Patent Publication Number: US-6985681-B2

Title: Image forming apparatus and image forming method

Description:
This is a continuation of U.S. application Ser. No. 10/372,658 filed Feb. 25, 2003. Now U.S. Pat. No. 6,801,728 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming utilizing electrophotography, such as a printer, a copier machine and a facsimile machine. 
   2. Description of the Related Art 
   In a conventional image forming apparatus utilizing electrophotography, developing means adheres toner to an electrostatic latent image which is formed on a photosensitive member by exposure means, a toner image is accordingly formed and then transferred onto a transfer paper, and the toner image formed on the transfer paper is fixed by fixing means on the transfer paper. Known in particular as an apparatus which permits to form a color image is an image forming apparatus in which a toner image which is formed on a photosensitive member is primarily transferred onto an intermediate transfer medium and thus primarily transferred image which is on the intermediate transfer medium is secondarily transferred onto a transfer paper. 
   There are two typical types of color image forming apparatuses. An image forming apparatus of one of the two types comprises one photosensitive member for example. In such an image forming apparatus, toner images of different colors are created one after another on the photosensitive member while primarily transferring each toner image onto an intermediate transfer medium every time a toner image in each color is formed, a color toner image, which is an overlap of the toner images of the plurality of colors, is accordingly formed on the intermediate transfer medium, and thus formed color toner image is secondarily transferred onto a transfer paper, whereby a color image is obtained. 
   Known as an image forming apparatus of the other type is an image forming apparatus of the so-called tandem type in which a plurality of photosensitive members are disposed in a direction of rotational driving of an intermediate transfer medium which rotates such that the photosensitive members are faced with the intermediate transfer medium. In such an image forming apparatus, toner images in different colors are created on the respective photosensitive members, thus formed toner images are primarily transferred on the rotating intermediate transfer medium so as to superimpose the toner images on top of the other, and a color toner image resulting from the superimposition is secondarily transferred onto a transfer paper. 
   By the way, primary transfer described above is realized as a primary transfer bias is applied between an intermediate transfer medium and a photosensitive member for instance, while secondary transfer described above is realized as a secondary transfer bias is applied between the intermediate transfer medium and a secondary transfer member, which is disposed to face with the intermediate transfer medium, with a transfer paper for instance interposed between the intermediate transfer medium and the secondary transfer member. 
   Among known as this type requires to output different primary transfer bias values for the different colors, or change an output value of the primary transfer bias or the secondary transfer bias in accordance with an environmental condition such as a temperature, a humidity level, etc. 
   In a configuration that the primary transfer bias is controlled to a constant voltage, since a potential difference is maintained constant in a primary transfer unit, even when an output value of the secondary transfer bias changes during application of the primary transfer bias, the change does not influence primary transfer almost at all. 
   On the contrary, when an output value of the primary transfer bias changes during application of the secondary transfer bias, since an electric field between the intermediate transfer medium and the secondary transfer member changes, there is a risk that secondary transfer will become instable. Particularly in a configuration that the intermediate transfer medium comprises a plurality of layers including a conductive layer, since application of the primary transfer bias upon the intermediate transfer medium is application upon the entire intermediate transfer medium which is not limited to the primary transfer unit but also includes the secondary transfer unit, the change of the output value of the primary transfer bias exerts a large influence over secondary transfer. 
   Hence, it is preferable to change the output value of the primary transfer bias in accordance with various conditions, since the materials of toner in the respective colors are different, since the toner in the respective colors is accumulated on the intermediate transfer medium, since the transfer efficiency changes because of a change in temperature or humidity, or for other reasons. It is also preferable to determine the timing of changing the output value of the primary transfer bias so that the timing will not adversely affect secondary transfer. However, the output value of the primary transfer bias needs be determined before starting primary transfer. 
   An image forming apparatus of the tandem type, a monochrome image forming apparatus or the like which comprises one photosensitive member in particular often uses a structure that an intermediate transfer medium moves passed a primary transfer unit immediately after moving passed a secondary transfer unit, in an attempt to reduce the size of the apparatus. Therefore, next primary transfer starts while secondary transfer is still ongoing depending on the size of a transfer paper, and the output value of the primary transfer bias will change during execution of secondary transfer but for countermeasure. Noting this, it is important to set up the timing of changing the output value of the primary transfer bias in such a manner that secondary transfer will not be negatively influenced. 
   SUMMARY OF THE INVENTION 
   A primary object of the present invention is to provide an image forming apparatus and an image forming method which prevent a change in output value of a primary transfer bias from adversely influencing secondary transfer in a configuration that an intermediate transfer medium comprises a plurality of layers including a conductive layer. 
   The present invention is directed to an image forming apparatus in which a toner image formed on a photosensitive member is transferred onto a transfer paper through an intermediate transfer medium which comprises a plurality of layers including a conductive layer and which moves from a primary transfer part to a secondary transfer part by rotation, said apparatus comprises: primary transfer means which primarily transfers the toner image from the photosensitive member onto the intermediate transfer medium in the primary transfer part by applying a primary transfer bias which is determined in advance upon the conductive layer of the intermediate transfer medium; secondary transfer means which secondarily transfers the toner image now on the intermediate transfer medium onto a transfer paper in the secondary transfer part; and bias control means which changes an output value of the primary transfer bias in accordance with a predetermined bias change condition when secondary transfer is not ongoing. 
   The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a drawing which shows an inner structure of a printer which is a preferred embodiment of an image forming apparatus according to the present invention; 
       FIG. 2  is a block diagram which shows an electric structure of the printer; 
       FIG. 3  is a cross sectional view of an intermediate transfer belt; 
       FIGS. 4A and 4B  are development views of the intermediate transfer belt; 
       FIG. 5  is a drawing which schematically shows an example of a structure of a transfer bias generating circuit; 
       FIG. 6  is a timing chart for describing operations, which shows time-induced changes appearing in the conditions of the respective portions of an engine part; 
       FIG. 7  is a timing chart for describing other operations, which shows time-induced changes appearing in the conditions of the respective portions of the engine part; 
       FIG. 8  is a timing chart for describing still other operations, which shows time-induced changes appearing in the conditions of the respective portions of the engine part; 
       FIG. 9  is a drawing of a modification; 
       FIG. 10  is a development view of an intermediate transfer belt; 
       FIG. 11  is a timing chart for describing operations, which shows time-induced changes appearing in the conditions of the respective portions of an engine part; 
       FIG. 12  is a drawing which shows an inner structure of a printer which is a preferred embodiment of an image forming apparatus according to the present invention; 
       FIG. 13  is a development view of an intermediate transfer belt; 
       FIG. 14  is a timing chart for describing operations, which shows time-induced changes appearing in the conditions of the respective portions of an engine part; 
       FIG. 15  is a flow chart which shows one example of the sequence of changing an output value of a primary transfer bias; and 
       FIG. 16  is a timing chart for describing an example of different operations, which shows time-induced changes appearing in the conditions of the respective portions of an engine part. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   &lt;First Preferred Embodiment&gt; 
   First, referring to  FIGS. 1 through 5 , a structure of a printer which is a preferred embodiment of an image forming apparatus according to the present invention will now be described.  FIG. 1  is a drawing which shows an inner structure of the printer,  FIG. 2  is a block diagram which shows an electric structure of the printer,  FIG. 3  is a cross sectional view of an intermediate transfer belt,  FIGS. 4A and 4B  are development views of the intermediate transfer belt, and  FIG. 5  is a drawing which schematically shows an example of a structure of a transfer bias generating circuit. 
   This printer is for superimposing toner in four colors, which are yellow (Y), magenta (M), cyan (C) and black (K), and thereby forming a full color image, or for forming a single-color image using only toner in the black color (K) for instance. In this printer, when a print instruction signal containing a video signal is fed to a main controller  100  from an external apparatus such as a host computer, an engine controller  110  controls each portion of an engine part  1  in accordance with a control signal from the main controller  100 , and the printer prints out an image corresponding to the video signal on a transfer paper  4  transported from a paper feeding cassette  3  which is disposed in a lower section of a main unit  2 . 
   The engine part  1  comprises a photosensitive member unit  10 , a rotary developer  20 , an intermediate transfer unit  30 , a fixing unit  40 , and an exposure unit  50 . The photosensitive member unit  10  comprises a photosensitive member  11 , an electrifier  12  and a cleaner  13 . The rotary developer  20  comprises a yellow developer unit  2 Y housing yellow toner, a magenta developer unit  2 M housing magenta toner, a cyan developer unit  2 C housing cyan toner, a black developer unit  2 K housing black toner, etc. The intermediate transfer unit  30  comprises an intermediate transfer belt  31 , a vertical synchronization sensor  32 , a belt cleaner  33 , a gate roller pair  34 , a secondary transfer roller  35 , a photosensitive member driving motor  36 , etc. These seven units  10 ,  2 Y,  2 M,  2 C,  2 K,  30  and  40  are formed so that these units can be freely attached to and detached from the main unit  2 . 
   This printer has such a structure which allows to primarily transfer a toner image currently on the photosensitive member  11  onto the intermediate transfer belt  31  and secondarily transfer the primarily transferred toner image onto the transfer paper  4 . An output value of a primary transfer bias is changed in accordance with a predetermined bias changing condition. 
   With the seven units  10 ,  2 Y,  2 M,  2 C,  2 K,  30  and  40  described above mounted to the main unit  2 , the photosensitive member  11  of the photosensitive member unit  10  is rotated by the photosensitive member driving motor  36  in the direction of an arrow  5 . Along the rotating direction  5  of the photosensitive member  11 , the electrifier  12 , the rotary developer  20  and the cleaner  13  are disposed around the photosensitive member  11 . 
   The electrifier  12  comprises a wire electrode to which a high voltage at a predetermined level is applied. Utilizing corona discharge for instance, the electrifier  12  uniformly electrifies an outer circumferential surface of the photosensitive member  11 . The cleaner  13  is disposed on the upstream side to the electrifier  12  in the rotating direction  5  of the photosensitive member  11 . The cleaner  13  scrapes off toner which remains on the outer circumferential surface of the photosensitive member  11  after primary transfer of a toner image onto the intermediate transfer belt  31  from the photosensitive member  11 , to thereby clean the surface of the photosensitive member  11 . 
   The exposure unit  50  comprises a laser light source  51  which is formed by a semiconductor laser for instance, a polygon mirror  52  which reflects laser light from the laser light source  51 , a scanner motor  53  which drives the polygon mirror  52  so that the polygon mirror  52  rotates at a high speed, a lens part  54  which converges the laser light reflected by the polygon mirror  52 , a plurality of reflection mirrors  55 , a horizontal synchronization sensor  56 , etc. Leaving the lens part  54  and the reflection mirrors  55  after reflected by the polygon mirror  52 , laser light  57  scans the surface of the photosensitive member  11  in a main scanning direction (a direction which is perpendicular to the plane of  FIG. 1 ), whereby an electrostatic latent image corresponding to the video signal is formed on the surface of the photosensitive member  11 . At this stage, the horizontal synchronization sensor  56  provides a synchronizing signal which is in the main scanning direction, i.e., a horizontal synchronizing signal. The exposure unit  50  functions as exposure means. 
   The rotary developer  20  is for making the toner in the respective colors adhere to the electrostatic latent image to thereby develop the electrostatic latent image. The yellow developer unit  2 Y, the magenta developer unit  2 M, the cyan developer unit  2 C and the black developer unit  2 K of the rotary developer  20  are disposed for free rotations about an axis. These developer units  2 Y,  2 M,  2 C and  2 K are movable to a plurality of predetermined positions, and are selectively located at an abutting position on the photosensitive member  11  and a separated position from the photosensitive member  11 . When a developing bias is applied which is a direct current component as it is alone or a direct current component on which an alternating current component is superimposed, from the developer unit which is at the abutting position relative to the photosensitive member  11 , the toner in the corresponding color adheres to the surface of the photosensitive member  11 . The rotary developer  20  functions as developing means. 
