Abstract:
An image heating apparatus includes a rotatable heating member configured to heat a toner image on a sheet; a belt unit including an endless belt configured to heat the rotatable heating member by contacting an outer surface of the rotatable heating member, and a supporting mechanism configured to rotatably supporting the endless belt; a detector configured to detect that the endless belt is out of a predetermined zone in a widthwise direction of the endless belt; and a tilting mechanism configured to tilt the belt unit in a direction of causing the endless belt to return into the predetermined zone based on an output of the detector.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image forming apparatus, such as a printer, a copying machine, a facsimile machine, and the like, which employs an electrophotographic image forming method or an electrostatic recording method. It relates to also an electrophotographic multifunction image forming apparatus and an electrostatic multifunction image recording apparatus which are capable of playing two or more roles of the abovementioned examples of image forming apparatus. Further, it relates to an image heating apparatus employable by an image forming apparatus such as the abovementioned ones. 
         [0002]    There have been known various types of image forming apparatuses. However, it is electrophotographic image forming apparatuses that are widely in current use. An image forming apparatus is desired to be high in productivity regardless of recording medium type. That is, not only is it desired to be high in productivity (in terms of number of prints per unit length of time) when recording medium is ordinary or thin paper, but also, when recording medium is cardstock or the like. 
         [0003]    In order for an image forming apparatus such as those mentioned above to be high in productivity even when recording medium is cardstock or the like, that is, recording medium which is significantly greater in basis weight, it has to employ a fixing device (image heating device) which is high in fixation speed. However, the amount by which a fixing device is robbed of heat by recording medium when cardstock or the like is used as recording medium is substantially greater than that when thin paper or the like is used as recording medium. In other words, the amount of heat required of a fixing device for image fixation when cardstock or the like is used as recording medium is substantially greater than that when thin paper or the like is used as recording medium. One of the known methods to deal with cardstock or the like is to reduce a fixing device (hence, image forming apparatus) in productivity (fixation speed; number of prints per unit length of time). 
         [0004]    Thus, there have been devised various methods for dealing with cardstock or the like without reducing a fixing device (image heating device) in productivity. One of them is disclosed in Japanese Laid-open Patent Application 2007-212896. According to this patent application, a fixing device is provided with an external heating means which is placeable in contact with the peripheral surface of the fixation roller of the fixing device to keep the temperature of the peripheral surface of the fixation roller at a preset target level. More specifically, in order to substantially improve a fixing device in terms of its ability to keep the fixation roller stable in surface temperature at a preset level, the fixing device is provided with an external heating belt (endless belt), instead of an external heating roller, because an external heating belt is substantially greater in the area of contact between the external heating means and fixation roller than an external heating roller. The external heating belt (endless belt) is suspended (supported) by a pair of belt supporting rollers in such a manner that it can be circularly movable in contact with the peripheral surface of the fixation roller to externally heat the fixation roller. 
         [0005]    However, unless it is ensured that the two belt supporting rollers remain perfectly parallel to each other, the external heating belt is made to shift in its widthwise direction, making it possible for the belt to become unstable in its movement. Yet, it is virtually impossible to construct a fixing device so precisely that a pair of its endless belt supporting rollers remain perfectly parallel to each other. One of the possible solutions to this problem is to structure a fixing device so that one of the pair of belt supporting rollers can be tilted relative to the other to control the external heating belt in positional deviation. This solution, however, is difficult to adopt, because the external heating belt is required to heat the fixation roller. More specifically, in the case of this method, that is, in the case where a fixing device is structured so that one of the belt supporting rollers can be tilted relative to the other, pivotally moving one of the belt supporting roller relative to the other possibly causes the heating range of the heating belt to partially disengage from the fixation roller, which in turn reduces the heating belt in performance. With the heating belt reduced in performance, the fixing device is likely to fail to properly fix an unfixed toner image. 
       SUMMARY OF THE INVENTION 
       [0006]    Thus, one of the primary objects of the present invention is to provide an image heating apparatus (device) which is superior to any image heating apparatus in accordance with the prior art, in terms of the stability of endless belt movement. 
         [0007]    Another object of the present invention is to provide an image forming apparatus which is superior to any image forming apparatus in accordance with the prior art, in terms of the stability of endless belt movement. 
         [0008]    According to an aspect of the present invention, there is provided, for example, an image heating apparatus comprising a rotatable heating member configured to heat a toner image on a sheet; a belt unit including an endless belt configured to heat said rotatable heating member by contacting an outer surface of said rotatable heating member, and a supporting mechanism configured to rotatably supporting said endless belt; a detector configured to detect that said endless belt is out of a predetermined zone in a widthwise direction of said endless belt; and a tilting mechanism configured to tilt said belt unit in a direction of causing said endless belt to return into the predetermined zone based on an output of said detector. 
         [0009]    According to another aspect of the present invention, there is provided, for example, an image forming apparatus comprising a belt unit including an endless belt and a supporting roller rotatably supporting said endless belt at an inner surface of said endless belt; a driving rotatable member contacted to an outer surface of the endless belt to rotate said endless belt; a detector configured to detect a position of said endless belt in a widthwise direction of said endless belt; and a tilting mechanism configured to tilt, in accordance with an output of the detector, said belt unit so that an axis of the supporting roller which press-contacts said endless belt to said heating rotatable member crosses with a generatrix of the heating rotatable member. 
         [0010]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic sectional view of a fixing device, in the first embodiment of the present invention, which has an external heating belt. It shows the general structure of the device. 
           [0012]      FIG. 2  is a schematic sectional view of an image forming apparatus which employs a fixing device in accordance with the present invention. It shows the general structure of the apparatus. 
           [0013]      FIG. 3  is a front view of the fixation roller heating external unit of the fixing device in the first embodiment of the present invention. It shows the general structure of the unit. 
           [0014]      FIG. 4  is a partially broken plan view of the fixation roller heating unit of the fixing device in the first embodiment, as seen from the direction perpendicular to the recording medium conveyance direction. It shows the general structure of the unit. 
           [0015]      FIG. 5(   a ) is a front view of the driving section of the fixation roller heating external unit in the first embodiment, and  FIG. 5(   b ) is a front view of the driving section of the fixation roller heating external unit, minus its sector gear, in the first embodiment. They show the general structure of the driving section. 
           [0016]      FIGS. 6(   a ) and  6 ( b ) are front views of the driving section of the fixation roller heating external unit in the first embodiment, as seen when the unit is being driven. 
           [0017]      FIG. 7  is a graph which shows the relationship between the distance by which the point of contact between the shaft  203  and sector gear  118 , and the amount of load to which the external heating belt is subjected in a manner to be laterally shifted. 
           [0018]      FIG. 8  is an external perspective view of the mechanism of the fixing device in the first embodiment, which is for detecting the amount of positional deviation (lateral shift) of the fixation roller heating external belt. 
           [0019]      FIGS. 9(   a ) and  9 ( b ) are plan views of the mechanism of the fixing device in the first embodiment, which is for detecting the lateral shift of the external heating belt of the fixing device, as seen when the mechanism is in action. 
           [0020]      FIG. 10  is a flowchart of the control sequence of the operation for controlling the positional deviation (lateral shift) of the fixation roller heating external belt in the first embodiment. 
           [0021]      FIG. 11  is a block diagram of the system for controlling the fixation roller heating external belt unit, in the first embodiment. 
           [0022]      FIG. 12  is a schematic front view of the fixation roller heating external unit of the fixing device in the second embodiment. It shows the general structure of the unit. 
           [0023]      FIG. 13  is a plan view of the partially broken fixation roller heating external unit of the second embodiment, as seen from the direction perpendicular to the recording medium conveyance direction. It shows the general structure of the unit. 
           [0024]      FIGS. 14(   a ) and  14 ( b ) are plan views of the external heating unit supporting units in the first and second embodiments, respectively, as seen when the two units are the same in the angle of intersection between the fixation roller and external heating unit supporting unit. 
           [0025]      FIGS. 15(   a ) and  15 ( b ) are plan views of the external heating unit supporting units in the second and third embodiments, respectively, as seen when the two units are the same in the angle of intersection between the fixation roller and external heating unit supporting unit. 
           [0026]      FIG. 16  is a table which shows the results of the experiment carried out for comparing the effects of the first, second, and third embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Hereinafter, embodiments of the present invention are described in detail with reference to the appended drawings. Incidentally, in the description of the following embodiments of the present invention, an image heating apparatus in accordance with the present invention is described as a fixing device for fixing an unfixed toner image to a sheet of recording medium (paper). However, the present invention is also applicable to an image heating device (apparatus) for applying heat and pressure to the fixed or semi-fixed image on a sheet of recording medium in order to modify the toner image in surface properties. 
       Embodiment 1 
       [0028]    To begin with, an image forming apparatus  100 , which is compatible with the present invention, is described with reference to  FIG. 2 , which is a schematic sectional view of the image forming apparatus  100  having an image heating device which functions as a fixing device. This image forming apparatus  100  is a color laser beam printer of the so-called tandem type, which has the first, second, third and fourth image formation stations Pa, Pb, Pc and Pd, which are aligned in the listed order, in the moving direction of its intermediary transfer belt  130 . Incidentally,  FIG. 2  does not show a fixation roller heating external unit  34 , which will be described later. 
       &lt;Image Forming Apparatus&gt; 
       [0029]    Referring to  FIG. 2 , the image forming apparatus  100  internally holds the first, second, third, and fourth image formation stations Pa, Pb, Pc and Pd which are aligned in tandem, and in which multiple (four) monochromatic toner images, different in color, are formed, one for one, sequentially through a process of forming a latent image, a process of developing the latent image, and a process of transferring the developed latent image. Each of these image formation stations Pa, Pb, Pc and Pd has its own electrophotographic photosensitive member (in this embodiment, photosensitive drums  3   a ,  3   b ,  3   c  and  3   d ), and forms a monochromatic toner image which is different in color from those formed in the other image formation stations. 
         [0030]    The image forming apparatus  100  is provided with the intermediary transfer belt  130 , which is positioned so that its outward surface contacts the peripheral surface of each of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d . The toner images, different in color, formed on the peripheral surfaces of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , one for one, are transferred (primary transfer) onto the intermediary transfer belt  130 , and then, are transferred (secondary transfer) onto a sheet P of recording medium, in the secondary transfer station. After the transfer of the toner images, different in color, onto the sheet P, the sheet P is conveyed to the fixing device  9 , in which the toner images are fixed to the sheet P by the application of heat and pressure to the sheet P and toner images thereon. After the fixation of the toner images to the sheet P, the sheet P is discharged as a finished print from the image forming apparatus  100 . A combination of the image formation stations Pa, Pb, Pc and Pd, and intermediary transfer belt  130  makes up an image forming unit. The abovementioned fixing device  9  is for fixing to the sheet P, the toner images formed on the sheet P by this image formation unit. 
