sets the charging bias to be applied to the cyan scorotron charger  12  to, for example, +700 V. 
     The charging bias controller  50  sets the charging bias to be applied to the black scorotron charger  12  to, for example, +700 V. 
     When the main casing  2  is in the low humidity environment, the charging bias controller  50  sets the charging bias to be applied to the yellow scorotron charger  12  to, for example, +700 V. 
     The charging bias controller  50  sets the charging bias to be applied to the magenta scorotron charger  12  to, for example, +750 V. 
     The charging bias controller  50  sets the charging bias to be applied to the cyan scorotron charger  12  to, for example, +750 V. 
     The charging bias controller  50  sets the charging bias to be applied to the black scorotron charger  12  to, for example, +800 V. 
     That is, the charging bias controller  50  sets the charging bias to be applied individually to the scorotron chargers  12  such that higher charging bias is applied to the scorotron chargers  12  in the order along the front-to-rear direction. That is, the charging bias to be applied to the scorotron charger  12  that is positioned further rearward is set to be higher than the charging bias to be applied to the scorotron charger  12  that is positioned further frontward. 
     The transfer bias controller  48  sets the transfer bias to be applied individually to the plurality of transfer rollers  19  in the same manner as in the first embodiment. 
     In the fourth embodiment, the front more side image forming units among the plurality of image forming units are an example of a first image forming unit, and the image forming units disposed rearward of the first image forming unit is an example of a second image forming unit. 
     For example, when a set of the yellow process unit  5 Y and the yellow transfer roller  19  is assumed to be the first image forming unit, a set of the magenta process unit  5 M and the magenta transfer roller  19 , a set of the cyan process unit  5  and the cyan transfer roller  19 , and a set of the black process unit  5 K and the black transfer roller  19 , which are disposed rearward of the set of the yellow process unit  5 Y and the yellow transfer roller  19 , each constitute the second image forming unit. 
     When the set of the magenta process unit  5 M and the magenta transfer roller  19  is assumed to be the first image forming unit, the set of the cyan process unit  5 C and the cyan transfer roller  19  and the set of black process unit  5 K and the black transfer roller  19 , which are disposed rearward of the set of the magenta process unit  5 M and the magenta transfer roller  19 , each constitute the second image forming unit. 
     When the set of the cyan process unit  5 C and the cyan transfer roller  19  is assumed to be the first image forming unit, the set of the black process unit  5 K and the black transfer roller  19 , which is disposed rearward of the set of the cyan process unit  5  and the cyan transfer roller  19 , constitute the second image forming unit. 
     In the low humidity environment, the charging bias to be applied to the first charging member is an example of a second charging bias, and the charging bias to be applied to the second charging member is an example of a third charging bias. 
     Specifically, when the set of the yellow process unit  5 Y and the yellow transfer roller  19  is assumed to be the first image forming unit, the charging bias of +700 V to be applied to the yellow scorotron charger  12  is an example of the second charging bias; and the charging bias of +750 V to be applied to the magenta scorotron charger  12 , the charging bias of +750 V to be applied to the cyan scorotron charger  12 , and the charging bias of +800 V to be applied to the black scorotron charger  12  are each an example of the third charging bias. 
     When the set of the magenta process unit  5 M and the magenta transfer roller  19  is assumed to be the first image forming unit, the charging bias of +750 V to be applied to the magenta scorotron charger  12  is an example of the second charging bias; and the charging bias of +750 V to be applied to the cyan scorotron charger  12 , and the charging bias of +800 V to be applied to the black scorotron charger  12  are each an example of the third charging bias. 
     When the set of the cyan process unit  5  and the cyan transfer roller  19  is assumed to be the first image forming unit, the charging bias of +750 V to be applied to the cyan scorotron charger  12  is an example of the second charging bias, and the charging bias of +800 V to be applied to the black scorotron charger  12  is an example of the third charging bias. 
     (3) Operational Advantages of Fourth Embodiment 
     (3-1) The printer  301  according to the fourth embodiment can obtain the same operational advantages described for the printer  1  according to the first embodiment. 
     8. Fifth Embodiment 
     A printer  401  as an image forming apparatus according to a fifth embodiment of the present invention will be described while referring to  FIG. 6 , wherein like parts and components are designated with the same reference numerals to avoid duplicating description. In the following description, only parts differing from those of the embodiment will be described in detail. 
     (1) Overview of Fifth Embodiment 
     In the first embodiment described above, the printer  1  is a direct tandem-type color printer. 
     On the other hand, the printer  401  according to the fifth embodiment is an intermediate transfer type color printer, as illustrated in  FIG. 6 . 
     (2) Structure of Fifth Embodiment 
     In the printer  401  of the fifth embodiment, each scorotron charger  12  is disposed opposite to the corresponding photosensitive drum  11  from the lower-rear side thereof and spaced apart therefrom. 
     The drum cleaning roller  24  is disposed above the corresponding scorotron charger  12  at a rear side of the corresponding photosensitive drum  11 . 
     The developing unit  10  is disposed at a lower-front side of the corresponding photosensitive drum  11 . 
     The developing roller  13  is rotatably supported at an upper end portion of the developing unit  10  so as to be exposed upward through an upper edge of the developing unit  10  and contacts the corresponding photosensitive drum  11  from a lower-front side thereof. 
     The supply roller  14  is rotatably supported at the developing unit  10  and contacts the corresponding developing roller  13  from a lower-front side thereof. 