   The intermediate transfer belt  31  of the intermediate transfer unit  30  stretches around a plurality of rollers, and is driven by the photosensitive member driving motor  36  and accordingly rotates together with the photosensitive member  11 . As shown in the cross sectional view in  FIG. 3 , the intermediate transfer belt  31  is formed by a resistance layer  81  on the surface, a conductive layer  82  which is in the middle, and a base material portion  83  which is the bottom most layer. The resistance layer  81  is made of synthetic resin (which may be urethane resin for instance) having predetermined thickness (which may be 20 μm for instance), and contains conductive particles (SnO 2  for instance)  84 , fluororesin (polytetrafluoroethylene for instance) particles  85 , etc. The resistance value of the resistance layer  81  is set to about 10 8  through 10 14  Ω since the conductive particles  84  are contained while frictional resistance is suppressed since the fluororesin particles  85  are contained, thereby preventing locking of the intermediate transfer belt  31  by the belt cleaner  33  (which will be described later). 
   The conductive layer  82  is formed by deposition of aluminum for example. The base material portion  83  has predetermined thickness (100 μm for instance), and is made of synthetic resin (which may be polyethylene terephthalate for instance). In this manner, a cost is reduced as the resistance layer  81  and the conductive layer  82 , which are layers realizing an electric function, are separated from the base material portion  83  which is a layer for providing mechanical strength. 
   In addition, as shown in the development views in  FIGS. 4A and 4B , the intermediate transfer belt  31  is formed as an endless belt which is obtained by joining an approximately rectangular sheet at a splice  71  so as to span over the length L 0 . In  FIGS. 4A and 4B , an arrow  72  denotes a direction of rotational driving, while an arrow  73  denotes a direction of rotation axis. On one edge side along the direction of rotation axis  73  (on the top side in  FIGS. 4A and 4B ), a projection  74  is disposed to the intermediate transfer belt  31 . 
   The intermediate transfer belt  31  contains a transfer protection area  75  and a transfer area  76 . The transfer protection area  75  is defined across one edge and the other edge along the direction of rotation axis  73  and within a predetermined range which stretches on the both sides to the splice  71 , and in the transfer protection area  75 , primary transfer of a toner image is prohibited. The transfer area  76  is an area other than the transfer protection area  75 , and expands in a rectangular area except for a one edge portion and other edge portion along the direction of rotation axis  73 . The transfer area  76  has a larger size than that of an A3 paper as it is placed with the longer sides aligned along the direction of rotational driving  72 . As shown in  FIG. 4A , it is possible to transfer an image  77  whose size is that of an A3 paper as it is placed with the longer sides aligned along the direction of rotational driving  72 . Further, as shown in  FIG. 4B , the transfer area  76  can be split into two sub areas  76 A and  76 B. Therefore it is possible to transfer two images  78  each having the size of an A4 paper with the shorter sides aligned along the direction of rotational driving  72 , while the intermediate transfer belt  31  rotates one round. 
   As shown in  FIGS. 4A and 4B , the conductive layer  82  is exposed at the surface on the other edge side (the bottom side in  FIGS. 4A and 4B ) of the intermediate transfer belt  31  along the direction of rotation axis  73 . A primary transfer bias is applied to the exposed portion through a bias applying member  31 A (See  FIG. 2 .) so that a toner image on the photosensitive member  11  will be primarily transferred onto the intermediate transfer belt  31  because of this primary transfer bias. The abutting position at which the photosensitive member  11  contacts the intermediate transfer belt  31  is provided within a primary transfer part  14 . 
   Referring to  FIGS. 1 and 2  again, the vertical synchronization sensor  32  is formed by a photo-interrupter which comprises a light emitter (such as an LED) and a light receiver (such as a photo diode) which are disposed so as to face each other for instance. The vertical synchronization sensor  32  is disposed on the one edge side of the rotating intermediate transfer belt  31  along the direction of rotation axis  73  and detects a passage of the projection  74 . The resulting detection signal is used as a vertical synchronizing signal (reference signal) which the engine controller  110  refers to when controlling formation of an image. The belt cleaner  33  is disposed so as to be switched by a cleaner contacting/clearing clutch between an abutting state (denoted by the solid line in  FIG. 1 ) abutting on the intermediate transfer belt  31  and a cleared-off state (denoted by the dotted line in  FIG. 1 ). In the abutting state, the belt cleaner  33  scrapes off toner which remains on the intermediate transfer belt  31 . When a gate clutch is turned on, the drive force of a transportation system driving motor  60  is transmitted to the gate roller pair  34  and the gate roller pair  34  accordingly rotates. 
   A contacting/clearing clutch for secondary transfer roller switches the secondary transfer roller  35  between an abutting state (denoted by the solid line in  FIG. 1 ) abutting on the intermediate transfer belt  31  and a cleared-off state (denoted by the dotted line in  FIG. 1 ). When applied with a predetermined secondary transfer bias in the abutting state abutting on the intermediate transfer belt  31 , the secondary transfer roller  35  secondarily transfers a primarily transferred toner image currently on the intermediate transfer belt  31  onto the transfer paper  4  while the transfer paper  4  is transported. This abutting position is located in a secondary transfer part  37 . 
   The fixing unit  40  comprises a heating roller  41  and a pressure roller  42 , and fixes a toner image on the transfer paper  4  by a heating roller fixing method while transporting the transfer paper  4  so that the toner image will be fixed to the transfer paper  4 . The fixing unit  40  therefore constitutes fixing means. 
   A crescent-shaped pick-up roller  61  and a feed roller pair  62  are disposed toward above from the front edge of the paper feeding cassette  3  (the right-most edge in  FIG. 1 ), and on the opposite side to the gate roller pair  34 , the secondary transfer roller  35  and the fixing unit  40 , a transportation roller pair  63  and a discharge roller pair  64  are disposed, whereby a transportation path for the transfer papers  4  (denoted at the chain line in  FIG. 1 ) is formed. The transfer papers  4  discharged by the discharge roller pair  64  accumulate in a discharging part  9 . 
   The pick-up roller  61  is driven by a pick-up solenoid. The feed roller pair  62 , the gate roller pair  34 , the secondary transfer roller  35 , the fixing unit  40 , the heating roller  41 , the transportation roller pair  63  and the discharge roller pair  64  are each linked to the same transportation system driving motor  60  via a drive force transmission mechanism. When a feed clutch is turned on, the drive force of the transportation system driving motor  60  is transmitted to the feed roller pair  62 , and the feed roller pair  62  accordingly rotates. The transportation system driving motor  60  transports the transfer paper  4  at a predetermined transportation speed. The feed roller pair  62 , the gate roller pair  34 , the transportation roller pair  63  and the discharge roller pair  64  constitute transporting means for the transfer papers  4 . 
   The engine part  1  comprises a temperature sensor  6  which detects the temperature of an atmosphere and a humidity sensor  7  which detects the humidity level of the atmosphere. The temperature sensor  6  and the humidity sensor  7  respectively constitute temperature detecting means and humidity detecting means each serving as environment condition detecting means. 
   In  FIG. 2 , the main controller  100  comprises a CPU  101 , an interface  102  which transfers a control signal with the external apparatus such as a host computer, and an image memory  103  which stores the video signal received through the interface  102 . Upon receipt of the print instruction signal containing the video signal from the external apparatus via the interface  102 , the CPU  101  converts the same into job data which are in a format appropriate to provide the engine part  1  with an instruction for operation, and sends the data to the engine controller  110 . 
   The engine controller  110  comprises the CPU  111 , a ROM  112 , a RAM  113 , etc. The ROM  112  stores a control program of the CPU  111 , etc. The RAM  113  temporarily stores control data of the engine part  1 , a result of computation by the CPU  111 , etc. 
   As input signals from the engine part  1 , the CPU  111  receives the vertical synchronizing signal Vsync from the vertical synchronization sensor  32 , the horizontal synchronizing signal Hsync from the horizontal synchronization sensor  56 , and information regarding environment conditions, i.e., the temperature of the atmosphere and the humidity level of the atmosphere, from the temperature sensor  6  and the humidity sensor  7 . Based on these input signals and the control program, the CPU  111  controls operations of the respective portions of the engine part  1 . 
   That is, the CPU  111  sends a control signal to a motor drive circuit  114  which drives the photosensitive member driving motor  36 , synchronizes the photosensitive member  11  and the intermediate transfer belt  31  to each other, and drives these. Further, the CPU  111  sends a control signal to a motor drive circuit  115  which drives the transportation system driving motor  60 , and controls feeding of the transfer paper  4  from the paper feeding cassette  3 . 
   In addition, the CPU  111  sends a control signal to a drive circuit which drives the cleaner contacting/clearing clutch, and controls clearing off of the belt cleaner  33  from the intermediate transfer belt  31  and abutting of the belt cleaner  33  on the intermediate transfer belt  31 . Still further, the CPU  111  sends a control signal to a drive circuit which drives the contacting/clearing clutch for secondary transfer roller, and controls clearing off of the secondary transfer roller  35  from the intermediate transfer belt  31  and abutting of the secondary transfer roller  35  on the intermediate transfer belt  31 . 
   The CPU  111  receives the content of an operation made on an operating key of an operation display panel  8  which is disposed on the surface of the main unit  2  for instance, and controls the content of what is displayed on a display part. When two or more images are to be formed in a size which can be transferred two images during one rotation of the intermediate transfer belt  31  (for instance, the A4 size or smaller size with the shorter sides aligned along the direction of rotational driving  72 ), the CPU  111  controls formation of images on the photosensitive member  11  such that toner images will be transferred one in the sub area  76 A and the other in the sub area  76 B within the transfer area  76 . 
   Still further, the CPU  111  sends a control signal to a primary transfer bias generating circuit  116  which generates the primary transfer bias, to thereby control application of the primary transfer bias upon the intermediate transfer belt  31 . The CPU  111  sends a control signal also to a secondary transfer bias generating circuit  117  which generates the secondary transfer bias, to thereby control application of the secondary transfer bias upon the secondary transfer roller  35 . 
   As shown in  FIG. 5 , the CPU  111  sends control data to a D/A convertor  121  of the primary transfer bias generating circuit  116 . The D/A convertor  121  is for control of a drive part  122  based on the control data received from the CPU  111 . The D/A convertor  121  controls the drive part  122  to a constant voltage (which is a voltage value which is set in advance within a range of about 50 to 400 V for instance), and hence, application of the primary transfer bias. 
   Further, the CPU  111  sends control data to a D/A convertor  123  of the secondary transfer bias generating circuit  117 . The D/A convertor  123  is for control of a drive part  124  based on the control data received from the CPU  111 . By means of constant current (a current value which is set in advance within a range of about 1 to 100 μA for instance) control added with lower limit voltage (a voltage value which is set in advance within a range of about 500 to 3000 V for instance) control, the D/A convertor  123  controls application of the secondary transfer bias. In other words, voltage control is performed until a lower limit voltage is reached and constant current control is thereafter performed. 
   In  FIG. 5 , a load  125  is equivalent to the resistance components of the photosensitive member  11 , the bias applying member  31 A and the like, and a load  126  is equivalent to the resistance components of the secondary transfer roller  35 , the intermediate transfer belt  31  and the like. 
   The CPU  111  also serves to change the output value of the primary transfer bias to the D/A convertor  121  as described below. In this embodiment in particular, when a non-image area on the intermediate transfer belt  31  bearing no transferred toner image is moving through the primary transfer part  14  and secondary transfer is not ongoing, the CPU  111  changes the output value of the primary transfer bias in accordance with a predetermined bias change condition. 
   The “non-image area” is the transfer protection area  75  for instance. Alternatively, the non-image area may merely be an area onto which no toner image has been transferred and which contains the transfer protection area  75 . Further, mentioned as “a non-image area is moving passed the primary transfer part  14 ” is a state that the rear edge of the transfer protection area  75 , namely, the front edge of the transfer area  76  (the downstream edge along the direction of rotational driving  72 ) for example has yet arrived at the primary transfer part  14 . 