         [0031]    The image forming apparatus  100  is also provided with drum charging devices  2   a ,  2   b ,  2   c  and  2   d , developing devices  1   a ,  1   b ,  1   c  and  1   d , primary transfer charging devices  24   a ,  24   b ,  24   c  and  24   d , and cleaners  4   a ,  4   b ,  4   c  and  4   d , which are positioned in the adjacencies of the peripheral surface of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. The image forming apparatus  100  has also laser scanners  5   a ,  5   b ,  5   c  and  5   d , which are in the top portion of the image forming apparatus  100 . 
         [0032]    Each of the laser scanners  5   a ,  5   b ,  5   c  and  5   d  internally holds unshown light source and a polygonal mirror. The beam of laser light emitted from the light source is deflected by the rotating polygonal mirror, deflected by a stationary mirror, and focused by the f-θ lens (unshown) onto the peripheral surfaces of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , so that the beam of laser light is made to scan (expose) the peripheral surface of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d . Thus, a latent image which reflects image formation signals is formed on the peripheral surface of each of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d.    
         [0033]    The developing devices  1   a ,  1   b ,  1   c  and  1   d  contain a preset amount of cyan, magenta, yellow, and black toners, respectively, which are delivered thereto by an unshown toner delivering devices. The developing devices  1   a ,  1   b ,  1   c  and  1   d  develop the latent images on the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , into visible images, that is, cyan, magenta, yellow and black toner images, respectively. 
         [0034]    The intermediary transfer belt  130  is circularly driven in the direction indicated by an arrow mark E in  FIG. 2 , at the same speed as the peripheral velocity of each of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d . While the cyan toner image, that is, the image formed on the photosensitive drum  3   a  in the first image formation station Pa, is conveyed through the nip between the photosensitive drum  3   a  and intermediary transfer belt  130 , it is transferred onto the outward surface of the intermediary transfer belt  130 , in terms of the loop which the intermediary transfer belt  130  forms, by a combination of the electric field formed by the primary transfer bias applied to the intermediary transfer belt  130 , and the pressure in the nip. 
         [0035]    Designated by a referential code  11  is a secondary transfer roller, which is supported by a pair of bearings, in parallel to the widthwise direction of the intermediary transfer belt  130  and in contact with the outward surface of the intermediary transfer belt  130 . The secondary transfer roller  11  is kept pressed against a roller  14 , which is one of the through rollers  13 ,  14  and  15 , by which the intermediary transfer belt  130  is suspended and kept tensioned, with the placement of the intermediary transfer belt  130  between the secondary transfer roller  11  and roller  14 . As it is kept pressed against the roller  14 , it forms the secondary transfer nip between itself and roller  14 . To the secondary transfer roller, a preset secondary transfer bias is applied by a secondary transfer bias power source. 
         [0036]    After the formation of a synthetic full-color toner image by the transfer in layers of the magenta, yellow, and black toner images onto the intermediary transfer belt  130  in such a manner that they are layered on the cyan toner image on the intermediary transfer belt  130 , the full-color toner image is transferred onto a sheet P of recording medium as follows. That is, the sheet P of recording medium is delivered to the nip between the intermediary transfer belt  130  and secondary transfer roller  11  from a sheet feeder cassette  10 , through the pair of registration rollers  12  and a pre-transfer sheet guide (unshown). Then, it is conveyed through the nip while the secondary transfer bias is applied to the secondary transfer roller  11  from the secondary bias power source. Thus, the synthetic full-color image is transferred onto the sheet P from the intermediary transfer belt  130  by the secondary transfer bias. 
         [0037]    Similarly, magenta, cyan and black toner images, that is, the toner images formed in the second, third, and fourth image formation stations Pb, Pc and Pd are transferred onto the intermediary transfer belt  130  in such a manner that they are layered on the cyan toner image on the intermediary transfer belt  130 . Consequently, a synthetic full-color image, which is virtually identical to the original image, is formed. The synthetic multicolor toner image is formed in such a manner that as it is transferred onto a sheet P of recording medium, a preset amount of margin will be left along the edges of the sheet P. 
         [0038]    After the primary transfer, the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d  are cleaned by the cleaners  4   a ,  4   b ,  4   c  and  4   d , respectively (toner remaining on peripheral surface of photosensitive drums  3   a ,  3   b ,  3   c  and  3   d  is removed by cleaners  4   a ,  4   b ,  4   c  and  4   d ), being thereby prepared for the subsequent formation of a latent image thereon. The toner remaining on the intermediary transfer belt  130  after the secondary transfer, and the like contaminants are wiped away by a cleaning web  19  (nonwoven cloth) which is placed in contact with the surface of the intermediary transfer belt  130 . 
         [0039]    After the second transfer, or the transfer of the multicolor toner image onto the sheet P of recording medium, the sheet P is introduced into the fixing device  9 , in which the unfixed multicolor toner image is fixed to the sheet P by the heat and pressure applied thereto by the fixing device. When the image forming apparatus is in the two-sided printing mode, the sheet P of recording medium is fed from the sheet feeder cassette  10  into the main assembly of the apparatus  100 , and conveyed further by the pair of registration rollers  12  and pre-transfer guide. Then, it is conveyed through the nip between the intermediary transfer belt  130  and secondary transfer roller  11 , in which unfixed multicolor toner image is fixed to one the two surfaces of the sheet P (first surface). Then, the sheet P is conveyed out of the fixing device  9 , and is guided into the sheet reversal passage  17  by the flapper  16  (sheet directing member). 
         [0040]    Then, the sheet P is changed in direction, and guided into the two-sided printing passage  30 , by a pair of sheet reversing rollers  18 . Then, it is conveyed by the pair of registration rollers  12 , guided by the pre-transfer guide, conveyed through the nip between the intermediary transfer belt  130  and secondary transfer roller  11 , and conveyed through the fixing device  9 , for the second time, in which the unfixed multicolor toner image on the second surface of the sheet P is fixed. While the second multicolor toner image is formed on the second surface of the sheet P, the flapper  16  (sheet directing member) is switched in position, so that after the fixation of the second image onto the second surface of the sheet P, the sheet P is discharged as a two-sided print from the image forming apparatus  100 . 
       [Fixing Apparatus] 
       [0041]    Next, referring to  FIG. 1 , the fixing device  9 , which functions as an image heating device, is described in detail.  FIG. 1  is a schematic sectional view of the fixing device  9  in this embodiment, which is equipped with an endless belt for externally heating the fixation roller of the fixing device  9 . It shows the general structure of the device  9 . As described above, the image forming apparatus  100  is equipped with the fixing device  9 , as an image heating device, which is in accordance with the present invention. 
         [0042]    Referring to  FIG. 2 , the fixing device  9  has a function of heating the unfixed toner image K on the sheet P of recording medium, with its fixation roller  101 , while the sheet P is conveyed through the fixation nip N. It has a fixation roller heating external unit  34 , the fixation roller  101 , the pressure roller  102 , and an unshown external frame in which the preceding components are encased. The fixation roller heating external unit  34  has a holding unit  43  for holding the external heating unit  34 . 
         [0043]    More specifically, the fixing device  9  has: the fixation roller  101  as a rotational heating member (heat roller) for heating the image on the sheet P of recording medium; pressure roller  102 , as a rotational pressure applying member (nip forming member), which is kept pressed upon the fixation roller  101  to form the fixation nip N between the peripheral surface of the fixation roller  101  and the peripheral surface of the pressure roller  102 ; and fixation roller heating external unit  34  (belt unit). The external heating unit holding frame  48 , as an external heating unit holding mechanism, is provided with a heating belt supporting rollers  103  and  104 , and a fixation roller heating external belt  105  (which hereafter may be referred to simply as external heating belt  105 ). The external heating belt  105  is suspended by the rollers  103  and  104 , which are held together by the fixation roller heating external unit holding frame  48  so that the rotational axes of the rollers  103  and  104  remain parallel to each other. 
         [0044]    The fixing device  9  is structured so that the fixation roller  101  is rotationally driven in the direction indicated by an arrow mark A at a preset peripheral velocity, by a fixation roller driving mechanism M ( FIG. 3 ) made up of a motor and a gear train. The fixation roller  101  has: a cylindrical metallic core (which in this embodiment is made of aluminum); a heat resistant elastic layer which is formed of silicone rubber, on the outward surface of the metallic core; and a heat resistant parting layer form of fluorinated resin (which in this embodiment is tube made of PFA (polytetrafluoroethylene) which covers the elastic layer to make it easier for toner to separate from the peripheral surface of the fixation roller  101 . 
         [0045]    The fixing device  9  is provided with a halogen heater  111 , as a heating means, which is in the hollow of the metallic core of the fixation roller  101 . The halogen heater  111  heats the fixation roller  101  from within the fixation roller  101  so that the surface temperature of the fixation roller  101  remains at a preset level. More specifically, the surface temperature of the fixation roller  101  is detected by a thermistor  121 , as a temperature detecting means, which is in contact with the peripheral surface of the fixation roller  101 . Based on this temperature detected by the thermistor  121 , the control section  40  ( FIG. 11 ) issues a command to a heater controlling section  140 , which is a fixation roller temperature controlling (adjusting) means, to turn on or off the halogen heater  111  through a heater controller  43  and heater driver  44  ( FIG. 11 ) so that the surface temperature of the fixation roller  101  remains at a preset target level. 
         [0046]    The pressure roller  102  forms the fixation nip N between itself and the fixation roller  101  by being pressured upon the fixation roller  101  by a preset amount of pressure applied to the pressure roller  102  by an unshown pressure applying means. It is rotated in the direction indicated by an arrow mark B at a preset peripheral velocity, by the rotation of the fixation roller  101  which is rotationally driven by the unshown driving section. 
         [0047]    The pressure roller  102  has: a cylindrical metallic core (which in this embodiment is made of aluminum); a heat resistant elastic layer which is formed of silicone rubber, on the outward surface of the metallic core; and a heat resistant parting layer formed of fluorinated resin (which in this embodiment is tube made of PFA) which covers the elastic layer to make it easier for toner to separate from the peripheral surface of the pressure roller  102 . 
         [0048]    The fixing device  9  is provided with a halogen heater  112 , as a heating means, which is in the hollow of the metallic core of the pressure roller  102 . The halogen heater  112  heats the pressure roller  102  from within the pressure roller  102  so that the surface temperature of the pressure roller  102  remains at a preset level. More specifically, the surface temperature of the pressure roller  102  is detected by a thermistor  122 , as a temperature detecting means, which is in contact with the peripheral surface of the pressure roller  102 . Based on this temperature detected by the thermistor  122 , the control section  40  issues a command to a heater controlling section  140  to turn on or off the halogen heater  112  through a heater controller  43  and heater driver  44  ( FIG. 11 ) so that the surface temperature of the pressure roller  102  remains at a preset target level. 