     The transfer unit  7  is disposed above and opposite to the plurality of process units  5 . The transfer unit  7  includes a belt unit  82  and a secondary transfer roller  83 . 
     The belt unit  82  includes the driving roller  16 , the driven roller  17 , an intermediate transfer belt  84  (transfer medium), and a plurality of primary transfer rollers  85 . 
     The driving roller  16  is rotatably supported at a rear end portion of the belt unit  82 . 
     The driven roller  17  is rotatably supported at a front end portion of the belt unit  82 . 
     The intermediate transfer belt  84  is looped around the driving roller  16  and the driven roller  17  in such a manner that a lower portion of the intermediate transfer belt  84  contacts all the photosensitive drums  11  from above. When the driving roller  16  is driven to rotate, the intermediate transfer belt  84  circulates so that its lower portion moves rearward, and the driven roller  17  rotates along the circulating movement of the intermediate transfer belt  84 . 
     Each of the plurality of primary transfer rollers  85  is disposed above and opposite to the corresponding photosensitive drum  11 , with the lower portion of the intermediate transfer belt  84  interposed between the bottom of the primary transfer roller  85  and the top of the corresponding photosensitive drum  11 . 
     Hereinafter, the primary transfer roller  85  corresponding to the yellow photosensitive drum  11  will be referred to as the yellow primary transfer roller  85 ; the primary transfer roller  85  corresponding to the magenta photosensitive drum  11  will be referred to as the magenta primary transfer roller  85 ; the primary transfer roller  85  corresponding to the cyan photosensitive drum  11  will be referred to as the cyan primary transfer roller  85 ; and the primary transfer roller  85  corresponding to the black photosensitive drum  11  will be referred to as the black primary transfer roller  85 . 
     (3) Image Forming Operation in Fifth Embodiment 
     When a print job is inputted to the printer  401 , toner accommodated in each developing unit  10  is tribo-charged between the supply roller  14  and the corresponding developing roller  13 , and is then carried on a surface of the developing roller  13  at a thin uniform thickness. 
     In the meantime, each scorotron charger  12  applies a uniform charge to a surface of the corresponding photosensitive drum  11 . Subsequently, the photosensitive drum  11  is exposed to light by the corresponding LED unit  6  based on predetermined image data. As a result, an electrostatic latent image based on the image data is formed on the surface of the photosensitive drum  11 . The toner carried on the developing roller  13  is then supplied to the electrostatic latent image formed on the surface of the corresponding photosensitive drum  11 . In this way, a toner image is carried on the surface of the photosensitive drum  11 . 
     The toner images carried on the surfaces of the respective photosensitive drums  11  are sequentially transferred onto the lower portion of the intermediate transfer belt  84 . As a result, a color image is formed on a surface of the intermediate transfer belt  84 . 
     The sheets P are accommodated in the sheet supply tray  22  provided at the bottom portion of the main casing  2  and conveyed above and rearward by various rollers. The sheets P are supplied, one at a time, between the intermediate transfer belt  84  and the secondary transfer roller  83  by a predetermined timing. The sheet P then passes between the intermediate transfer belt  84  and the secondary transfer roller  83  from a lower side to an upper side thereof. At this time, the color image is transferred onto the sheet P. 
     The sheet P is subjected to heat and pressure while passing between the heating roller  20  and the pressure roller  21  of the fixing unit  8 , thereby thermally fixing the color image onto the sheet P. Thereafter, the sheet P is discharged to the discharge tray  23 . 
     (4) Setting of Transfer Current and Charging Bias 
     In the fifth embodiment, the charging bias controller  50  sets the charging bias to be applied individually to the plurality of scorotron chargers  12  in the same manner as in the first embodiment. 
     The transfer bias controller  48  sets the transfer current to be made to flow between each of the plurality of photosensitive drums  11  and its corresponding primary transfer roller  85  in the same manner as in the first embodiment. 
     (5) Operational Advantages of Fourth Embodiment 
     The printer  401  according to the fifth embodiment can obtain the same operational advantages described for the printer  1  according to the first embodiment. 
     9. Variations of Embodiments 
     (1) Although the control substrate  42  includes the CPU in each of the above-described embodiments, the control substrate  42  may include an ASIC, i.e., an application specific integrated circuit, in place of the CPU. 
     (2) In each of the above-described embodiments, the residual toner adhered onto the photosensitive roller  11  may be collected also by means of the corresponding developing roller  13 . 
     In this case, in a state where the surface potential of the photosensitive drum  11  is prevented from being excessively lowered due to an increase in the value of the transfer current, the residual toner adhered onto the photosensitive drum  11  can reliably be cleaned by means of the corresponding drum cleaning roller  24  and the corresponding developing roller  13 . 
     (3) In each of the above-described embodiments, when continuous printing for the plurality of sheets P is instructed in the print job, the transfer bias controller  48  may increase the value of the transfer current and the charging bias controller  50  may increase the value of the charging bias, for the second and subsequent sheets P. 
     (4) Although the printer  1  is designed as a color printer in each of the above-described embodiments, the printer  1  may be designed as a monochromatic printer. 
     (5) The above-described embodiments may be combined each other to form another embodiment of the invention. 
     For example, the first, second, and third embodiments may be combined to one another. 
     Further, for example, to the fifth embodiment, at least one of the first, second, third, and fourth embodiments may be applied. 
     While the present invention has been described in detail with reference to the embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the present invention.