   Used as the predetermined bias change conditions described above are environment conditions including the temperature of the atmosphere and the humidity level of the atmosphere obtained by the temperature sensor  6  and the humidity sensor  7 . Also used as the predetermined bias change conditions for formation of a color image is the order of primary transfer of the toner which will be superimposed with each other on the intermediate transfer belt  31 , and the output value of the primary transfer bias is changed for every primary transfer of a toner image. 
   Even when it is not necessary to change the output value of the primary transfer bias in accordance with the bias change conditions, the CPU  111  sends control data to the D/A convertor  121  of the primary transfer bias generating circuit  116 . Hence, even if a noise for example creates a garbage content in the control data fed to the D/A convertor  121 , it is possible to prevent the primary transfer bias generating circuit  116  from continuously operating using such abnormal data. 
   The intermediate transfer belt  31  corresponds to an intermediate transfer medium, while the bias applying member  31 A and the primary transfer bias generating circuit  116  correspond to primary transfer means, and the secondary transfer roller  35  and the secondary transfer bias generating circuit  117  correspond to secondary transfer means. Meanwhile, the CPU  111  corresponds to bias control means, toner image formation control means, bias change judging means and second transfer judging means. 
   Referring to  FIG. 6 , an operation of this printer will now be described.  FIG. 6  is a timing chart which shows time-induced changes appearing in the conditions of the respective portions of the engine part  1 . The illustrate example is a situation that four images of such a size that can be transferred two images during one rotation of the intermediate transfer belt  31  are to be formed, and therefore, the output value of the primary transfer bias is changed for every primary transfer of a toner image which will be superimposed. The video signal is activated in response to a video request signal Vreq and in synchronization to the rotating intermediate transfer belt  31 , and therefore, the timing of the turning on of the video signal is in a predetermined delay from the timing of the video request signal Vreq. However, for convenience of illustration, the video signal turns on in synchronization to the video request signal Vreq in  FIG. 6 . 
   As the print instruction signal containing the video signal is fed to the main controller  100  from the external apparatus such as a host computer, in response to the control signal received from the main controller  100 , the engine controller  110  starts operating the respective portions of the engine part  1 . At this stage, if the size of the transfer papers  4  stacked up in the paper feeding cassette  3  fails to match with the size designated by the print instruction signal, the operation display panel  8  shows a message which encourages to replace the paper feeding cassette. Although  FIG. 1  shows the printer as a printer which comprises one paper feeding cassette  3 , this is not limiting. Instead, the printer may comprise a plurality of paper feeding cassettes. 
   When the size of the transfer papers  4  stacked up in the paper feeding cassette  3  matches with the size designated by the print instruction signal, by means of the laser light  57  emitted from the exposure unit  50 , an electrostatic latent image corresponding to the video signal described above is formed on the surface of the photosensitive member  11  which is uniformly electrified by the electrifier  12 . The rotary developer  20  develops the electrostatic latent image, thereby forming a toner image. In the primary transfer part  14 , the toner image thus formed on the photosensitive member  11  is primarily transferred onto the intermediate transfer belt  31 . 
   That is, the photosensitive member driving motor  36  rotates the intermediate transfer belt  31  at a predetermined peripheral velocity, and the vertical synchronizing signal Vsync is outputted at the time t 1 , t 2 , t 3 , t 4 , t 5 , t 6  and t 7  as shown in  FIG. 6 . After a predetermined period T 1  since the falling edges of the vertical synchronizing signal Vsync at t 1 , t 2 , t 3  and t 4 , the video request signal Vreq for the first image is outputted. In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to the video signal representing the first image is started, concurrently with which the developing bias is turned on. Meanwhile, after a predetermined period T 2  (&gt;T 1 ) since the falling edges of the vertical synchronizing signal Vsync, the video request signal Vreq for the second image is outputted. In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to the video signal representing the second image is started. 
   The developing units of the rotary developer  20  switch over with each other at the time t 1 , t 2 , t 3  and t 4 , whereby toner images in the respective colors are formed on the photosensitive member  11  and primarily transferred one after another onto the intermediate transfer belt  31 . At this stage, after a predetermined period T 3  from the time t 1 , t 2 , t 3  and t 4 , the output value from the primary transfer bias generating circuit  116  is changed. 
   In this embodiment, the primary transfer bias for the first image (Y) is set to a voltage V 1  (V 1 =220 V for example), the primary transfer bias for the second image (C) is set to a voltage V 2  (V 2 =245 V for example), the primary transfer bias for the third image (M) is set to a voltage V 3  (V 3 =270 V for example), and the primary transfer bias for the fourth image (K) is set to a voltage V 4  (V 4 =300 V for example). 
   The predetermined period T 3  is set in advance such that changing of the primary transfer bias to be applied upon the intermediate transfer belt  31  will have completed before the front edge of a toner image on the photosensitive member  11  which was formed to match in terms of timing with the sub area  76 A of the intermediate transfer belt  31  (the downstream edge along the direction of rotational driving  72 ) reaches the primary transfer part  14  (i.e., while the transfer protection area  75  serving as the non-image area is moving through the primary transfer part  14 ). Further, since secondary transfer is not ongoing after the predetermined period T 3  since the time t 1 , t 2 , t 3  and t 4 , the output value of the primary transfer bias is changed to V 1 , V 2 , V 3  and V 4 , respectively. 
   Since the secondary transfer roller  35  stays cleared off from the intermediate transfer belt  31  during this, the toner images in the respective colors are superimposed one atop the other on the intermediate transfer belt  31 . The developing bias is turned off after a predetermined period of time which is determined in advance depending on the size of the transfer papers since the falling edges of the vertical synchronizing signal Vsync at the time t 1 , t 2 , t 3  and t 4 . 
   As a result, a color image which is toner images Y 1 , C 1 , M 1  and K 1  as they are superimposed one atop the other is primarily transferred onto the sub area  76 A which is on the downstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 , and a color image which is toner images Y 2 , C 2 , M 2  and K 2  as they are superimposed one atop the other is primarily transferred onto the sub area  76 B which is on the upstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   On the other hand, the top-most transfer paper  4  among the bundle of transfer papers housed in the paper feeding cassette  3  is taken out by the pick-up roller  61 , transported by the feed roller pair  62  at a predetermined speed, and nipped by the gate roller pair  34 . The gate clutch turns on in synchronization to a toner image on the intermediate transfer belt  31 , and the transfer paper  4  is transported toward the secondary transfer part  37  from the gate roller pair  34  at a predetermined speed. 
   The contacting/clearing clutch for secondary transfer roller turns on at the time t 11  which is after a predetermined period from the time t 4 , and the secondary transfer roller  35  accordingly abuts on the intermediate transfer belt  31 . Following this, at the time t 12  which is after a predetermined period since the time t 4 , application of the secondary transfer bias from the secondary transfer bias generating circuit  117  upon the secondary transfer roller  35  is activated. 
   This realizes transfer onto the first transfer paper  4  of the color image which is the toner images Y 1 , C 1 , M 1  and K 1  as they are superimposed one atop the other and which was primarily transferred onto the sub area  76 A which is on the downstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   The gate clutch is temporarily turned off after discharging of the first transfer paper  4 . A period during which the secondary transfer bias is applied is set in advance in accordance with the size of the transfer papers  4 . At this stage, the next transfer paper  4  is taken out by the pick-up roller  61 , transported by the feed roller pair  62  at a predetermined speed, and nipped by the gate roller pair  34 . 
   After turning off of the gate clutch and inactivation of application of the secondary transfer bias, the gate clutch turns on in synchronization to the next toner image and the next transfer paper  4  is transported, and application of the secondary transfer bias turns on at the time t 13  which is after a predetermined period since the time t 4 . When the preset period during which the secondary transfer bias is applied elapses, application of the secondary transfer bias turns off, and at the time t 14  which is after a predetermined period since the time t 4 , the contacting/clearing clutch for secondary transfer roller turns on and the secondary transfer roller  35  leaves the intermediate transfer belt  31 . 
   This realizes transfer onto the second transfer paper  4  of the color image which is toner images Y 2 , C 2 , M 2  and K 2  as they are superimposed one atop the other and which was primarily transferred onto the sub area  76 B which is on the upstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   In the fixing unit  40 , this toner image is fixed on the transfer paper during transportation of the transfer paper  4 . The transfer paper  4  is further transported by the transportation roller pair  63  and discharged by the discharge roller pair  64  into the discharging part  9  which is disposed to an upper section of the main unit  2 . 
   While the third and the fourth images are formed on the photosensitive member  11  following this, since the period during which the secondary transfer bias is applied is determined in advance in accordance with the size of the transfer papers  4 , at the time t 5  which is the next outputting of the vertical synchronizing signal Vsync, it is already possible to determine whether the secondary transfer bias is on after the predetermined period T 3  from the time t 5 . In  FIG. 6 , since the secondary transfer bias is on after the predetermined period T 3  from the time t 5 , the output value of the primary transfer bias is not changed, and therefore, formation of image is held off by the time t 6  at which the vertical synchronizing signal Vsync is outputted next time. 
   After a predetermined period since the time t 6 , toner images Y 3  and Y 4  are formed which are to form the third and the fourth images, and after the predetermined period T 3  since the time t 6 , the output value of the primary transfer bias is changed to V 1  and an operation similar to the above is performed. 
   As described above, during the operations shown in  FIG. 6 , since changing of the output value of the primary transfer bias completes while the transfer protection area  75  serving as the non-image area is still moving through the primary transfer part  14 , it is possible to change the output value without fail prior to the start of primary transfer. 
   Further, since the primary transfer bias is changed for every primary transfer of a toner image, it is possible to primarily transfer the toner of the respective colors onto the intermediate transfer belt  31  in an excellent manner in accordance with the toner transfer efficiency which changes depending on the thickness of the stacked-up toner on the intermediate transfer belt  31 , and hence, to obtain a color image having a high image quality on the transfer paper  4 . 
   In addition, since the output value of the primary transfer bias is changed when secondary transfer is not ongoing, it is possible to prevent a change to the primary transfer bias from adversely affecting secondary transfer, and hence, the quality of an image transferred onto the transfer paper  4  from deteriorating. 
   Referring to  FIG. 7 , other operations of this printer will now be described.  FIG. 7  is a timing chart which shows time-induced changes appearing in the conditions of the respective portions of the engine part  1 . The illustrate example is a situation that one wishes to form four images of such a size that can be transferred two images during one rotation of the intermediate transfer belt  31  as monochrome images and then to form a monochrome image whose size permits to be transferred only one image during one rotation of the intermediate transfer belt  31 (e.g., the A3 size). The same symbols as those used in  FIG. 6  denote the same timing. For convenience of illustration,  FIG. 7  shows as if the video signal turns on in synchronization to the video request signal Vreq as in  FIG. 6 . 
   In  FIG. 7 , the primary transfer bias is changed depending on the environment conditions representing the temperature and the humidity level. For convenience of description, the output value is changed always at the timing of changing the primary transfer bias assuming that there is a change to the environment conditions. Further,  FIG. 7  assumes that the printer comprises two paper feeding cassettes  3  in which the transfer papers  4  of the sizes described above are held. In addition, since a monochrome image (in K for instance) is formed in  FIG. 7 , the black developer unit  2 K remains to serve as the rotary developer  20 . Hence, the developing bias stays turned on until an image has been formed. Since superimposition of toner images is not performed, until an image has been formed, the secondary transfer roller  35  is maintained abutting on the intermediate transfer belt  31 . 
   The table below shows one example of the output value of the primary transfer bias in accordance with the environment conditions representing the temperature and the humidity level during the operation shown in  FIG. 7 . In Table 1, the symbol TP denotes the temperature while the symbol HM denotes the humidity. Since the transfer efficiency decreases as the temperature and the humidity increase, the output value of the primary transfer bias is increased as the temperature and the humidity increase as shown in Table 1. 
   