       [Fixation Roller Heating External Unit] 
       [0049]    Next, referring to  FIG. 1 , the fixation roller heating external unit  34  (belt unit) with which the fixing device  9  is provided is described in detail. 
         [0050]    Referring to  1 , the fixing device  9  is provided with the fixation roller heating external belt  105 , which is an endless belt for heating the fixation roller  101  by being placed in contact with the peripheral surface of the fixation roller  101 . The belt  105  is suspended and kept stretched by the belt supporting upstream and downstream rollers  103  and  104 , respectively, in terms of the rotational direction of the fixation roller  101 , which function together as a belt supporting mechanism. The rollers  103  and  104  are positioned upstream and downstream, respectively, in terms of the moving direction of the belt  105 , and suspend and keep stretched the belt  105 . The fixing device  9  is structured so that the belt supporting rollers  103  and  104  circularly move the belt  105  while keeping the belt  105  pressed upon the peripheral surface of the fixation roller  101 . Further, the fixing device  9  is structured so that the belt  105  is circularly moved by the rotational movement of the fixation roller  101 , and the belt suspending rollers  103  and  104  are rotated by the circular movement of the belt  105 . 
         [0051]    The belt suspending rollers  103  and  104  are positioned in such a manner that their rotational axes are parallel to each other. They are kept pressed against the peripheral surface of the fixation roller  101  by a preset amount of pressure generated by a pressure applying section  204  ( FIG. 3 ) such as a pair of compression springs, with the presence of the fixation roller heating external belt  105  between the two rollers  103  and  104 , and the fixation roller  101 . Thus, the outward surface of the belt  105  is kept pressed upon the peripheral surface of the fixation roller  101 . Further, the fixing device  9  is structured so that the external heating belt  105  can be placed in contact with, or separated (retracted) from, the fixation roller  101 , and also, so that as the external heating belt  105  is placed in contact with the fixation roller  101 , it forms a heating nip Ne, between itself and fixation roller  101 . Further, the fixing device  9  is structured to suspend the belt suspending rollers  104  and  105  in such a manner that as the external heating belt  105  is pressed upon the peripheral surface of the fixation roller  101 , it is circularly movable by the rotation of the fixation roller  101 . 
         [0052]    The external heating belt  105  is made up of a substrate layer and a surface layer. The substrate layer is made of a metallic substance (stainless steel, nickel, or the like) or a resinous substance (PI or the like). The surface layer is for preventing toner from adhering to the external heating belt  105 . It is formed of fluorinated resin (in this embodiment, substrate layer is covered with pieces of PFA tube). The external heating belt  105  heats the fixation roller  101  while remaining in contact with the peripheral surface of the fixation roller  101 , being thereby circularly moved in the direction indicated by an arrow mark C in  FIG. 1 , at a preset peripheral velocity, by the rotation of the fixation roller  101 . 
         [0053]    The fixing device  9  is also provided with a cleaning roller  108 , which is positioned in contact with the outward surface of the external heating belt  105 . In terms of the rotational direction of the fixation roller  101 , the cleaning roller  108  is between the fixation roller  101  and thermistor  121 . More specifically, the cleaning roller  108  is on the upstream side of the thermistors  123  and  124  in terms of the moving direction of the external heating belt  105 , and is kept pressed upon the external heating belt  105  by a preset amount of pressure. It is made up of a metallic core, and a porous surface layer formed of sponge or the like. It cleans the outward surface of the external heating belt  105  while remaining pressed upon the external heating belt  105  by a preset amount of pressure generated by an unshown pressure applying means. 
         [0054]    In terms of the rotational direction of the fixation roller  101 , the heating belt supporting roller  104 , which is one of the rollers by which the external heating belt  105  is suspended, is on the downstream side, relative to the other heating belt supporting roller. The heating belt supporting roller  104  is made up of a metallic core, and a surface layer for minimizing the friction between the roller  104  and inward surface of the external heating belt  105 . In this embodiment, the surface layer is a piece of tube made of PFA. 
         [0055]    Further, the fixing device  9  is provided with a halogen heater  114 , as a heating means, which is positioned in the hollow of the metallic core of the belt supporting roller  104  to internally heat the supporting roller  104  so that the surface temperature of the external heating belt  105  remains at a preset level. 
         [0056]    Similarly, the heating belt supporting roller  103 , which is one of the rollers by which the external heating belt  105  is suspended, internally heats the external heating belt  105  by being kept in contact with the inward surface of the external heating belt  105 . The supporting roller  103  is made up of a metallic core, and a surface layer for minimizing the friction between the roller  103  and inward surface of the external heating belt  105 . In this embodiment, the surface layer is a piece of tube made of PFA. 
         [0057]    Further, the fixing device  9  is provided with a halogen heater  113 , as a heating means (heater), which is positioned in the hollow of the metallic core of the belt supporting roller  103  to internally heat the supporting roller  103  so that the surface temperature of the external heating belt  105  remains at a preset level. 
         [0058]    The surface temperature of the external heating belt  105  is detected by the thermistors  123  and  124 . The thermistor  123 , which is a temperature detecting means, is kept in contact with the external heating belt  105 , in a range D 1  of the area of contact between the belt supporting roller  103  and external heating belt  105 . The thermistor  124 , which also is a temperature detecting means, is kept in contact with the external heating belt  105 , in a range D 2  of the area of contact between the belt supporting roller  104  and external heating belt  105 . It is based on the temperature levels detected by the thermistors  123  and  124  that the control section  140  ( FIG. 11 ) issues a command to the heater control section  140  to make the control section  140  to turn on or off the halogen heaters  113  and  114  through the heater controller  34  and heater driver  44  so that the surface temperature of the external heating belt  105  reaches and remains at a preset level. 
         [0059]    The target level for the temperature of the fixation roller heating external belt  105  is set to be higher than that of the fixation roller  101  for the following reason. That is, keeping the heating belt  105  higher in temperature than the fixation roller  101  makes the heating belt  105  quicker in its response (accuracy in thermal response) to the drop in the surface temperature of the fixation roller  101  attributable to a sheet of recording medium which is being conveyed through the fixing device  9 ; heat is quickly (efficiently) transferred from the heating belt  105  to the fixation roller  101 . 
         [0060]      FIG. 3  is a front view of the fixation roller heating external unit, in this embodiment, having the heating belt  105 . It shows the structure of the unit.  FIG. 4  is a plan view of the fixation roller heating external unit, in this embodiment, having the heating belt  105 , as seen from the direction perpendicular to the lengthwise direction of the unit. It shows the structure of the unit. 
         [0061]    Referring to  FIGS. 3 and 4 , the fixing device  9  is structured so that its fixation roller heating external unit  34  is rotationally (pivotally) movable by the mechanism (which will be described next), so that the axial lines of the heating belt supporting rollers  103  and  104  intersect with the direction of generatrix of the peripheral surface of the fixation roller  101  (direction indicated by arrow mark X in  FIGS. 4 and 8 ), while making the rollers  103  and  104  keep the heating belt  105  pressed upon the fixation roller  101 . That is, the fixation roller heating external unit  34  has an external heating unit supporting frame  48 , and a shaft  203  positioned between the lateral plates  202   a  and  202   b  of the external frame (casing) of the fixing device  9 . The shaft  203  is supported by a side plate  202   a , that is, one of the lateral plates of the external frame of the fixing device  9 , by one of its lengthwise ends, in such a manner that it can be pivotally moved in the left or right direction of  FIG. 4 , about a pivot  33  of a shaft supporting member  39  which is a device for holding the fixation roller heating external unit  34  in such a manner that the unit  34  can be rotationally (pivotally) moved. The other end of the shaft  203  is put through a through hole  38  with which the lateral plate  202   b  of the external frame of the fixing device  9 . The diameter of the through hole  38  is greater than the external diameter of the shaft  203 . The abovementioned lengthwise end of the shaft  203  is supported by the pivot  33 . Therefore, the shaft  203  is pivotally movable in the direction indicated by an arrow mark E in  FIG. 4 , or the direction indicated by an arrow mark F in  FIG. 4 . As described above, the frame  48  for holding the fixation roller heating external unit  34  is supported by the end (top end in  FIG. 4 ) in terms of the direction of the rotational axis of the fixation roller  101  (top-to-bottom direction in  FIG. 4 ) so that it can be pivotally moved about the top end. 
         [0062]    Further, the fixing device  9  is provided with a pair of pressure application arms  117   a  and  117   b  which are between the lateral plates  202   a  and  202   b  of the external frame of the fixing device  9 . The pressure application arms  117   a  and  117   b  are rotationally (pivotally) supported by the shaft  203  which extends from one end of the fixing device  9  to the other. They are kept pressed toward the fixation roller  101  by the pressure from aforementioned pressure applying section  204 . The pressure application arm  117   a  is positioned next to the lateral plate  202   a  of the external frame of the fixing device  9 , in such manner that it extends in the lengthwise direction of the lateral plate  202   b.    
         [0063]    That is, the holding unit  48  is supported by a pair of shafts  32 , by its supporting members  206   a  and  206   b  (lateral plates), in such a manner that it is rotationally (pivotally) movable about the shafts  32 . Further, the shafts  32  are attached to the roughly center portions of the pressure application arms  117   a  and  117   b , respectively, in terms of the lengthwise direction of the arms  117   a  and  117   b . Further, the aforementioned heating belt supporting rollers  103  and  104 , by which the fixation roller heating external belt  105  is suspended, are rotatably supported by the lateral plates  206   a  and  206   b  of the supporting unit  48 . The lateral plates  206   a  and  206   b  are connected to each other by their top portions, by a plate  49  which bridges between the two lateral plates  206   a  and  206   b.    
         [0064]    Further, the fixation roller heating external unit supporting unit  34  is provided with a roughly elliptic cam  205 , which is rotatably supported by a shaft  45 . The cam  205  is below the front end portion (left end portion in  FIG. 3 ) of the pressure application arm  117   b , or the front arm, in  FIG. 3 . The cam  205  functions as a part of the mechanism which moves the pressure application arms  117   a  and  117   b  to place the heating belt  105  in contact with, or separated from, the fixation roller  101 . That is, the cam  205  moves the pressure application arms  117  by pressing the pressure application arm  117   b  upward against the resiliency of the pressure applying section  204 , or allowing the pressure application arm  117   b  to be moved downward by the pressure generated by the resiliency of the pressure application section  204 . Thus, the fixation roller heating external belt  105  indirectly held by the lateral plates  202   a  and  202   b , through the shafts  32  and  32 , lateral plates  206   a  and  206   b , and belt supporting rollers  103  and  104 , can be placed in contact with, or separated from, the fixation roller  101 . 