     
       
         
             
             
           
             
                 
               TABLE 1 
             
           
          
             
                 
                 
             
             
                 
               Humidity (%) 
             
          
         
         
             
             
             
          
             
               Temperature (° C.) 
               HM &lt; 70 
               70 ≦ HM 
             
             
                 
             
             
               TP &lt; 28 
               220 (V) 
               240 (V) 
             
             
               28 ≦ TP &lt; 31 
               300 
               320 
             
             
               31 ≦ TP 
               350 
               370 
             
             
                 
             
          
         
       
     
   
   During the operations shown in  FIG. 7 , the CPU  111  accepts input data from the temperature sensor  6  and the humidity sensor  7  for every vertical synchronizing signal Vsync and judges whether it is necessary to change the output value of the primary transfer bias. 
   In  FIG. 7 , the vertical synchronizing signal Vsync is outputted each at the time t 1 , t 2 , t 3 , t 4 , t 5 , t 6  and t 7 . The video request signal Vreq for the first image is outputted after the predetermined period T 1  from the falling edge of the vertical synchronizing signal Vsync at the time t 1 . In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to the video signal representing the first image is started, concurrently with which the developing bias is turned on. Meanwhile, after the predetermined period T 2  from the falling edge of the vertical synchronizing signal Vsync at the time t 1 , the video request signal Vreq for the second image is outputted. In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to the video signal representing the second image is started. 
   Since secondary transfer is not ongoing after the predetermined period T 3  from the time t 1 , the output value from the primary transfer bias generating circuit  116  is changed to V 12  from V 11  in accordance with a change to the environment conditions. 
   As a result, a toner image K 1  is primarily transferred onto the sub area  76 A which is on the downstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 , and a toner image K 2  is primarily transferred onto the sub area  76 B which is on the upstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   Meanwhile, the transfer paper  4  is transported toward the secondary transfer part  37  from the paper feeding cassette  3  at a predetermined speed. The contacting/clearing clutch for secondary transfer roller turns on at the time t 15  which is after a predetermined period from the time t 1 , and the secondary transfer roller  35  accordingly abuts on the intermediate transfer belt  31 . Following this, at the time t 16  and t 17  after predetermined periods from the time t 1 , application of the secondary transfer bias from the secondary transfer bias generating circuit  117  upon the secondary transfer roller  35  is activated. This realizes transfer onto the first transfer paper  4  of the first toner image K 1  which was primarily transferred onto the sub area  76 A which is on the downstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 , while realizing transfer onto the second transfer paper  4  of the second image K 2  which was primarily transferred onto the sub area  76 B which is on the upstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 . The toner images K 1  and K 2  are fixed on the transfer papers  4  respectively and the transfer papers  4  are discharged. 
   At the time t 2  which is outputting of the vertical synchronizing signal Vsync, whether it is necessary to change the output value of the primary transfer bias in accordance with the environment conditions is judged. When it is necessary to change, whether the secondary transfer bias is on after the predetermined period T 3  from the time t 2  is judged. 
   In  FIG. 7 , the output value needs be changed, and the secondary transfer bias is on after the predetermined period T 3  from the time t 2 . Hence, formation of image is not performed at the time t 2  but waits until the time t 3  at which the vertical synchronizing signal Vsync is outputted the next time. After a predetermined period since the time t 3 , toner images K 3  and K 4  for forming the third and the fourth images are formed, and after the predetermined period T 3  from the time t 3 , the output value of the primary transfer bias is changed to V 13  from V 12  and operations similar to the above are performed. 
   While formation of image is performed for the transfer paper  4  whose size is large, since it is possible to judge that the secondary transfer bias is on after the predetermined period T 3  from the time t 4  in a similar manner to the above, the output value of the primary transfer bias can not be changed. Hence, formation of image is not performed at the time t 4  but waits until the time t 5  at which the vertical synchronizing signal Vsync is outputted the next time. 
   The video request signal Vreq is outputted after the predetermined period T 1  from the falling edges of the vertical synchronizing signal Vsync at t 5 . In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to this video signal is started, concurrently with which the developing bias is turned on. 
   Since secondary transfer is not ongoing after the predetermined period T 3  from the time t 5 , the output value from the primary transfer bias generating circuit  116  is changed to V 12  from V 13  in accordance with a change to the environment conditions. 
   Meanwhile, the transfer paper  4  is transported toward the secondary transfer part  37  from the paper feeding cassette  3  at a predetermined speed. At the time t 18  after a predetermined period from the time t 5 , application of the secondary transfer bias from the secondary transfer bias generating circuit  117  upon the secondary transfer roller  35  is activated. This realizes transfer onto the transfer paper  4  of the toner image K 5  which was primarily transferred onto the transfer area  76  of the intermediate transfer belt  31 . The toner image K 5  is then fixed on the transfer paper  4  and the transfer paper  4  is discharged. 
   At the time t 6  which is outputting of the vertical synchronizing signal Vsync, whether the secondary transfer bias is on after the predetermined period T 3  from the time t 6  is judged. Since the secondary transfer bias is off after the predetermined period T 3  from the time t 6  in  FIG. 7 , a toner image K 6  for forming the next image is formed after a predetermined period from the time t 6 . After the predetermined period T 3  from the time t 6 , the output value of the primary transfer bias is changed to V 11  from V 12  and operations similar to the above are performed. Secondary transfer of the toner image K 6  finishes, and at the time t 19  which comes after turning off of the secondary transfer bias, the contacting/clearing clutch for secondary transfer roller turns on, and the secondary transfer roller  35  accordingly leaves the intermediate transfer belt  31 . 
   As described above, during the operation shown in  FIG. 7 , as in the example shown in  FIG. 6 , since changing of the output value of the primary transfer bias completes while the transfer protection area  75  serving as the non-image area is still moving through the primary transfer part  14 , it is possible to change the output value without fail prior to the start of primary transfer. 
   Further, as in the example shown in  FIG. 6 , since the primary transfer bias is changed when secondary transfer is not performed, it is possible to prevent a change to the primary transfer bias from adversely affecting secondary transfer, and hence, the quality of an image transferred onto the transfer paper  4  from deteriorating. 
   While an unnecessary toner image will be formed in the configuration that formation of image is continued at the time of detection of the vertical synchronizing signal Vsync and therefore the output value of the primary transfer bias can not be changed although necessary because of the ongoing secondary transfer, during the operations shown in  FIG. 7 , since whether it is necessary to change the output value of the primary transfer bias in accordance with the bias change conditions is judged when the vertical synchronizing signal Vsync is detected, it is possible to prevent from forming an unnecessary toner image. 
   Referring to  FIG. 8 , still other operations of this printer will now be described.  FIG. 8  is a timing chart which shows time-induced changes appearing in the conditions of the respective portions of the engine part  1 . The illustrate example is a situation that four monochrome images of such a size that can be transferred two images during one rotation of the intermediate transfer belt  31  are to be formed. The same symbols as those used in  FIG. 6  denote the same timing. For convenience of illustration,  FIG. 8  shows as if the video signal turns on in synchronization to the video request signal Vreq as in  FIG. 6 . 
   As in  FIG. 7 , the primary transfer bias is changed in accordance with the environment conditions representing the temperature and the humidity level in  FIG. 8 . In addition, since a monochrome image (in K for instance) is formed in  FIG. 8  as in  FIG. 7 , the black developer unit  2 K remains serving as the rotary developer  20 . Hence, the developing bias stays turned on until an image has been formed. Since superimposition of toner images is not performed, until an image has been formed, the secondary transfer roller  35  is maintained abutting on the intermediate transfer belt  31 . Further, the values shown in Table 1 described earlier are used as the output value of the primary transfer bias which is in accordance with the temperature and the humidity. 
   In  FIG. 8 , the vertical synchronizing signal Vsync is outputted each at the time t 1 , t 2 , t 3  and t 4 . The video request signal Vreq for the first image is outputted after the predetermined period T 1  from the falling edge of the vertical synchronizing signal Vsync at the time t 1 . In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to the video signal representing the first image is started, concurrently with which the developing bias is turned on. Meanwhile, after the predetermined period T 2  from the falling edge of the vertical synchronizing signal Vsync at the time t 1 , the video request signal Vreq for the second image is outputted. In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to the video signal representing the second image is started. 
   Since secondary transfer is not ongoing after the predetermined period T 3  from the time t 1 , the output value from the primary transfer bias generating circuit  116  is changed to V 12  from V 11  in accordance with a change to the environment conditions. 
   As a result, a toner image K 1  is primarily transferred onto the sub area  76 A which is on the downstream side in transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 , and a toner image K 2  is primarily transferred onto the sub area  76 B which is on the upstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   Meanwhile, the transfer paper  4  is transported toward the secondary transfer part  37  from the paper feeding cassette  3  at a predetermined speed. At the time t 21  after a predetermined period from the time t 1 , the contacting/clearing clutch for secondary transfer roller turns on, and the secondary transfer roller  35  accordingly abuts on the intermediate transfer belt  31 . At the time t 22  and t 23  after predetermined periods from the time t 1 , application of the secondary transfer bias from the secondary transfer bias generating circuit  117  upon the secondary transfer roller  35  is activated. This realizes transfer onto the first transfer paper  4  of the toner image K 1  which was primarily transferred onto the sub area  76 A which is on the downstream side in transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 , while realizing transfer onto the second transfer paper  4  of the second image K 2  which was primarily transferred onto the sub area  76 B which is on the upstream side in the transfer area  76  of the intermediate transfer belt  31  along the direction of rotational driving  72 . The toner images K 1  and K 2  are fixed on the transfer papers  4  respectively and the transfer papers  4  are discharged. 
   At the time t 2  at which the vertical synchronizing signal Vsync is outputted the next time, based on the period during which the secondary transfer bias is applied is determined in advance in accordance with the size of the transfer papers  4 , whether the secondary transfer bias is on after the predetermined period T 3  from the time t 2  is judged. Since the secondary transfer bias is on after the predetermined period T 3  from the time t 2  in  FIG. 8 , an image is not formed at the time t 2  but the printer stays on stand-by. After the predetermined period T 2  from the time t 2 , whether the secondary transfer bias is on is judged. 
   Since the secondary transfer bias is off after the predetermined period T 2  from the time t 2  in  FIG. 8 , for the purpose of primary transfer onto the sub area  76 B which is on the upstream side in the transfer area  76  along the direction of rotational driving  72 , the video request signal Vreq is outputted after the predetermined period T 2  has elapsed since the time t 2 . In synchronization to falling of this video request signal Vreq, formation of an electrostatic latent image corresponding to the video signal representing the third image is started and a toner image K 3  is formed. After a predetermined period T 4  from the time t 2 , the output value of the primary transfer bias is changed to V 13  from V 12  and primary transfer of the toner image K 3  is performed, followed by secondary transfer. 
   The predetermined period T 4  is determined in advance such that changing of the primary transfer bias to be applied upon the intermediate transfer belt  31  will have completed before the front edge of a toner image on the photosensitive member  11  which was formed to match in terms of timing with the sub area  76 B of the intermediate transfer belt  31  (the downstream edge along the direction of rotational driving  72 ) reaches the primary transfer part  14  (i.e., while the sub area  76 A not bearing a transferred toner image and serving as the non-image area is moving through the primary transfer part  14 ). 
   At the time t 3  at which the vertical synchronizing signal Vsync is outputted the next time, whether there is a change to the environment conditions and it is necessary to change the output value of the primary transfer bias is judged, and it is judged that it is not necessary to change the output value in this example. The video request signal Vreq for the fourth image is outputted after the predetermined period T 1  from the time t 3 , and a toner image K 4  is accordingly formed. Although secondary transfer is ongoing after the predetermined period T 3  from the time t 3 , since there is no change to the environment conditions and the output value of the primary transfer bias is not changed, the operation of forming an image is performed. Although the output value is not changed as described above, the same data as the previous data are fed to the D/A convertor  121  from the CPU  111  as control data after the predetermined period T 3  from the time t 3 . 
   Secondary transfer of the toner image K 4  finishes, and at the time t 24  which comes after turning off of the secondary transfer bias, the contacting/clearing clutch for secondary transfer roller turns on, and the secondary transfer roller  35  accordingly leaves the intermediate transfer belt  31 . 
   As described above, during the operations shown in  FIG. 8 , as in the example shown in  FIG. 6 , since the output value of the primary transfer bias is changed when secondary transfer is not ongoing, it is possible to prevent a change to the primary transfer bias from adversely affecting secondary transfer, and hence, the quality of an image transferred onto the transfer paper  4  from deteriorating. 
   Further, during the operations shown in  FIG. 8 , the printer waits only for the sub area  76 A, which does not bear a toner image and which serves as the non-image area, to move passed instead of remaining on stand-by until the next vertical synchronizing signal Vsync. Hence, the throughput improves for this amount than in the operations shown in  FIG. 7 . 
   In addition, during the operations shown in  FIG. 8 , whether it is necessary to change the output value of the primary transfer bias in accordance with the bias change conditions is judged when the vertical synchronizing signal Vsync is detected. Therefore, when there is no change to the environment conditions and the output value of the primary transfer bias needs not be changed, it is possible to continue forming an image and prevent a deterioration in throughput. 
   Although the preferred embodiment above uses the intermediate transfer belt  31  which is formed by an endless belt joined at the splice  71 , the intermediate transfer medium used in the present invention is not limited to this. Instead, the intermediate transfer medium may be an intermediate transfer belt which is formed by a seamless endless belt having no splice, an intermediate transfer drum having a cylindrical shape, or the like for instance, in which case the transfer protection area  75  may be provided as an area in which the belt cleaner  33  abuts on and leaves the intermediate transfer belt  31 . 
   Further, although the output value of the primary transfer bias is changed in all of the first through the fourth rounds of primary transfer during the operations according to the preferred embodiment above shown in  FIG. 6 , this is not limiting. The output value may be the same value from the first through the third rounds of primary transfer and the output value of the primary transfer bias for the fourth round of primary transfer alone may be changed, for example. Alternatively, the output value of the primary transfer bias for the first round of primary transfer alone may be changed while the same value may be used for the second through the fourth rounds of primary transfer. 
   While the preferred embodiment above is directed to a color printer, the operations shown in  FIGS. 7 and 8  are not limited to this but may be applicable to a monochrome printer as well. Further, the foregoing has described the preferred embodiment above in relation to a color printer which comprises one photosensitive member and requires to continuously rotate the intermediate transfer belt  31  for superimposition of toner images, the operations shown in  FIG. 8  are not limited to this but may be applicable to a color printer of the so-called tandem type which comprises a plurality of photosensitive members which are aligned along an intermediate transfer belt. 
   Alternatively, an embodiment as that shown in  FIG. 9  may be used. The following relationship holds in the embodiment shown in FIG.  9 :
 
 L   1 &gt; L   3 
 
where L 0  is the total length of the intermediate transfer belt  31  ( FIGS. 4A  and  4 B), L 1  is the size of the transfer protection area  75  along the direction of rotational driving  72  as shown in  FIG. 9 , L 2  is the size of the transfer area  76  (L 0 =L 1 +L 2 ), and L 3  is a distance between the primary transfer part  14  and the secondary transfer part  37 .
 
   The CPU  111  performs the control for changing the output value of the primary transfer bias, while the transfer protection area  75  is passing through both the primary transfer part  14  and the secondary transfer part  37 . In this modified embodiment, it is possible to prevent the output value of the primary transfer bias from getting changed without fail during secondary transfer. This allows the configuration for controlling to be simple. 
   &lt;Second Preferred Embodiment&gt; 
   A second preferred embodiment of the image forming apparatus according to the present invention will now be described. A major difference in structure of the second preferred embodiment from the first preferred embodiment is that the intermediate transfer belt  31  is formed by an endless belt having no splice (seamless). In the second preferred embodiment, a transfer area  79  of the intermediate transfer belt  31  has a larger size than the size of an A3 paper as it is placed with the longer sides aligned along the direction of rotational driving  72  for example. It is possible to split the transfer area  79  into two sub areas  79 A and  79 B, so as to transfer during one rotation of the intermediate transfer belt  31  two toner images  78  having the A4 size with the shorter sides aligned along the direction of rotational driving  72 . The other structures are the same, and therefore, will not be described yet denoted at the same reference symbols. 
   The CPU  111  changes the output value of the primary transfer bias to the D/A convertor  121  as in the first preferred embodiment, and the output value is changed when secondary transfer is not ongoing according to the second preferred embodiment. When a plurality of images are to be formed in a row for instance, the output value of the primary transfer bias for the fourth color (which is K for example in  FIG. 11  which will be described later) used to form a previous image is changed to the output value of the primary transfer bias for the first color (which is Y for example in  FIG. 11 ) used to form the next image, after completion of secondary transfer which is for formation of the previous image. 
   At this stage, the CPU  111  judges the completion timing of secondary transfer onto the transfer paper  4  based on the size of the transfer paper  4 , and starts the operation of forming an image on the photosensitive member  11  during execution of secondary transfer so that the next toner image in the first color on the photosensitive member  11  will arrive at the primary transfer part  14  immediately after thus judged completion timing. 
   The intermediate transfer belt  31  corresponds to the intermediate transfer medium, while the bias applying member  31 A and the primary transfer bias generating circuit  116  correspond to the primary transfer means, and the secondary transfer roller  35  and the secondary transfer bias generating circuit  117  correspond to secondary transfer means. Meanwhile, the CPU  111  corresponds to the bias control means, completion judging means and image formation control means. 
   Referring to  FIG. 11 , an operation of this printer will now be described.  FIG. 11  is a timing chart which shows time-induced changes appearing in the conditions of the respective portions of the engine part  1 . The illustrate example is a situation that four color images of such a size that can be transferred two images during one rotation of the intermediate transfer belt  31  (the A4 size for example) are formed and that the output value of the primary transfer bias is changed for every primary transfer of a toner image which will be superimposed. The video signal and the developing bias are turned on at predetermined timing in response to the video request signal Vreq, and therefore, this timing is in a predetermined delay from the timing of the video request signal Vreq. However, for convenience of illustration,  FIG. 11  shows as if these are turned on in synchronization to the video request signal Vreq. 
   As the print instruction signal containing the video signal is fed to the main controller  100  from the external apparatus such as a host computer, in response to the control signal received from the main controller  100 , the engine controller  110  starts operating the respective portions of the engine part  1 . At this stage, if the size of the transfer papers  4  stacked up in the paper feeding cassette  3  fails to match with the size designated by the print instruction signal, an operation display panel  8  shows a message which encourages to replace the paper feeding cassette. 
   When the size of the transfer papers  4  stacked up in the paper feeding cassette  3  matches with the size designated by the print instruction signal (i.e., when a plurality of paper feeding cassettes include a cassette which holds the transfer papers  4  of the size designated by the print instruction signal), by means of the laser light  57  emitted from the exposure unit  50 , an electrostatic latent image corresponding to the video signal described above is created on the surface of the photosensitive member  11  which is uniformly electrified by the electrifier  12 . The rotary developer  20  develops the electrostatic latent image, thereby forming a toner image. In the primary transfer part  14 , the toner image thus formed on the photosensitive member  11  is primarily transferred onto the intermediate transfer belt  31 . 
   That is, the photosensitive member driving motor  36  rotates the intermediate transfer belt  31  at a predetermined peripheral velocity, and the video request signal Vreq is outputted each at the time t 1 , t 2 , t 3 , t 4 , t 5 , t 6 , t 7  and t 8  as shown in  FIG. 11 . In  FIG. 11 , the following holds:
 
 L   0 =2 ·T   0   ·S   1 
 
where T 0  denotes the cycle of outputting the video request signal Vreq and S 1  denotes the peripheral velocity of the intermediate transfer belt  31 . As shown in  FIG. 10 , two toner images having the A4 size are transferred as the intermediate transfer belt  31  rotates one round.
 