         [0065]    The lengthwise end portion of the shaft  203 , which protrudes outward of the lateral plate  202   b  through the through hole  38  of the lateral plate  202   b , is rotatably supported by a bearing  126 , which is on the outward side of the lateral plate  202   b . It is also put through the elongated hole  115  with which a sector gear  118  (fan-shaped gear) is provided, in such a manner that the lengthwise end portion of the shaft  203  is allowed to slide along the edge of the elongated hole  115 . That is, the lengthwise end portion of the shaft  203  is put through the bearing  126 , which is positioned between the lateral plate  202   b  of the external frame of the fixing device  9 , and the sector gear  118 . 
         [0066]    The sector gear  118  is on the outward side of the lateral plate  202   b  of the external frame of the fixing device  9 , and is rotatably supported by a shaft  119  attached to the lateral plate  202   b . It has: a downwardly facing toothed section  118   b ; the aforementioned elongated hole  115 , the lengthwise direction (long axis) of which coincides with the axial line of the shaft  119 ; and a light blocking section  118   a  which is next to the toothed section  118   b  and extends roughly downward. Thus, as the sector gear  118  is pivotally moved about the shaft  119 , its light blocking section  118   a  moved into, or out of, the slit between the light emitting portion and light sensing portion of a photo-interrupter  135  ( FIG. 5 ). Next, referring to  FIGS. 5(   a ) and  5 ( b ), the photo-interrupter  135  is attached to the lateral plate  202   b  of the external frame of the fixing device  9 , with the use of a bracket  35 , so that it coincides in position to the light blocking portion  118   a.    
         [0067]    The fixation roller heating external unit supporting unit  34  is provided with a motor  125  which is supported by the lateral plate  202   b  of the external frame of the fixing device  9 , being positioned adjacent to the sector gear  118 . To the axle  125   a  of the motor  125 , a worm gear  120  is solidly attached. The motor  125 , worm gear  118 , shaft  203 , etc., make up the mechanism  51  for pivotally moving the external heating unit holding frame  48 . This mechanism  51  is enabled to pivotally move the fixation roller heating external unit  34  (external heating unit holding frame  48 ) so that the axial lines of the belt supporting rollers  103  and  104  intersect with the axial line of the fixation roller  101  (generatrix direction), while keeping the external heating belt  105  pressed upon the peripheral surface of the fixation roller  101 . 
         [0068]    Next, referring to  FIG. 3 , the rotational axis Ce of the external heating unit holding frame  48  is perpendicular to the area of contact (nip Ne) between the external heating belt  105  and fixation roller  101 . That is, the fixing device  9  is structured so that the rotational axis Ce coincides with the center of the area of contact between the external heating belt  105  and fixation roller  101 , in terms of the rotation direction of the fixation roller  101 , and is parallel to the line normal to a line  53  which is tangent to the peripheral surface of the fixation roller  101  at the center of the area of contact between the external heating belt  105  and fixation roller  101 . In other words, the rotational axis Ce is virtually parallel to the normal line to the portion (portion W in  FIG. 3 ) of the external heating belt  105 , which is the flat portion of the belt  105  between the heat belt supporting rollers  103  and  104 , that is, the portion which is not in contact with the fixation roller  101 . 
         [0069]    In this embodiment, the axial line Ce is realized by the pressure application arms  117   a  and  117   b  which support the external heating unit holding frame  48 , and the pivot  33  around which the shaft  203  by which the pressure application arms  117   a  and  117   b  are supported by one of their lengthwise ends. The axial line Ce is at one of the lengthwise ends of the fixation roller  101 , in terms of the direction of its axial line (top-to-bottom direction in  FIG. 4 ). Further, the axial line Ce is perpendicular to the axial line (which extends in front-to-rear direction) of the fixation roller  101  (image heating member). 
         [0070]      FIG. 5(   a ) is a front view of the portion (mechanism  51 ) of the fixing device  9 , which is for driving the fixation roller heating external unit  34  in this embodiment, and  FIG. 5(   b ) is a front view of the same portion of the fixing device  9  as  FIG. 5(   a ), minus the sector gear  118 .  FIG. 6(   a ) is a front view of the portion (mechanism  51 ) of the fixing device  9 , which is for driving the fixation roller heating external unit  34  in this embodiment, as seen while the portion (mechanism  51 ) is pivotally moving the external heating unit  34  (external heating unit holding frame  48 ). First, the case in which the fixation roller heating external unit  34  (external heating unit holding frame  48 ) is pivotally moved in such a direction that the front end of the unit  34  moves upstream (direction indicated by arrow mark E in  FIG. 4) , is described. 
         [0071]    First, the motor  125  is driven to rotate the worm gear  120 , whereby the sector gear  118  is rotated in the direction indicated by an arrow mark G in  FIG. 6(   a ). The elongated hole  115  of the sector gear  118  extends in the direction parallel to the line which connects the center of the shaft  203  and pivot  119  of the sector gear  118 . 
         [0072]    Next, referring to  FIG. 5(   b ), the mechanism  51  for moving the fixation roller heating external unit  34  is provided with a pair of straight guides  127  and  127  which are parallel to each other. The two guides  127  and  127  are on the outward side of the lateral plate  202   b  of the external frame of the fixing device  9 , and are angled so that their left ends are positioned higher than their right ends. One of the guides  127  is on the top side of the bearing  126 , and the other is on the bottom side of the bearing  126 . Thus, the bearing  126  is allowed to move not only in the direction parallel to the two guides  127  and  127 , and also, in the direction which is intersectional to the guides  127  and  127 , by a distance equal to the clearance between the shafts  203  and the wall of through hole  38 . Thus, the bearing  126 , through which the shaft  203  is put, is allowed to move in the direction parallel to the lengthwise direction of the guides  127  and  127  while being regulated in its vertical movement by the guides  127  and  127 . 
         [0073]    Therefore, as the motor  125  is rotated forward, the sector gear  118  is rotated, through the worm gear  120 , in the direction indicated by the arrow mark G in  FIG. 6(   a ), causing thereby the shaft  203  to linearly move in the preset direction (indicated by arrow mark H in  FIG. 6(   a )). This linear movement of the shaft  203  is realized by the coordination among the guides  127  and  127 , the bearing  126 , and the shaft  203  pivotally attached to the pivot  33  by one end and put through the elongated hole  115  of the lateral plate  202   b  of the external frame of the fixing device  9  by the other end. 
         [0074]    That is, as the worm gear  120  is rotated by the forward rotation of the motor  125 , the sector gear  118  is pivoted in the direction indicated by the arrow mark G in  FIG. 6(   a ), causing thereby the bearing  126  to move along the guides  127  and  127 . Thus, the shaft  203  is moved in the direction indicated by the arrow mark H by the bearing  126  which moves along the guides  127  and  127 , while being guided by the elongated hole  115  which extends in the direction parallel to the line which connects the center of the shaft  203  and center of the pivot  119 . Consequently, the shaft  203  is pivoted in such a manner that the opposite end the shaft  203  from the pivot  119  linearly moves leftward in  FIG. 6(   a ). This linear movement of the opposite end of the shaft  203  causes the external heating unit holding frame  48  to pivot in such a manner that its front end moves upstream (direction indicated by arrow mark E in  FIG. 4) , along with the shaft  203 . 
         [0075]    On the other hand, if it is necessary for the external heating unit holding frame  48  to be pivoted in such a manner that its front end moves downstream (direction indicated by arrow mark F in  FIG. 4 ), all that is necessary is to rotate the motor  125  in reverse, that is, the direction opposite to the above described direction. As the worm gear  120  is made to pivot by the reverse rotation of the motor  125 , the sector gear  118  is made to pivotally move in the direction indicated by an arrow mark I in  FIG. 6(   b ), whereby the bearing  126  is made to linearly moved in the direction indicated by an arrow mark J, that is, the opposite direction from the above described direction, along the guides  127  and  127 . Thus, the opposite end of the shaft  203  from the pivot  119  linearly move rightward in  FIG. 6(   b ). This linear movement of the opposite end of the shaft  203  causes the external heating unit holding frame  48  to pivotally move in such a manner that its front end moves downstream (direction indicated by arrow mark F in  FIG. 4)  along with the shaft  203 . 
         [0076]    As described above, as the shaft  203  is made to pivot in such a manner that its front end portion, to which the external heating unit holding frame  48  is attached, moves upstream or downstream, the frame  48 , which is supported by the shaft  203 , rotationally moves about the rotational axis Ce (about pivot  33 , in reality). Consequently, the angle of intersection between the heating belt supporting rollers  103  and  104 , and the fixation roller  101  changes. 
         [0077]    It has been known that in the case of the fixing device  9  equipped with the fixation roller heating external device which uses an endless heating belt, there is a correlation between the angle of the fixation roller  101  with which the external heating belt  105  is placed in contact, and the external heating belt  105  (belt supporting rollers  103  and  104 ), and the amount by which the external heating belt  105  laterally shifts (deviates in position) as it is circularly moved. Thus, the fixing device  9  can be controlled in the unwanted lateral shift of the external heating unit  105 , by pivotally moving the shaft  203  in such a manner that the portion of the shaft  203 , to which the external heating unit holding frame  48  is attached, moves upstream or downstream in terms of the direction in which recording medium is conveyed, because moving the portion of the shaft  203 , to which the front end of the external heating unit holding frame  48  is attached, upstream or downstream changes the angle of intersection between the external heating belt  105  suspended by the belt supporting rollers  103  and  104 , and the fixation roller  101 , which in turn controls the lateral shifting of the external heating belt  105 . 
         [0078]    Here, referring to  FIG. 7 , the relationship between the amount by which the front end of the external heating unit holding frame  48  is moved, and the amount of force generated in the direction to laterally shift the external heating belt  105  is described.  FIG. 7  shows the relationship between the amount of force generated in the direction to laterally shift the external heating belt  105 , and the amount by which the portion of the shaft  203 , to which the front end of the external heating unit holding frame  48  is attached, is moved. 
         [0079]    The amount of force by which the external heating belt  105  was made to laterally shift (deviate) was measured by placing a pair of rollers in contact with the lateral edges of the external heating belt  105  one for one. That is, as the external heating belt  105  was circularly moved by the rotation of the fixation roller  101 , the amount of load to which one of the rollers was subjected by the lateral shift (deviation) of the heating belt  105  in the direction parallel to the rotational axis of the belt supporting rollers  103  and  104 , was measured with a load cell (unshown). 
         [0080]    The horizontal axis of the graph in  FIG. 7  stands for the amount [mm] by which the front end of the external heating unit holding frame  48  (shaft  203 ) moved, and the vertical axis stands for the amount [N] of force by which the external heating belt  105  is made to laterally shift. In  FIG. 7 , a point (0, 0) is the idealistic point, that is, a point at which the circular movement of the external heating belt  105  does not cause the belt  105  to laterally shift. 