   Formation of an electrostatic latent image corresponding to the video signal representing the first image is started in response to receipt of the video request signal Vreq at the time t 1 , and the developing bias is turned on. Following this, in response to receipt of the video request signal Vreq at the time t 2 , formation of an electrostatic latent image corresponding to the video signal representing the second image is started. 
   The developer units of the rotary developer  20  switch over with each other at the time t 1 , t 3 , t 5  and t 7 , whereby toner images in the respective colors are formed on the photosensitive member  11  and primarily transferred one after another onto the intermediate transfer belt  31 . At this stage, after the predetermined period T 2  from the time t 1 , t 3 , t 5  and t 7 , the output value from the primary transfer bias generating circuit  116  is changed. 
   In this embodiment, the primary transfer bias for the first image (Y) is set to a voltage V 1  (V 1 =220 V for example), the primary transfer bias for the second image (C) is set to a voltage V 2  (V 2 =245 V for example), the primary transfer bias for the third image (M) is set to a voltage V 3  (V 3 =270 V for example), and the primary transfer bias for the fourth image (K) is set to a voltage V 4  (V 4 =300 V for example). 
   The predetermined period T 2  is set in advance such that changing of the primary transfer bias to be applied to the intermediate transfer belt  31  will have completed before the front edge of a toner image on the photosensitive member  11  (the downstream edge along the direction of rotational driving  72 ) reaches the primary transfer part  14 . 
   Since primary transfer is not complete yet after the predetermined period T 2  from the time t 1 , t 3 , t 5  and t 7 , secondary transfer is not performed. Hence, even if the output value of the primary transfer bias is changed, there will be no problem. In short, since the secondary transfer roller  35  stays cleared off from the intermediate transfer belt  31  during this, the toner images in the respective colors are superimposed one atop the other on the intermediate transfer belt  31 . 
   The developing bias is turned off after a predetermined period of time which is determined in advance depending on the size of the transfer papers since the falling edges of the video request signal Vreq at the time t 1 , t 3 , t 5  and t 7 . 
   As a result, a color image which is toner images Y 1 , C 1 , M 1  and K 1  as they are superimposed one atop the other is primarily transferred onto the sub area  79 A which is on the downstream side in the transfer area  79  of the intermediate transfer belt  31  along the direction of rotational driving  72 , and a color image which is toner images Y 2 , C 2 , M 2  and K 2  as they are superimposed one atop the other is primarily transferred onto the sub area  79 B which is on the upstream side in the transfer area  79  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   On the other hand, the top-most transfer paper  4  among the bundle of transfer papers housed in the paper feeding cassette  3  is taken out by the pick-up roller  61 , transported by the feed roller pair  62  at the predetermined speed S 1 , and nipped by the gate roller pair  34 . The gate clutch turns on in synchronization to a toner image on the intermediate transfer belt  31 , and the transfer paper  4  is transported toward the secondary transfer part  37  from the gate roller pair  34  at the predetermined speed S 1 . 
   The contacting/clearing clutch for secondary transfer roller turns on at the time t 9  which is after a predetermined period since the time t 7 , and the secondary transfer roller  35  accordingly abuts on the intermediate transfer belt  31 . Following this, at the time t 10  which is after a predetermined period from the time t 7 , application of the secondary transfer bias from the secondary transfer bias generating circuit  117  upon the secondary transfer roller  35  is activated. 
   This realizes transfer onto the first transfer paper  4  of the color image which is toner images Y 1 , C 1 , M 1  and K 1  as they are superimposed one atop the other and which was primarily transferred onto the sub area  79 A which is on the downstream side in the transfer area  79  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   The gate clutch is temporarily turned off after discharging of the first transfer paper  4 . A period during which the secondary transfer bias is applied is set in advance in accordance with the size of the transfer papers  4 . At this stage, the next transfer paper  4  is taken out by the pick-up roller  61 , transported by the feed roller pair  62  at the predetermined speed S 1 , and nipped by the gate roller pair  34 . 
   After turning off of the gate clutch and inactivation of application of the secondary transfer bias, the gate clutch turns on in synchronization to the next toner image and the next transfer paper  4  is transported, and application of the secondary transfer bias turns on at the time till which is after a predetermined period from the time t 7 . When the preset period during which the secondary transfer bias is applied elapses (the time t 12 ), application of the secondary transfer bias turns off, and after a predetermined period from the time t 7 , the contacting/clearing clutch for secondary transfer roller turns on and the secondary transfer roller  35  leaves the intermediate transfer belt  31 . 
   This realizes transfer onto the second transfer paper  4  of the color image which is toner images Y 2 , C 2 , M 2  and K 2  as they are superimposed one atop the other and which was primarily transferred onto the sub area  79 B which is on the upstream side in the transfer area  79  of the intermediate transfer belt  31  along the direction of rotational driving  72 . 
   In the fixing unit  40 , this toner image is fixed on the transfer paper  4  during transportation of the transfer paper  4 . The transfer paper  4  is further transported by the transportation roller pair  63  and discharged by the discharge roller pair  64  into a discharging part  9  which is disposed to an upper section of the main unit  2 . 
   While the third and the fourth images are formed on the photosensitive member  11  following this, since the period during which the secondary transfer bias is applied is determined in advance in accordance with the size of the transfer papers  4 , after the predetermined cycle T 1  from the time t 8  (i.e., at the time t 13 ), it is already possible to determine whether the secondary transfer bias is on further after the predetermined period T 2  (i.e., whether t 13 +T 2  comes earlier than the time t 12 ). In  FIG. 11 , since the secondary transfer bias is on after the predetermined period T 2  from the time t 13  and the CPU  111  learns of this upon judgement at the time t 13 , the video request signal Vreq is not outputted at the time t 13  and the printer remains on stand-by for a predetermined standby period. 
   In the predetermined cycle T 1 , the video request signal Vreq is outputted at the time t 14 , t 15 , . . . whereby toner images Y 3  and Y 4  which are to form the third and the fourth images are formed. At the time t 16  after the predetermined period T 2  from the time t 14 , the output value of the primary transfer bias is changed to V 1  and operations similar to the above are performed. The time t 14  is set such that the secondary transfer bias is off again at the time t 16  which is after the predetermined period T 2  from the time t 14  (such that the time t 16  comes later than the time t 12 ). 
   Since formation of the third and the fourth images at and after the time t 14  is in a delay equivalent to the stand-by period as compared to formation of the first and the second images, the time at which the belt cleaner  33  abuts on the intermediate transfer belt  31  and the time at which the belt cleaner  33  leaves the intermediate transfer belt  31  may be changed considering the delay. 
   As described above, during the operation shown in  FIG. 11 , since the output value of the primary transfer bias is changed for every primary transfer of a toner image, it is possible to primarily transfer the toner in the respective colors onto the intermediate transfer belt  31  in an excellent manner in accordance with the toner transfer efficiency which changes depending on the thickness of the stacked-up toner on the intermediate transfer belt  31 , and hence, to obtain a color image having a high image quality on the transfer paper  4 . 
   Further, since the output value of the primary transfer bias is changed when secondary transfer is not ongoing (after completion of secondary transfer), it is possible to prevent a change to the primary transfer bias from adversely affecting secondary transfer, and hence, the quality of an image transferred onto the transfer paper  4  from deteriorating. 
   In addition, since the time t 14  for resuming outputting of the video request signal Vreq is set such that the secondary transfer bias is already off again at the time t 16  which is after the predetermined period T 2  from the time t 16  (such that the time t 16  comes later than the time t 12 ), it is possible to increase or decrease the stand-by period in accordance with the size of the transfer papers  4 , and hence, to suppress a deterioration in throughput as much as possible. 
   For instance, although the preferred embodiment above uses an endless belt having no splice as the intermediate transfer belt  31 , the intermediate transfer medium used in the present invention is not limited to this. Instead, an intermediate transfer drum having a cylindrical shape may be used for instance. 
   Further, although the preferred embodiment above requires to change the output value of the primary transfer bias during primary transfer of each one of toner images in the first through the fourth colors, this is not limiting. For example, the same output value may be used for primary transfer of the first through the third toner images, and the output value may be changed for primary transfer of only the fourth toner image. Alternatively, the output value may be changed for primary transfer of only the first toner image and the same output value may be used for primary transfer of the second through the fourth toner images. In short, the only requirement is to change the output value between primary transfer of the first toner image and primary transfer of the last toner image. 
   Still further, the time t 14  for resuming outputting of the video request signal Vreq is set such that the secondary transfer bias is already off again at the time t 16  which is after the predetermined period T 2  from the time t 14  (such that the time t 16  comes later than the time t 12 ) according to the preferred embodiment above, this is not limiting. Instead, outputting of the video request signal Vreq may be resumed in synchronization to the time t 12  at which the secondary transfer bias turns off again. Such simplifies the control sequence and makes it easy to design the control program. 
   &lt;Third Preferred Embodiment&gt; 
   A third preferred embodiment of the image forming apparatus according to the present invention will now be described. A major difference in structure of the third preferred embodiment from the first preferred embodiment is that a tandem-type structure is used. The major difference will be mainly described in the following, and the same structures will not be described yet denoted at the same reference symbols. 
   The engine part  1  comprises the exposure unit  50 , photosensitive member units  10 Y,  10 C,  10 M and  10 K, an intermediate transfer unit  30  and the fixing unit  40  as shown in  FIG. 12 . The exposure unit  50  comprises a laser light source, a horizontal synchronization sensor, etc. Each one of the photosensitive member units  10 Y,  10 C,  10 M and  10 K comprises a photosensitive member  11 , an electrifier  12 , a developer  15  and a cleaner  13 . The intermediate transfer unit  30  comprises the intermediate transfer belt  31 , the bias applying member  31 A, the belt cleaner  33 , the secondary transfer roller  35 , the photosensitive member driving motor  36 , etc. The developers  15  of the photosensitive member units  10 Y,  10 C,  10 M and  10 K house yellow toner, cyan toner, magenta toner and black toner, respectively. The photosensitive members  11  of the photosensitive member units  10 Y,  10 C,  10 M and  10 K are arranged one next to the other along the intermediate transfer belt  31 . 
   This printer uses a structure of the so-called tandem type that toner images in the respective colors are formed on the photosensitive members  11  of the photosensitive member units  10 Y,  10 C,  10 M and  10 K, the toner images on the photosensitive members  11  are primarily transferred onto the intermediate transfer belt  31  so that the toner images will be superimposed one atop the other, and thus primarily transferred toner image is secondarily transferred onto the transfer paper  4  in the secondary transfer part  37 . In this printer, the output values of the primary transfer biases are changed in accordance with results of a temperature sensor  6  and a humidity sensor  7 , and as described later, the output value is changed after the transported transfer paper  4  is discharged from the secondary transfer part  37  but before the next transfer paper  4  is loaded into the secondary transfer part  37 . 