         [0081]    Also in the graph, the plus and minus directions correspond to the upstream and downstream directions (indicated by arrow marks E and F, respectively, in  FIG. 4 ) in which the free end (front end) of the shaft  203  moves, respectively. Regarding the amount [N] of force by which the external heating belt  105  is moved, and which is represented by the vertical axis of the graph, the positive side corresponds to the force which works in the direction to move the external heating belt  105  frontward of the fixing device  9  (direction indicated by arrow mark L in  FIG. 4 ), and the negative side corresponds to the force which works in the direction to move the external heating belt  105  rearward of the fixing device  9  (direction indicated by arrow mark M in  FIG. 4 ). 
         [0082]    It has been confirmed that as the point of attachment of the front end of the external heating unit holding frame  48  to the shaft  203  shifts upstream from the ideal point, the amount of force which works in the direction to shift the external heating belt  105  rearward in terms of the lengthwise direction of the fixation roller  101  (direction indicated by arrow mark M in  FIG. 4 ) increases, whereas as the point of attachment of the front end of the external heating unit holding frame  48  to the shaft  203  shifts downstream from the ideal point, the amount of force which works in the direction to shift the external heating belt  105  frontward in terms of the lengthwise direction of the fixation roller  101  (direction indicated by arrow mark L in  FIG. 4 ) increases, as is evident from the graph in  FIG. 7 . Thus, the direction in which the external heating belt  105  shifts can be reliably controlled by pivotally moving the shaft  203 , with the use of the mechanism  51  for pivotally moving the shaft  203 , which is structured as in this embodiment. 
       [System for Detecting Shifting of External Heating Belt] 
       [0083]    Next, referring to  FIGS. 8 ,  9 ( a ) and  9 ( b ), the system, in this embodiment, for detecting the position of the external heating belt  105  in terms of its widthwise direction is described.  FIG. 8  is an external perspective view of the system, in this embodiment, for detecting the position of the external heating belt  105 .  FIGS. 9(   a ) and  9 ( b ) are plan views of the system, shown in  FIG. 8 , while the system is in action. 
         [0084]    In this embodiment, the external heating belt  105  is controlled so that while it is circularly moved by the rotation of the fixation roller  101 , it remains within a preset range (normal range) in terms of the widthwise direction of the external heating belt  105  (lengthwise direction of fixation roller  101 ). Thus, the fixing device  9  is provided with a system (detector) for detecting whether or not the external heating belt  105  is within the preset range. The system for detecting the position of the external heating belt  105  is structured so that it can detect that the external heating belt  105  is outside the preset range. If it detects that the external heating belt  105  is out of the preset range, it makes the shaft  203  (fixation roller heating external unit  34 ) pivot in the direction for shifting the external heating belt  105  back into the preset range. More concretely, it pivotally moves the shaft  203  (fixation roller heating external unit  34 ) in such a direction that the axial lines of the heating belt supporting rollers  103  and  104 , by which the external heating belt  105  is kept pressed upon the peripheral surface of the fixation roller  101 , intersect with the generatrix of the peripheral surface of the fixation roller  101 . In this embodiment, the range in which the angle θ, at which the axial lines of the external belt supporting rollers  103  and  104  intersect with the direction of the generatrix of the peripheral surface of the fixation roller  101 , is kept within 1.25° (±1.25° (+corresponds to clockwise direction)). 
         [0085]    More concretely, the system for detecting the heating belt shift, which is a detector, has a roller  128  and an arm  129 , which are positioned at one of the lateral edges of the external heating belt  105  (in terms of direction intersectional (perpendicular) to circular movement of external heating belt  105 ). The roller  128  is rotatably attached to the arm  129  so that it remains in contact with the lateral edge of the external heating belt  105 . The arm  129  is positioned at one of the lateral edges of the connective plate  49  of the external heating unit holding frame  48  of the external heating unit  34 . The arm  129  is enabled to pivotally move about the shaft  136 , and is kept pressed in the direction indicated by an arrow mark Q in  FIG. 8  by a pressure applying member  131 , such as a spring, which generates roughly 200 gf of force. 
         [0086]    The arm  129  is in connection to a sensor flag  132 , which has two slits. This sensor flag  132  is supported so that it is movable by the pivotal movement of the arm  129 . It is provided with photo-interrupters  133  and  134  ( FIGS. 9(   a ) and  9 ( b )). 
         [0087]    As the external heating belt  105  shifts frontward in terms of the axial line (lengthwise direction) of the fixation roller  101  (direction indicated by arrow mark L in  FIG. 4 ), the external heating belt  105  pushes the roller  128  in the direction indicated by an arrow mark R in  FIG. 8 . Consequently, the arm  129  is subjected to a force which is greater than the force generated by the resiliency of the pressure applying member  131 , being thereby pivotally moved about the shaft  136  (pivot) in the direction indicated by an arrow mark S in  FIG. 9(   a ). 
         [0088]    Thus, the sensor flag  132  is rotationally moved by the pivotal movement of the arm  129  in the direction indicated by an arrow mark G in  FIG. 9(   a ), being thereby made to enter into the slit (unshown) between the light emitting and light sensing portions of the photo-interrupter  133 , that is, one of the pair of photo-interrupters positioned at the two sides of the shaft  136  (pivot), and therefore, blocking the light emitted by the light emitting portion. The signal outputted by the photo-interrupter  133  as the light emitted by the light emitting portion is interrupted by the flag  132  is received by the control section  40  ( FIG. 11) . Thus, the control section  40  determines that the external heating belt  105  has shifted frontward (direction indicated by arrow mark L in  FIG. 4 ), and issues a command for making the belt shift control portion  54  of the fixing device  9  begin controlling the external heating belt  105  in lateral shift. Thus, the belt shift control portion  54  drives the motor  125 , through the motor controller  41  and motor driver  42 , in the direction to shift the external heating belt  105  in the direction opposite to the direction in which it has been shifting. 
         [0089]    In this embodiment, the combination of the mechanism  51  for pivotally moving the shaft  203 , and shift control portion  54 , functions as the means for adjusting the external heating belt  105  in the position in terms of the widthwise direction of the external heating belt  105 . This heating belt position adjusting means adjusts the external heating belt  105  in position in terms of the widthwise direction of the belt  105 , which is intersectional (perpendicular) to the moving direction of the external heating belt  105 , by pivotally moving the shaft  203 , rotationally moving thereby the external heating unit holding frame  48  about the rotational axis Ce, which is perpendicular to the area of contact (Ne) between the fixation roller  101  and external heating belt  105 . Further, the mechanism  51  for pivotally moving the shaft  203  functions as a means for rotationally moving the external heating unit holding frame  48 , whereas the belt shift control portion  54  functions as a means for controlling the amount (angle) by which the external heating unit holding frame  48  is to be rotationally moved by the mechanism  51 . 
         [0090]    On the other hand, if the external heating belt  105  shifts rearward in terms of the axial line (lengthwise direction) of the fixation roller  101  (direction indicated by arrow mark M in  FIG. 4 ), the external heating belt  105  moves in the direction to move away from the roller  128 . Consequently, the arm  129 , which is under the pressure generated by the pressure applying member  131  in the direction to pivotally move the arm  129  in the direction indicated by the arrow mark Q, pivots in the direction indicated by an arrow mark U in  FIG. 9(   b ). 
         [0091]    Thus, the sensor flag  132  is rotationally moved by the pivotal movement of the arm  129  in the direction indicated by an arrow mark V in  FIG. 9(   b ), being thereby made to come out of the slit (unshown) between the light emitting and light sensing portions of the photo-interrupter  133 . As soon as the flag  132  is made to come out of the slit, it is made to move into the slit (unshown) between the light emitting portion and light sensing portions of the photo-interrupter  134 , blocking thereby the light emitted by the light emitting portion. The signal outputted by the photo-interrupter  134  as the light emitted by the light emitting portion is interrupted by the flag  132  is received by the control section  40 . Thus, the control section  40  determines that the external heating belt  105  has shifted rearward (direction indicated by arrow mark M in  FIG. 4) , and issues to the belt shift control portion  54  of the fixing device  9 , a command to make the belt shift control portion  54  begin controlling the external heating belt  105  in lateral shift. Thus, the belt shift control portion  54  drives the motor  125 , through the motor controller  41  and motor driver  42 , in the direction to shift the external heating belt  105  in the direction opposite to the direction in which it has been shifting. 
       [Shift Control System] 
       [0092]    The shift control system is structured to support the shaft  203  by the sector gear  118  in such a manner that when the shaft  203  is in its home position, the fixation roller heating external heating unit  34  (external heating unit holding frame  48 ) is in its home position, and the rotational axis of the fixation roller  101  is parallel to the rotational axes of the heating belt supporting rollers  103  and  104 . Whether the external heating unit holding frame  48  is in its home position or not is determined by the control section  40  (controller) based on the output of the photo-interrupter  135  ( FIGS. 6 and 11 ) attached to the sector gear  118 . 
         [0093]    Next, referring to  FIG. 4 , the photo-interrupter  135  is positioned so that it can detect the movement of the pressure application arm  117   b  (shaft  203 ) relative to the lateral plate  202   b  of the external frame of the fixing device  9 . That is, as described with reference to  FIGS. 5(   a ) and  5 ( b ), the shaft  203  (pressure application arm  117   b ) is put through the elongated hole  115  of the sector gear  118 , and the position of the sector gear  118  in terms of the direction of its pivotal movement is detected as the light blocking portion  118   a  of the sector gear  118  is detected by the photo-interrupter  135 . Therefore, the amount of the movement of the pressure application arm  117   b  relative to the lateral plate  202   b  of the external frame of the fixing device  9  can be detected by detecting the angular position of the sector gear  118 . 
         [0094]    Regarding the operation of the shift control system, the external heating belt  105  is rotated by the rotation of the fixation roller  101 , possibly shifting frontward or rearward in terms of the widthwise direction of the belt  105  (lengthwise direction of fixation roller  101 ). If the belt  105  happens to shift frontward or rearward, the control section  40  moves the point of attachment of the external heating unit holding frame  48  to the shaft  203  in such a direction that a force which is opposite in direction to the force which has been working in the direction to laterally shift the belt  105  is generated. That is, in the case of this shift control system, the photo-interrupters  133  and  134  are positioned so that as the external heating belt  105  laterally shifts by a preset amount, for example, 5 mm, from the home position, the shifting of the belt  105  can be detected. Further, the amount by which the shaft  203  is allowed to pivotally move before the belt shift control system begins to respond is set to 2 mm from the abovementioned home position, either upstream or downstream. 
         [0095]    As described above, in this embodiment, the belt shift control system and shift detection system are expertly combined as described above. Therefore, the fixing device  9  is smoothly and accurately controlled in terms of the lateral shifting of its fixation roller heating external belt  105 . 
       [Control of Belt Shift Control System] 
       [0096]    Next, referring to  FIG. 11 , the control of the belt shift control system (system for controlling lateral shift of external heating belt  105 ) in this embodiment is described.  FIG. 11  is a block diagram of the control sequence of the belt shift control system. 
         [0097]    The control section  40  made up of a CPU or the like for integrally controlling the operation of each of the components of the fixing device  9  (image forming apparatus  100 ) is within the main assembly of the image forming apparatus  100 . The control section  40  is in connection to the belt shift control portion  54  of the fixing device  9 , and also, to the heater control section  140  which controls the heating of the fixation roller  101 , pressure roller  102 , and external heating belt  105  of the fixing device  9 . The belt shift control portion  54  controls the external heating belt  105  in lateral shift by controlling the external heating unit holding frame  48  of the fixation roller heating external unit  34  in its attitude relative to the fixation roller  101 . Further, the control section  40  is in connection to the photo-interrupters  133 ,  134  and  135 , and thermistors  121 ,  122 ,  123  and  124 . 
         [0098]    The belt shift control portion  54  is in connection to the motor controller  41 . The aforementioned roller  128  for detecting the position of the external heating belt  105 , and the photo-interrupters  133  and  134  make up the means for detecting the lateral shifting of the external heating belt  105 . The belt shift control portion  54  (controlling means) controls the mechanism  51  for rotationally moving the fixation roller heating external unit  34 , based on the amount of the lateral shift of the belt  105  detected by the belt deviation detecting means ( 128 ,  133  and  134 ). The motor controller  41 , which also was mentioned previously, drives the motor  125 , through the motor driver  42 , in response to the signal outputted by the belt shift control portion  54  in response to the command issued by the control section  40 . 
         [0099]    The heater control portion  140  is in connection to the heater controller  43 , which turns on or off the halogen heaters  111 ,  112 ,  113  and  114 , through the heater driver  44 , in response to the signals outputted by the heater control portion  140  in response to the command issued by the control section  40 . This is how the temperatures of the fixation roller  101 , pressure roller  102 , and external heating belt  105  are increased to, and kept at, their target levels. 
       [Belt Shift Control Sequence] 
       [0100]    Next, referring to  FIGS. 10 and 11 , the operation of the fixing device  9  in this embodiment is described.  FIG. 10  is a flowchart of the control sequence for controlling the fixing device  9  in this embodiment in the lateral shifting of its fixation roller heating external belt  105 . According to this flowchart, the control section  40  (controller) controls various portions of the fixing device  9 , in order to control the fixing device  9  in the lateral shifting of the external heating belt  105 . 
         [0101]    As the fixing device  9  is put on standby in Step S 1 , the control section  40  drives the motor  125  to put the point of attachment (that is, sector gear  118 ) of the front end of the external heating unit holding frame  48  to the shaft  203 , in its home position. That is, the control section  40  detects the position of the external heating unit holding frame  48  relative to the fixation roller  101 , based on the output of the photo-interrupter  135  (Step S 2 ). 
         [0102]    As the heater control portion  140  receives the command issued by the control section  40  based on the outputs of the thermistors  121 - 124 , it flows electric current through the halogen heaters  111 - 114  to heat the fixation roller  101 , pressure roller  102 , and heating belt supporting rollers  103  and  104 . That is, the heater control portion  140  begins to adjust each of the rollers  101 ,  102 ,  103  and  104  in temperature (Step S 3 ). 
         [0103]    Then, as an image formation job is started (Yes in Step S 4 ), the cam  205  is rotated by the unshown cam driving power source, whereby the external heating belt  105  is placed in contact with the fixation roller  101  (Step S 5 ). Then, fixation roller  101  is rotated by the unshown fixation roller driving power source (Step S 6 ), whereby the external heating belt  105  is rotated by the rotation of the fixation roller  101 . 
         [0104]    If the external heating belt  105  shifts frontward (direction indicated by arrow mark L in  FIG. 4 ) of the fixation roller heating external unit  34  while it is circularly moved by the rotation of the fixation roller  101 , the roller  128  which is in contact with the front edge of the external heating belt  105  is pushed by the external heating belt  105 , causing thereby the sensor flag  132  to rotationally move, and block the light emitted by the light emitting portion of the photo-interrupter  133  (Yes in S 7 ). Thus, the control section  40  issues a command to shift control portion  54  to make the belt shift control portion  54  to rotate the motor  125  in the direction to pivotally move the sector gear  118  so that the shaft  203  put through the elongated hole  115  of the sector gear  118  is moved to change the point of attachment of the heating unit  34  to the shaft  203  in order to cause the external heating belt  105  to shift rearward (direction indicated by arrow mark M in  FIG. 4 ) (Step S 8 ). 
         [0105]    On the other hand, if the external heating belt  105  shifts rearward (direction indicated by arrow mark M in  FIG. 4 ) of the external heating unit  34  while it is circularly moved by the rotation of the fixation roller  101 , the roller  128  pivotally moves by being made to follow the external heating belt  105  by the resiliency of the pressure applying member  131 , causing thereby the sensor flag  132  to rotationally move, and block the light emitted by the light emitting portion of the photo-interrupter  134  (Yes in S 9 ). Thus, the control section  40  issues a command to the belt shift control portion  54  to make the belt shift control portion  54  rotate the motor  125  in the direction to pivotally move the sector gear  118  so that the shaft  203  put through the elongated hole  115  of the sector gear  118  is moved to move the point of attachment of the external heating unit holding frame  48  to the shaft  203 , in order to cause the external heating belt  105  to shift frontward (direction indicated by arrow mark L in  FIG. 4 ) (Step S 10 ). 
         [0106]    The above-described operation for controlling the external heating belt  105  in its lateral shift is continued till the end of the image formation job (Step S 11 ). 
         [0107]    As soon as the image formation job is completed (Yes in Step  11 ), the cam  205  which functions as the mechanism for moving the external heating belt  105  away from the fixation roller  101  is rotated by the cam driving power source. Thus, the external heating belt  105  retracts from the fixation roller  101  (Step S 12 ). Then, in order to move the point of attachment of the external heating unit holding frame  48  to the shaft  203  (sector gear  118 ), to the home position, the control section  40  drives the motor  125  through the belt shift control portion (Step S 13 ). Thus, the position of the external heating unit holding frame  48  relative to the fixation roller  101  is detected by the photo-interrupter  135 . 
         [0108]    As described above, this embodiment can minimize the external heating belt  105  in the lateral shift, that is, the shift in the direction parallel to the axial line of the fixation roller  101 , by changing the angle of intersection between the external heating belt  105  and fixation roller  101  (±1.25°, in this embodiment) without changing the positional relationship between the heating belt supporting rollers  103  and  104 , and the external heating belt  105 , and therefore, can keep the external heating belt  105  more stable while it is circularly moved. Therefore, it can infallibly control the external heating belt  105  in unwanted lateral shift. In other words, this embodiment can prevent the pressure distribution between the external heating belt  105 , and the heating belt supporting rollers  103  and  104 , from becoming nonuniform. Therefore, it can keeps external heating belt  105  more uniform in its surface temperature in terms of its widthwise direction than any prior art. 
         [0109]    Therefore, the external heating belt  105  is kept uniform, in terms of its widthwise direction, in the amount of the heat which it supplies to the fixation roller  101 , being enabled to keep the fixation roller  101  more uniform in surface temperature, in terms of the lengthwise direction of the fixation roller  101 . Therefore, the fixing device  9  in this embodiment is uniform and stable in the amount of heat it applies to a toner image (toner images 0  on the sheet P of recording medium. Therefore, it is unlikely for the fixing device  9  (image forming apparatus  100 ) in this embodiment to output an image which is nonuniform in gloss, and/or an image which suffers from the like defects. 
       Embodiment 2 
       [0110]    Next, referring to  FIGS. 12 and 13 , the second embodiment of the present invention is described. In the following description of the second embodiment, the components of the fixing device in this embodiment, which are the same in structure are given the same referential codes as those given to the counterparts in the first embodiment, and are not going to be described.  FIG. 12  is a schematic sectional view of the fixing device equipped with the external heating belt in accordance with the present invention. It shows the general structure of the device.  FIG. 3  is a plan view of the fixation roller heating external unit  37 , as seen from the direction perpendicular to the lengthwise direction of the unit  37 . It shows the general structure of the unit. 
         [0111]    In the first embodiment, the external heating unit  34  was structured so that the angle of intersection between the external heating belt  105  and fixation roller  101  is changed by moving the point of attachment of one end of the external heating unit holding frame  48  to the shaft  203 . Therefore, while the external heating belt  105  is controlled in lateral shift, the angle of intersection between the fixation roller  101  and external heating belt  105  (heating belt supporting rollers  103  and  104 ) changed, with the rear end of the external heating unit holding frame  48  (rear end of shaft  33  in  FIG. 4 ) functioning as a pivot. 
         [0112]    It has been confirmed that there is a relationship between the amount of pressure between the external heating belt  105  and fixation roller  101 , and the amount by which the external heating belt  105  supplies heat to the fixation roller  101 . That is, the greater the amount of pressure by which the external heating belt  105  is pressed upon the fixation roller  101 , the tighter the contact between the external heating belt  105  and fixation roller  101 , and therefore, the wider the nip Ne between the belt  105  and roller  101 , and therefore, the greater the amount by which heat is supplied to the fixation roller  101  by the external heating belt  105 . That is, it has been known that the greater the amount of pressure by which the external heating belt  105  is pressed upon the fixation roller  101 , the greater the amount by which heat is supplied from the external heating belt  105  to the fixation roller  101 . In other words, in the case of the fixing device  9 , in the first embodiment, structured as described above, it is likely for the fixing device  9  to become nonuniform in the pressure applied to the fixation roller  101  by the heating belt supporting rollers  103  and  104 , in terms of the lengthwise direction of the fixation roller  101 ; the front and rear sides of the fixing device  9  are likely to become different in the contact pressure between the belt supporting rollers  103  and  104 , and the fixation roller  101 . 
         [0113]    Thus, in this embodiment, the fixing device  9  is structured so that the external heating unit holding frame  48  is rotationally supported by a holding frame supporting device, at the center in terms of the direction parallel to the rotational axis of the fixation roller  101 . In other words, the fixing device  9  in this embodiment is structured so that the rotational axis Ce, which is the axis about which the external heating belt  105  and fixation roller  101  is pivotally moved relative to each other to change the angle of intersection between the external heating belt  105  and fixation roller  101 , coincides with the rotational axis  209 . That is, the external heating unit holding frame  48  is rotatably supported in such a manner that its pivot coincides with the center of the fixation roller  101  in terms of the direction parallel to the axial line of the fixation roller  101 . Therefore, this embodiment is better than the first embodiment, in terms of the changes which occur to the front and rear sides of the fixing device  9  in the amount of pressure applied to the fixation roller  101  by the external heating belt  105  (belt supporting rollers  103  and  104 ), when the fixing device  9  is changed in the angle of intersection between the external heating belt  105  and fixation roller  101 . 