   The exposure unit  50  comprises a laser light source which is formed by a semiconductor laser for instance, a polygon mirror which reflects laser light from the laser light source, a scanner motor which drives the polygon mirror so that the polygon mirror rotates at a high speed, a lens part which converges the laser light reflected by the polygon mirror, the horizontal synchronization sensor  56  and the like as one set, and four such sets respectively for the photosensitive member units  10 Y,  10 C,  10 M and  10 K. Laser light  16  reflected by the polygon mirror and emitted through the lens part scans the surfaces of the photosensitive members  11  in the main scanning direction (a direction which is perpendicular to the plane of  FIG. 12 ), whereby electrostatic latent images corresponding to video signals are formed on the surfaces of the photosensitive members  11 . At this stage, the horizontal synchronization sensors  56  provide synchronizing signals which are in the main scanning direction, i.e., horizontal synchronizing signals. The exposure unit  50  functions as exposure means. 
   The photosensitive member units  10 Y,  10 C,  10 M and  10 K have the same structure, and the respective photosensitive members  11  is rotated by the photosensitive member driving motor  36  in the direction denoted at arrows. The electrifiers  12 , the developers  15  and the cleaners  13  are arranged around the respective photosensitive members  11  along the rotating direction of the photosensitive members  11 . For convenience of illustration,  FIG. 12  omits reference symbols denoting the respective portions of the photosensitive member units  10 C and  10 M. 
   The electrifiers  12  comprise wire electrodes to which a high voltage at a predetermined level is applied. Utilizing corona discharge for instance, the electrifiers  12  uniformly electrify outer circumferential surfaces of the photosensitive members  11 . The developers  15  make toner of the respective colors adhere to electrostatic latent images formed by the exposure unit  50  to thereby form toner images. As developing biases, which are direct current components as they are alone or direct current components on which alternating current components are superimposed, are applied, the toner in the respective colors from the developers  15  adhere to the surfaces of the photosensitive members  11 . The cleaners  13  are disposed immediately on the upstream side to the electrifiers  12  in the rotating direction of the photosensitive members  11 , and scrape off the toner remaining on the outer circumferential surfaces of the photosensitive members  11  to thereby clean the surfaces of the photosensitive members  11  after primary transfer of a toner image onto the intermediate transfer belt  31  from the photosensitive members  11 . The developers  15  function as developing means. 
   The intermediate transfer belt  31  is formed by an endless belt having no splice (seamless), and as shown in the development view in  FIG. 13 , has the total length of L 0 . In  FIG. 13 , an arrow  72  denotes the direction of rotational driving while an arrow  73  denotes the direction of rotation axis. The transfer area  79  of the intermediate transfer belt  31  has a larger size than the size of an A3 paper as it is placed with the longer sides aligned along the direction of rotational driving  72  for example. 
   The belt cleaner  33  is disposed abutting on a portion where the intermediate transfer belt  31  is wound around a drive roller, the portion being the downstream side to the secondary transfer part  37  along the direction of rotational driving. The belt cleaner  33  scrapes off toner which remains on the outer circumferential surface of the intermediate transfer belt  31  after secondary transfer. 
   The crescent-shaped pick-up roller  61  and the gate roller pair  34  are disposed toward above from the front edge of the paper feeding cassette  3  (the right-most edge in  FIG. 12 ), and a discharge roller pair  64  is disposed on the opposite side to the secondary transfer roller  35  and the fixing unit  40 , whereby a transportation path for the transfer papers  4  is formed. On the transportation path and immediately on the downstream side to the secondary transfer part  37  along the transfer paper transporting direction, there is a post-transfer paper sensor  65 . 
   The pick-up roller  61  is driven by a pick-up solenoid. The gate roller pair  34 , the secondary transfer roller  35 , the heating roller  41  of the fixing unit  40  and the discharge roller pair  64  are each linked to the same transportation system driving motor  60  via a drive force transmission mechanism. The transportation system driving motor  60  transports the transfer paper  4  at the predetermined speed S 1 . When a gate clutch is turned on, the drive force of a transportation system driving motor  60  is transmitted to the gate roller pair  34  so that the gate roller pair  34  accordingly rotates. The post-transfer paper sensor  65  is formed by an actuation piece which is revolved by the moving transfer paper  4  for instance and a photo-interrupter which detects revolutions of the actuation piece, and detects a passage of the transfer paper  4 . Discharged by the discharge roller pair  64 , the transfer papers  4  are stacked up in a discharging part  9  which is disposed to an upper section of the main unit  2 . The gate roller pair  34  and the discharge roller pair  64  constitute transporting means for the transfer papers  4 . 
   The CPU  111  receives atmosphere temperature data from the temperature sensor  6  as input signals from the engine part  1 , atmosphere humidity data from the humidity sensor  7 , the horizontal synchronizing signal Hsync from the horizontal synchronization sensor  56 , and a detection signal regarding whether the transfer paper  4  has passed from the post-transfer paper sensor  65 . Based on these input signals and the control program, the CPU  111  controls operations of the respective portions of the engine part  1 . 
   In other words, the CPU  111  sends a control signal to the motor drive circuit  114  which drives the photosensitive member driving motor  36 , synchronizes the photosensitive members  11  and the intermediate transfer belt  31  to each other, and drives these. Further, the CPU  111  sends a control signal to the motor drive circuit  115  which drives the transportation system driving motor  60 , and controls feeding of the transfer paper  4  from the paper feeding cassette  3 . The CPU  111  sends a control signal also to the gate clutch and controls the timing of transporting the transfer papers  4  toward the secondary transfer part  37 . The CPU  111  also controls the operations of the respective photosensitive members  11  to form images, such that primary transfer toner images primarily transferred onto the intermediate transfer belt  31  from the respective photosensitive members  11  will be superimposed one atop the other on the intermediate transfer belt  31 . 
   Further, the CPU  111  sends a control signal to the primary transfer bias generating circuit  116  which generates the primary transfer bias, to thereby control application of the primary transfer bias upon the intermediate transfer belt  31 . The CPU  111  sends a control signal also to the secondary transfer bias generating circuit  117  which generates the secondary transfer bias, to thereby control application of the secondary transfer bias upon the secondary transfer roller  35 . 
   The CPU  111  changes the output value of the primary transfer bias to the D/A convertor  121  in accordance with a predetermined bias change condition. Used as the predetermined bias change condition is an environment condition regarding the temperature of an atmosphere detected by the temperature sensor  6  and the humidity level of the atmosphere detected by the humidity sensor  7 . In this case, as described later, the CPU  111  accepts input data from the temperature sensor  6  and the humidity sensor  7  at predetermined timing, and judges whether it is necessary to change the output values of the primary transfer biases. 
   The CPU  111  judges whether the transfer paper  4  has been loaded into the secondary transfer part  37 , and when the transfer paper  4  has not been loaded into the secondary transfer part  37  yet, the CPU  111  changes the output values of the primary transfer biases. However, the CPU  111  changes the output values when primary transfer is not ongoing. 
   The timing of loading of the transfer paper  4  into the secondary transfer part  37  is determined based on the elapsed time from turning on of the gate clutch which drives the gate roller pair  34 . Since a distance from the gate roller pair  34  to the secondary transfer part  37  and the transportation speed for the transfer papers  4  are known, a period of time needed for the transfer paper  4  to enter the secondary transfer part  37  since turning on of the gate clutch is also known. Meanwhile, the timing of departure of the transfer paper  4  from the secondary transfer part  37  is determined from a passage of the rear edge of the transfer paper  4  and switching of the post-transfer paper sensor  65  from on to off. 
   One example of the output values of the primary transfer biases in accordance with the environment condition regarding the temperature of an atmosphere and the humidity level of the atmosphere is as shown in Table 1. 
   Even when it is not necessary to change the output values of the primary transfer biases in accordance with the bias change condition, the CPU  111  sends control data to the D/A convertor  121  of the primary transfer bias generating circuit  116 . Hence, even if a noise for example creates a garbage content in the control data fed to the D/A convertor  121 , it is possible to prevent the primary transfer bias generating circuit  116  from continuously operating using such abnormal data. 
   The intermediate transfer belt  31  corresponds to the intermediate transfer medium, while the bias applying member  31 A and the primary transfer bias generating circuit  116  correspond to the primary transfer means, and the secondary transfer roller  35  and the secondary transfer bias generating circuit  117  correspond to secondary transfer means. The CPU  111  corresponds to the bias control means, transfer paper judging means, image formation control means and bias change judging means. 
   An example of operations of the printer will now be described with reference to  FIGS. 14 and 15 .  FIG. 14  is a timing chart which shows time-induced changes appearing in the conditions of the respective portions of the engine part  1 .  FIG. 15  is a flow chart which shows an example of the sequence of changing the output value of the primary transfer bias. 
   When a print instruction signal containing a video signal is fed to the main controller  100  from the external apparatus such as a host computer, the engine controller  110  controls an operation of each portion of the engine part  1  in accordance with a control signal from the main controller  100 . At this stage, when the size of the transfer papers  4  housed in the paper feeding cassette  3  fails to match with the size designated by the print instruction signal, the operation display panel  8  shows a message which encourages to replace the paper feeding cassette. Although  FIG. 12  shows the printer as a printer which comprises one paper feeding cassette  3 , this is not limiting. Instead, the printer may comprise a plurality of paper feeding cassettes. 
   When the size of the transfer papers  4  housed in the paper feeding cassette  3  matches with the size designated by the print instruction signal (or when a plurality of paper feeding cassettes include a cassette which holds the transfer papers  4  of the size designated by the print instruction signal), by means of each laser light  16  emitted from the exposure unit  50 , electrostatic latent images corresponding to the video signal described above are created on the surfaces of the photosensitive members  11  which are uniformly electrified by the electrifiers  12 . Developer units  15  make the toner in the respective colors adhere to these electrostatic latent images, thereby forming toner images in the respective colors. In the respective primary transfer parts  14 , thus formed toner images on the photosensitive members  11  are then primarily transferred onto the intermediate transfer belt  31  so that the toner images will be superimposed one atop the other. 
   In other words, the intermediate transfer belt  31  is rotated by the photosensitive member driving motor  36  at a predetermined peripheral velocity (which is the same as the transportation speed S 1  for the transfer papers  4  in this embodiment), and as shown in  FIG. 14 , at the time of forming the first image, environment conditions are loaded and determined at the time t 1 . Based on the environment condition, the output value of the primary transfer bias is changed from V 2  to V 3  and the video request signal Vreq is outputted. 