         [0114]    Thus, not only can this embodiment control the fixing device  9  in the lateral shifting of its external heating belt  105 , but also, it can keep the fixing device  9  more uniform, in terms of the lengthwise direction of the fixation roller  101 , in the amount of heat which the external heating belt  105  supplies to the fixation roller  101 , than the first embodiment. 
         [0115]    Next, the fixing device  9  in this embodiment, which is structured so that the rotational axis of the external heating unit holding frame  48  coincides with the center of the fixation roller heating external unit  37 , is described in detail. 
         [0116]    This embodiment is roughly the same as the first embodiment, in the structure of the external heating unit holding frame  48  for keeping the external heating belt  105  pressed upon the fixation roller  101 . It is different from the first embodiment, only in that the fixing device  9  in this embodiment is structured as follows: The fixing device  9  is provided with a pressure applying frame  201 , which is supported by the casing (external frame) of the fixing device  9  and functions as a device for supporting the external heating unit holding frame  48 , and the external heating unit holding frame  48  is rotatable (pivotally movable) about the shaft  209  which is roughly vertical, being therefore movable relative to the pressure application frame  201 . 
         [0117]    More concretely, the heating belt supporting rollers  103  and  104  are rotatably supported by their lengthwise ends, by a pair of supporting members  206   a  and  206   b , respectively, which are rotatably supported by a pair of shaft  207  and  207  attached to an intermediary rectangular frame  208  (as seen from above). Thus, the supporting members  206   a  and  206   b  are rotatable relative to the pressure application frame  201  about the shafts  207  and  207 , at the lengthwise ends (top and bottom ends in  FIG. 13 ) of the intermediary frame  208 . 
         [0118]    The external heating unit holding frame  48  supported by the intermediary frame  208  with the presence of the shafts  207  and  207  between itself and intermediary frame  208  rotatably holds the heating belt supporting rollers  103  and  104  by which the external heating belt  105  is suspended, between its belt supporting members  206   a  and  206   b . Further, there is the cam  205  which is below the front end (left end in  FIG. 12 ) of the pressure application frame  201  which is under the pressure from the pressing application section (member)  204 . The cam  205  is supported by its shaft  45  so that it can be eccentrically rotated about the shaft  45 . 
         [0119]    The pressure application frame  201  is rotatably supported at its right end in terms of the left-right direction in  FIG. 13 , by the shafts  212  and  212 , about which the pressure application frame  201  is rotationally movable. The shafts  212  and  212  are held to the lateral plates  202   a  and  202   b  of the external frame of the fixing device  9 , with a pair of shaft holding members  47 , which are solidly attached to the lateral plates  202   a  and  202   b , with the use of screws  47 . Further, the fixation roller heating external unit  37  is provided with a roughly vertical shaft  209  about which the external heating unit holding frame  48  is rotationally movable. More specifically, in terms of widthwise direction of external heating belt  105 , the shaft  209  is put through roughly the center of the connective plate  49  which bridges between the belt supporting roller supporting members  206   a  and  206   b . In terms of the direction in which the external heating belt  105  is circularly moved, the shaft  209  is on the left side of the connective plate  49 . 
         [0120]    Further, the belt shift control system (mechanism  52 ) is provided with two pairs of rollers  210  and  210 , which are rotationally supported by the pressure application frame  201 , in contact with the lengthwise ends (top and bottom ends), one for one, of the intermediary frame  208 . In terms of the direction parallel to the circular movement of the external heating belt  105 , the rollers  210  and  210  are located roughly in the middle of the pressure application frame  201 . The pressure application frame  201  is fitted around the shaft  209  which extends from the bottom side of the pressure application frame  201  to the top side of the pressure application frame  201 . Thus, the intermediary frame  208  is allowed to horizontally rotate about the shaft  209  while keeping a preset distance between itself and the pressure application frame  201 . 
         [0121]    The shaft  209 , about which the external heating unit holding frame  48  rotationally moves, is attached to the intermediary frame  208  so that its axial line is parallel to the direction perpendicular to the tangential line  53  to the external nip Ne between the fixation roller  101  and external heating belt  105 . In terms of the direction (top-to-bottom direction in  FIG. 13 ) parallel to the axial line of the fixation roller  101 , the shaft  209  is at the center of the fixation roller  101 , being therefore at the center of the external heating belt  105 . Thus, this embodiment can stabilize the fixing device  9  in the balance between the front and rear sides of the fixing device  9 , in terms of the contact pressure between the fixation roller  101  and external heating belt  105 . 
         [0122]    The pressure application frame  201  is allowed to rotationally move about the shaft  212  and  212  supported between the lateral plates  202   a  and  202   b  of the external frame of the fixing device  9 , and is kept pressured toward the fixation roller  101  by the pressure from the pressure applying section  204  (springs). Thus, as the cam  205  is rotated, the pressure application frame  201  is pivotally moved in such a manner that its front end moves upward or downward to make the external heating belt  105  come into contact with, or separated from, the fixation roller  101 . 
         [0123]    To reiterate, the fixation roller heating external unit  37  is held between the lateral plates  202   a  and  202   b  with a presence of a preset amount of clearance between the unit  37  and the lateral plates  202   a  and  202   b . The external heating unit holding frame  48  has a shaft  137  which protrudes from one end (bottom end in  FIG. 13 ) of the intermediary frame  208 . More concretely, the shaft  137  is solidly attached to the intermediary frame  208  by one end, and is loosely put through the through hole  38  of the lateral plate  202   b  of the external frame of the fixing device  9 , with the presence of a preset amount of gap between the shaft  137  and lateral plate  202   b , that is, the diameter of the through hole  38  is greater than the external diameter of the shaft  137 . Thus, the shaft  137  is allowed to move in the directions indicated by arrow marks E and F. 
         [0124]    The end portion of the shaft  137 , which is put through the through hole  38 , is rotatably supported by the bearing  126 , which is on the outward side of the lateral plate  202   b . Further, it is put through the elongated hole  115  of the sector gear  118 , which is on the outward side of the bearing  126 . Thus, the shaft  137  is pivotally movable in such a manner that its end portion put through the through hole  38  moves along the edge of the elongated hole  38 . Further, the sector gear  118  is supported as it is in the first embodiment. That is, it is pivotally supported by the shaft  119  attached to the outward side of the lateral plate  202   b . Thus, the pivotal movement of the sector gear  118  (that is, angle of external heating unit holding frame  48  relative to fixation roller  101 ) can be detected with the use of the same method as that in the first embodiment. 
         [0125]    The motor  125  is attached to the lateral plate  202   b , being placed next to the sector gear  118 . A worm gear  120  is solidly attached to the output shaft  125   a  of the motor  125 . More specifically, the motor  125  is solidly attached to the lateral plate  202   b , being positioned so that the worm gear  20  can be meshed with the toothed portion  118   b  of the sector gear  118   b . In other words, the fixing device  9  in this embodiment is structured so that the external heating unit holding frame  48  is pivotally movable by the combination of the motor  125 , worm gear  120 , sector gear  118 , shaft  137 , etc. 
         [0126]    In this embodiment, the combination of the belt shift control system (mechanism  52 ) and control section  40  makes up a fixing device adjusting means. The belt shift control system (mechanism  52 ) is the means for pivotally moving the external heating unit holding frame  48 . The belt shift control portion  54  functions as the means for controlling the amount by which the external heating unit holding frame  48  is pivotally moved by the belt shift control system (mechanism  52 ). 
         [0127]    The control sequence carried out by the control section  40  to control the external heating belt  105  in lateral shift is the same as that in the first embodiment. That is, the control section  40  pivotally moves the shaft  137  by driving the motor  125  to change the angle of the external heating unit holding frame  48  relative to the axial line of the fixation roller  101 . The effects obtainable by this embodiment are the same as those obtainable by the above-described first embodiment. 
         [0128]    Here, referring to  FIGS. 14(   a ) and  14 ( b ), a case in which the angle of intersection between the external heating belt supporting rollers  103  and  104 , and fixation roller  101  is set to an angle θ in order to control the lateral shift of the external heating belt  105  of the fixing device  9  structured as in the first or second embodiments is described. The arrow marks a and b in  FIGS. 14(   a ) and  14 ( b ) indicate the directions in which the belt supporting rollers  103  and  104  are pivotally moved about the rotational axis C 3 , and the arrow marks V and W indicate the amount by which the two rollers  103  and  104  are pivotally moved about the rotational axis Ce. 
         [0129]    In the first embodiment, in order to allow the external heating unit holding frame  48  to pivotally move in such a manner that the angle of intersection between the external heating belt supporting rollers  103  and  104 , and the fixation roller  101  changes, the external heating unit holding frame  48  of the fixation roller heating external unit  34  is attached by one end (rear end) to the shaft  203  which is pivotally attached to the rear plate of the external frame of the fixing device  9 . Therefore, it was likely for the external heating belt supporting rollers  103  and  104  to deviate in position from the fixation roller  101  on their front side, in terms of their lengthwise direction, than on their rear side. 
         [0130]    In comparison, in the second embodiment, the fixation roller heating external unit  37  is provided with the intermediary frame  208 , by which the pressure application frame  201  directly supported by the lateral plates  202   a  and  202   b  of the external frame of the fixing device  9 , is separated from the external heating unit holding frame  48  which is pivotally movable relative to the fixation roller  101 . Further, in terms of the lengthwise direction of the external heating unit holding frame  48  (top-bottom direction in  FIG. 13 ), the shaft  209  which functions as the rotational axis Ce about which the external heating belt  105  rotates is at the center of the external heating unit holding frame  48 . Therefore, in a case where the angle θ of intersection between the external heating unit holding frame  48  and fixation roller  101  in the second embodiment is set as it is set in the first embodiment, the amount by which the front end (and rear end) of the external heating unit holding frame  48  moves relative to the fixation roller  101  when the external heating belt  105  is controlled in its lateral shift is a half of that in the first embodiment, as shown in  FIGS. 14(   a ) and  14 ( b ). 