   In response to the video request signal Vreq outputted at the time t 1 , after the predetermined period T 1  from the time t 1 , formation of an electrostatic latent image which corresponds to the video signal representing the color Y is started. After the predetermined period T 2  from the time t 1 , formation of an electrostatic latent image which corresponds to the video signal representing the color C is started. After the predetermined period T 3  from the time t 1 , formation of an electrostatic latent image which corresponds to the video signal representing the color M is started. After the predetermined period T 4  from the time t 1 , formation of an electrostatic latent image which corresponds to the video signal representing the color K is started. The predetermined periods T 1 , T 2 , T 3  and T 4  are determined in advance based on the distances between the respective primary transfer parts  14  and the peripheral velocity of the intermediate transfer belt  31 , so that toner images on the photosensitive members  11  will be superimposed one atop the other when primarily transferred onto the intermediate transfer belt  31 . 
   The printer according to this embodiment uses a structure of the so-called tandem type as shown in  FIG. 12 . Since the intermediate transfer belt  31  is fed toward the primary transfer parts  14  immediately after cleaned by the belt cleaner  33  upon secondary transfer, even when secondary transfer is still ongoing, it is possible to continuously proceed to next image formation as soon as primary transfer ends. 
   Noting this, at the time t 2  which corresponds to the end of the first primary transfer (in Y in this example), environment conditions are loaded (# 1  in  FIG. 15 ) and whether it is necessary to change the output value of the primary transfer bias is determined (# 2 ). When it is not necessary to change the output value (NO at # 2 ), the same value is outputted as the output value (# 3 ) and outputting of the video request signal Vreq is permitted (# 4 ). This enables to perform next image formation. 
   On the contrary, when it is necessary to change the output value (YES at # 2 ), outputting of the video request signal Vreq is prohibited (# 5 ). Illustrated in  FIG. 14  is a situation that it is necessary to change the output value and therefore the video request signal Vreq is not outputted at the time t 2 . Further, since primary transfer is still ongoing at the time t 2 , the output value of the primary transfer bias is not changed. 
   Following this, at the time t 3  which corresponds to the end of the last primary transfer (which is the fourth primary transfer and transfer in K in this example), environment conditions are loaded again (# 6 ), and whether it is necessary to change the output value of the primary transfer bias is determined (# 7 ). When it is not necessary to change the output value (NO at # 7 ), the sequence proceeds to # 3 . 
   On the contrary, when it is necessary to change the output value (YES at # 7 ), whether the transfer paper  4  has been already loaded into the secondary transfer part  37  is judged (# 8 ). When the transfer paper  4  has been already loaded into the secondary transfer part  37  (YES at # 8 ), the printer remains on stand-by until discharging of the transfer paper  4  from the secondary transfer part  37 . When the transfer paper  4  has not been loaded into the secondary transfer part  37  yet (NO at # 8 ), the output value is changed (# 9 ), the sequence proceeds to # 4 , and outputting of the video request signal Vreq is permitted. At the time t 3  in  FIG. 14 , since it is YES at # 7  and NO at # 8 , the output value of the primary transfer bias is changed from V 3  to V 2  and the video request signal Vreq is outputted. 
   Meanwhile, the top-most transfer paper  4  among the bundle of transfer papers housed in the paper feeding cassette  3  is taken out by the pick-up roller  61  and nipped by the gate roller pair  34 . The gate clutch turns on at the time t 4 , which is after a predetermined period from the time t 1 , in synchronization to a color toner image on the intermediate transfer belt  31 , and the transfer paper  4  is transported toward the secondary transfer part  37  from the gate roller pair  34  at the predetermined speed S 1 . 
   At the time t 5  after a predetermined period from the time t 1 , application of the secondary transfer bias from the secondary transfer bias generating circuit  117  upon the secondary transfer roller  35  is activated. This realizes transfer onto the transfer paper  4  of the color image which is toner images Y, C, M and K as they are superimposed one atop the other and which was primarily transferred onto the intermediate transfer belt  31 . 
   The gate clutch turns off after discharging of the transfer paper  4 . The period during which the secondary transfer bias is applied is determined in advance in accordance with the size of the transfer papers  4 . The secondary transfer bias is turned off at the time t 6  which is after thus determined application period from the time t 1 . In the fixing unit  40 , this toner image is fixed on the transfer paper  4  during transportation of the transfer paper  4 . The transfer paper  4  is further discharged by the discharge roller pair  64  into the discharging part  9 . 
   In response to the video request signal Vreq outputted at the time t 3  described above, next toner images Y, C, M and K are formed, and the sequence shown in  FIG. 15  is similarly executed at the time t 7  (which is the end of primary transfer in the first color Y). While it is necessary to change the output value at the time t 7  shown in  FIG. 14 , since primary transfer is still ongoing, outputting of the video request signal Vreq is prohibited. 
   Following this, it is judged at the time t 8  (which is the end of primary transfer in the last color K) again whether it is necessary to change the output value. However, since the transfer paper  4  has been already loaded into the secondary transfer part  37  (YES at # 7  and YES # 8 ), the printer remains on stand-by. In short, the output value of the primary transfer bias is not changed and the video request signal Vreq is not outputted. 
   At the time t 9  which is switching of the post-transfer paper sensor  65  from on to off, the primary transfer bias is changed from V 2  to V 1  and the video request signal Vreq is outputted. After this, the sequence shown in  FIG. 15  is similarly executed at the time t 10  (which is the end of primary transfer in the first color Y). 
   An example of different operations of the printer will now be described with reference to  FIGS. 15 and 16 .  FIG. 16  is a timing chart which shows time-induced changes appearing in the conditions of the respective portions of the engine part  1 . The illustrated example is an example that the transfer paper size is larger than that in  FIG. 14 . 
   As in  FIG. 14 , toner images Y, C, M and K are formed at the predetermined timing after the time t 1 , and at the time t 2  which corresponds to the end of the first primary transfer (in Y in this example), environment conditions are loaded (# 1  in  FIG. 15 ) and whether it is necessary to change the output value of the primary transfer bias is determined (# 2 ). In  FIG. 16 , although it is necessary to change the output value, since primary transfer is still ongoing, the output value is not changed at the time t 2 . 
   Following this, the gate clutch turns on at the time t 3 , which is after a predetermined period from the time t 1 , in synchronization to a color toner image on the intermediate transfer belt  31 , and at the time t 4  which is after a predetermined period from the time t 1 , application of the secondary transfer bias from the secondary transfer bias generating circuit  117  upon the secondary transfer roller  35  is turned on. This starts secondary transfer although primary transfer is ongoing, whereby a color image, which is toner images Y, C, M and K as they are superimposed one atop the other and which was primarily transferred onto the intermediate transfer belt  31 , is transferred from the front edge onto the transfer paper  4 . 
   At the time t 5  which corresponds to the end of the last primary transfer (which is the fourth primary transfer and transfer in K in this example), environment conditions are loaded again (# 6  in  FIG. 15 ). In  FIG. 16 , although it is determined that it is necessary to change the output value of the primary transfer bias (YES at # 7 ), since secondary transfer which has started already is still ongoing and since it is determined that the transfer paper  4  has been already loaded into the secondary transfer part  37  (YES at # 8 ), the output value is not changed and the printer is held off for image formation. 
   At the time t 6  at which the post-transfer paper sensor  65 , passed by the rear edge of the transfer paper  4 , switches from on to off, the output value of the primary transfer bias is changed from V 3  to V 2  and the video request signal Vreq for next image formation is outputted. 
   As described above, according to this embodiment, since the output value of the primary transfer bias is changed when the transfer paper  4  has not been loaded into the secondary transfer part  37  yet, it is possible to change the output value when secondary transfer is not ongoing without fail. This makes it possible to securely prevent a change to the primary transfer bias from adversely affecting secondary transfer, and hence, the quality of an image transferred onto the transfer paper  4  from deteriorating. 
   Further, discharging of the transfer paper  4  from the secondary transfer part  37  is judged referring to switching of the post-transfer paper sensor  65  from on to off, and loading of the transfer paper  4  into the secondary transfer part  37  is judged based on the time required by the transfer paper  4  to arrive at the secondary transfer part  37  from the gate roller pair  34 . Hence, it is possible to judge whether the transfer paper  4  has been loaded into the secondary transfer part  37  or not without fail. 
   In addition, since environment conditions fed from the temperature sensor  6  and the humidity sensor  7  are judged and the output value of the primary transfer bias is changed in accordance with the result of the judgment, it is possible to perform primary transfer in an excellent manner independently of a change in transfer efficiency. 
   The present invention is not limited to the preferred embodiments described above. The preferred embodiments described above may be modified in various manners to the extent not deviating from the object of the invention. 
   For instance, although the preferred embodiments described above use the intermediate transfer belt  31  which is formed by an endless belt which does not have a splice, the intermediate transfer medium of the present invention is not limited to this. Instead, the intermediate transfer medium may be an intermediate transfer drum which has a cylindrical shape. 
   Further, although the preferred embodiments described above require to change the output value of the primary transfer bias in accordance with detection results of both the temperature sensor  6  and the humidity sensor  7 , this is not limiting. For example, only one of the temperature sensor  6  and the humidity sensor  7  may be disposed and the output value may be changed in accordance with a detection result of the temperature sensor  6  or the humidity sensor  7 . 
   The sequence of changing the output value of the primary transfer bias is not limited to the routine which is shown in  FIG. 15 . For instance, # 6  and # 7  may be omitted so as to proceed directly to # 8  at the end of the last primary transfer. Alternatively, environment conditions may be loaded after YES at # 8  and stand-by until discharging of the transfer paper  4  from the secondary transfer part  37  and whether it is necessary to change the output value may then be determined. 
   Further, while the foregoing has described the preferred embodiments above in relation to a color printer of the so-called tandem type in which the plurality of photosensitive members  11  are disposed one next to the other along the intermediate transfer belt  31 , this is not limiting. The preferred embodiments above may be applied to a monochrome printer which comprises one photosensitive member. 
   &lt;Other&gt; 
   The present invention is not limited to the preferred embodiments above. The preferred embodiments may be modified in various manners to the extent not deviating from the object of the invention. 
   In addition, while the foregoing has described the preferred embodiments above in relation to a printer which prints on a transfer paper an image which is fed from an external apparatus such as a host computer. The present invention is not limited to this, but may be applied to an electrophotographic image forming apparatus in a general use, such as a printer, a copier machine and a facsimile machine. 
   Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as other embodiments of the present invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.