         [0131]    Further, the front and rear sides of the heating belt supporting rollers  103  and  104  in terms of their lengthwise direction are the same in the amount by which they are moved when the external heating belt  105  is controlled in lateral shift. Therefore, the front and rear sides of the heating belt supporting rollers  103  and  104  become the same in the amount of pressure applied to the fixation roller  101  by the rollers  103  and  104 . Therefore, the fixing device  9  in the second embodiment is more uniform in terms of the lengthwise direction of the fixation roller  101 , in the amount of heat supplied to the fixation roller  101  by the external heating belt  105 , being therefore more uniform in the amount of heat which the fixation roller  101  gives to a sheet of recording medium and the toner image(s) thereon, than the fixing device  9  in the first embodiment. Therefore, the images outputted by an image forming apparatus employing the fixing device  9  in the second embodiment suffer far less from the image defects, more specifically, the nonuniformity in the gloss attributable to the fixing device, than an image forming apparatus employing the fixing device  9  in the first embodiment. 
       Embodiment 3 
       [0132]    Next, referring to  FIG. 15 , the third embodiment of the present invention is described. The components of the fixing device  9  in this embodiment, which are the same in structure as the counterparts in the first and second embodiments, are given the same referential codes as those given to the counterparts, and are not going to be described here.  FIGS. 15(   a ) and  15 ( b ) are plan views of the fixation roller heating external units in the second and third embodiments, in a case where the two units are made the same in the angle of intersection between the external heating belt  105  and fixation roller  101  in order to compare the two embodiments. 
         [0133]    In this embodiment, the fixation roller heating external unit ( 34 ,  37 ) is structured so that it becomes more uniform in terms of the lengthwise direction of the fixation roller  101 , in the amount of pressure applied to the fixation roller  101  by the external heating belt supporting rollers  103  and  104 , than the fixation roller heating external unit  37  in the second embodiment. More specifically, in this embodiment, the rollers  103  and  104  by which the external heating belt  105  is suspended are given such an overall contour that makes the rollers  103  and  104  nonuniform in diameter in terms of their lengthwise direction; they are formed so that they gradually reduce in diameter toward their center starting from their lengthwise end, in terms of the lengthwise direction of the rollers; in their sectional view at the plane which coincides with their axial line, their peripheral surface are concave. 
         [0134]    In this embodiment, the diameter of the center of each of the belt supporting rollers  103  and  104 , and the diameter of the lengthwise ends of the rollers  103  and  104 , are set according to the angle of intersection will be between the fixation roller  101  and external heating unit holding frame  48  during the operation for controlling the external heating belt  105  in lateral shift. In terms of the structure of the fixation roller heating external unit and the method for controlling the external heating belt  105  in lateral shift, this embodiment is the same as the first and second embodiment. 
         [0135]      FIGS. 15(   a ) and  15 ( b ) shows the fixation roller heating external units  34  and  37  in the first and second embodiments, respectively, as seen when the angle of intersection between the fixation roller  101  and external heating unit holding frame  48  is set to an angle θ in order to control the external heating belt  105  in lateral shift. The arrow marks c and d in the drawings indicate the direction in which the external heating belt supporting rollers  103  and  104  are pivotally moved about the rotational axis Ce, respectively. 
         [0136]    Referring to  FIG. 15(   a ), in the second embodiment, the belt supporting rollers  103  and  104  are uniform in diameter in terms of their lengthwise direction. Therefore, as the rollers  103  and  104  (external heating unit holding frame  48 ) are angled relative to the fixation roller  101 , the amount by which the lengthwise ends of each of the rollers  103  and  104  rotationally move about the lengthwise center of the rollers  103  and  104  is relatively large. In other words, the amount by which the lengthwise ends of the rollers  103  and  104  move away from the corresponding lengthwise ends of the fixation roller  101  is substantial, making the fixation roller  101  nonuniform in the amount of pressure applied to the fixation roller  101  by the belt supporting rollers  103  and  104 , in terms of the lengthwise direction of the fixation roller  101 (making lengthwise ends of fixation roller  101  different in amount of pressure applied to fixation roller  101  by belt supporting rollers  103  and  104 , from center of fixation roller  101 ). 
         [0137]    In comparison, in the third embodiment, the belt supporting rollers  103  and  104  are shaped so that as they are seen from the direction perpendicular their axial lines, they are concave. Therefore, even as the rollers  103  and  104  are tilted relative to the fixation roller  101  in such a manner that the angle of intersection between the roller  103  ( 104 ) and the fixation roller  101  becomes an angle θ, the lengthwise ends of the roller  103  ( 104 ) remain tightly pressed upon the fixation roller  101 . Thus, this embodiment can keep the fixing device  9  more uniform in terms of the lengthwise direction of the fixation roller  101 , in the amount of pressure applied to the fixation roller  101  by the roller  103  ( 104 ), than the first and second embodiment. 
         [0138]    The more uniform the fixation roller  101  in terms of its lengthwise direction, in the amount of heat given thereto by the external heating belt  105 , the more uniform the fixation roller  101  in terms of its lengthwise direction, in the surface temperature, and therefore, the more uniform, the fixation roller  101  in terms of its lengthwise direction, in the amount of heat its gives to a sheet of recording medium and the toner image(s) thereon. Thus, an image forming apparatus employing the fixing device  9  in this embodiment can output images which suffer far less from the defects, in particular, the nonuniformity in gloss than those outputted by an image forming apparatus employing the fixing device  9  in the first or second embodiments. 
         [0139]    In other words, this embodiment is more effective to minimize the problem that as the belt supporting rollers  103  and  104  are tilted relative to the fixation roller  101  at an angle of θ in order to control the external heating belt  105  in lateral shift, the lengthwise end portions of the fixation roller  101  become smaller in the amount of the pressure applied to the fixation roller  101  by the rollers  103  and  104  than the center of the fixation roller  101 . Thus, it can keep the fixing device  9  more uniform in its lengthwise direction, in the distribution of the pressure applied to the fixation roller  101  by the rollers  103  and  104 , and therefore, can keep the fixing device  9  uniform in the amount of the heat supplied to the fixation roller  101  by the external heating belt  105 , than the first and second embodiments. 
         [0140]    In this (third) embodiment, the belt supporting rollers  103  and  104  are shaped so that as they are seen from the direction perpendicular to their axial lines, they are concave. However, this embodiment is not intended to limit the present invention in the shape of the belt supporting roller  103  ( 104 ). For example, the present invention is also applicable to a fixing device, only one of the belt supporting rollers  103  and  104  of which is shaped so that its peripheral surface concaves. The effects of the application of the present invention to such a fixing device are roughly the same as those obtainable by this (third) embodiment. 
         [0141]    At this time, referring to  FIG. 16 , the results of the experiment carried out to compare the first, second, and third embodiments are described. In the experiment, the total amount of load (pressure) applied to the fixation roller  101  by the external heating belt  105  was pressed upon the fixation roller  101  was set to 10 kgf. The recording medium was sheets of coated paper which were A3 in size and 300 g in basis weight.  FIG. 16  shows the lowest temperatures of the peripheral surface of each of the fixation rollers  101  in the first, second, and third embodiments, measured at the front end, center, and rear end of each fixation roller  101  immediately after 50 sheets of coated paper were conveyed through the fixing device  9 . 
         [0142]    As will be evident from  FIG. 16 , in the first embodiment, the pivot of the fixation roller heating external unit (fixation roller heating external belt unit) was at one end of the unit in terms of the lengthwise direction of the fixing device  9 . In the second embodiment, it was at the center of the unit. In the third embodiment, it was also at the center of the unit. As for the contour of the belt supporting rollers  103  and  104 , the rollers  103  and  104  in the first and second embodiments were uniform in diameter in terms of their lengthwise direction. The rollers  103  and  104  in the third embodiment were shaped so that their peripheral surface concave. 
         [0143]    The fixing devices  9  in the first, second and third embodiments were set up so that the contact pressure between the rollers  103  ( 104 ) and fixation roller  101  became 100% at the center of the fixation nip Ne in terms of the lengthwise direction of the fixation roller  101 . In the case of the fixing device  9  in the first embodiment, the lowest temperatures at the front end, center, and rear end of the peripheral surface of the fixation roller  101  in terms of the lengthwise direction of the fixation roller  101  were 166.8° C., 167.9° C. and 170.2° C., respectively. In the case of the fixing device  9  in the second embodiment, they were 168.0° C., 168.5° C. and 168.0° C., respectively. In the case of the fixing device  9  in the third embodiment, they were 168.3° C., 168.3° C. and 168.3° C., respectively. 
         [0144]    It is evident from the results of the above described experiment that the fixing device  9  in the second embodiment is more uniform in the surface temperature of the fixation roller  101 , and smaller in the difference in the peripheral temperature between the front and rear sides of the fixation roller  101 , than the fixing device  9  in the first embodiment. It is also evident from the results of the above described experiment that in the case of the fixing device  9  in the third embodiment, there was no difference in the surface temperature of the fixation roller  101  between the front and rear ends of the fixing device  9 , whereas in the case of the fixing device  9  in the second embodiment, there was still a small amount of difference in surface temperature of the fixation roller  101  between the front and rear ends of the fixation roller  101 . That is, the former is more uniform in surface temperature of the fixation roller  101  than the latter. 
         [0145]    In the first to third embodiments of the present invention described above, the rotational heating member of the fixing device, which is to be heated by the external heating belt, was the fixation roller. However, the present invention is also applicable to a fixing device which employs a fixation belt. 
         [0146]    Also in the first to third embodiments, the fixing devices employed the external heating belt. However, the present invention is also applicable to a fixing device structured as follows. For example, the present invention is applicable to a fixing device, the pressure applying member of which is a pressure application belt supported by a pair of belt supporting rollers, and which is structured so that the pressure application belt is rotated by the rotation of the fixation roller, and also, so that the pair of belt supporting rollers are rotationally (pivotally) movable about a preset axis to be tilted together relative to the direction of the generatrix of the peripheral surface of the fixation roller (axial line of fixation roller). That is, the present invention is applicable to the mechanism (system) for controlling the pressure application belt in lateral shift. 
         [0147]    Further, in the first to third embodiments described above, it was to an image heating device (fixing device) that the present invention was applied. However, the present invention is also applicable to an image forming apparatus which employs an intermediary transfer member, which is in the form of an endless belt, supported by a pair of belt supporting rollers, and rotated by the rotation of the photosensitive member of the apparatus, and which is structured so that the two rollers are tilted together relative to the direction of the generatrix of the peripheral surface of the photosensitive member. In such a case, the present invention is applicable as a mechanism (system) for controlling the intermediary transfer belt in lateral shift. In addition, the present invention is also applicable to an image forming apparatus which employs an endless belt supported by a pair of belt supporting rollers and circularly moved by a belt driving rotational member. In such a case, the image forming apparatus is structured so that the pair of rollers by which the endless belt is suspended (supported) can be tilted together relative to the direction of the generatrix of the peripheral surface of the belt driving rotational member (axial line of belt driving rotational member). 
         [0148]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0149]    This application claims priority from Japanese Patent Application No. 029191/2012 filed Feb. 14, 2012, which is hereby incorporated by reference.