Patent Publication Number: US-8126384-B2

Title: Fixing device and image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2009-041362 filed Feb. 24, 2009. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a fixing device and an image forming apparatus. 
     2. Related Art 
     Fixing devices using an electromagnetic induction heating method are known as the fixing devices each to be installed in an image forming apparatus such as a copy machine and a printer using an electrophotographic method. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a fixing device comprising: a fixing member that has a conductive layer, and that fixes toner onto a recording medium by heat generation of the conductive layer through electromagnetic induction; a magnetic field generating member that generates an alternate-current magnetic field crossing the conductive layer of the fixing member; a magnetic path forming member that is arranged so as to face the magnetic field generating member through the fixing member, that forms a magnetic path of the alternate-current magnetic field generated by the magnetic field generating member within a temperature range not greater than a permeability change start temperature at which permeability starts to decrease, and that causes the alternate-current magnetic field generated by the magnetic field generating member to go through the magnetic path forming member within a temperature range exceeding the permeability change start temperature; and a heat radiation member that is arranged to be in contact with the magnetic path forming member in order to radiate heat generated in the magnetic path forming member toward a direction opposite to the fixing member with reference to the magnetic path forming member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a diagram showing a configuration example of an image forming apparatus to which a fixing device of the exemplary embodiments is applied; 
         FIG. 2  is a front view of the fixing unit of the exemplary embodiments; 
         FIG. 3  is a cross sectional view of the fixing unit, taken along the line III-III in  FIG. 2 ; 
         FIG. 4  is a configuration diagram showing cross sectional layers of the fixing belt; 
         FIG. 5A  is a side view of one of the end caps, and  FIG. 5B  is a plain view of the end cap when viewed from a VB direction of  FIG. 5A ; 
         FIG. 6  is a cross sectional view for explaining a configuration of the IH heater; 
         FIG. 7  is a diagram for explaining a multi-layer structure of the IH heater; 
         FIG. 8  is a diagram for explaining the state of the magnetic field lines in a case where the temperature of the fixing belt is within the temperature range not greater than the permeability change start temperature; 
         FIG. 9  is a diagram showing a summary of a temperature distribution in the width direction of the fixing belt when the small size sheets are successively inserted into the fixing unit; 
         FIG. 10  is a diagram for explaining a state of the magnetic field lines when the temperature of the fixing belt at the non-sheet passing regions is within the temperature range exceeding the permeability change start temperature. 
         FIGS. 11A and 11B  are diagrams showing slits formed in the temperature-sensitive magnetic member; 
         FIGS. 12A to 12C  are views for explaining the heat radiation path in the first exemplary embodiment; 
         FIGS. 13A to 13C  are diagrams for explaining the heat radiation path in the second exemplary embodiment; 
         FIGS. 14A to 14C  are diagrams for explaining the heat radiation path in the third exemplary embodiment; 
         FIGS. 15A to 15C  are diagrams for explaining the heat radiation path in the fourth exemplary embodiment; 
         FIGS. 16A and 16B  are diagrams for explaining the heat radiation path in the fifth exemplary embodiment; and 
         FIGS. 17A to 17C  are diagrams for explaining the heat radiation path in the sixth exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 
     &lt;Description of Image Forming Apparatus&gt; 
       FIG. 1  is a diagram showing a configuration example of an image forming apparatus to which a fixing device of the exemplary embodiments is applied. An image forming apparatus  1  shown in  FIG. 1  is a so-called tandem-type color printer, and includes: an image forming portion  10  that performs image forming on the basis of image data; and a controller  31  that controls operations of the entire image forming apparatus  1 . The image forming apparatus  1  further includes: a communication unit  32  that communicates with, for example, a personal computer (PC)  3 , an image reading apparatus (scanner)  4  or the like to receive image data; and an image processor  33  that performs image processing set in advance on image data received by the communication unit  32 . 
     The image forming portion  10  includes four image forming units  11 Y,  11 M,  11 C and  11 K (also collectively referred to as an “image forming unit  11 ”) as examples of a toner image forming unit, which are arranged side by side at certain intervals. Each of the image forming units  11  includes: a photoconductive drum  12  as an example of an image carrier that forms an electrostatic latent image and holds a toner image; a charging device  13  that uniformly charges the surface of the photoconductive drum  12  at a predetermined potential; a light emitting diode (LED) print head  14  that exposes, on the basis of color image data, the photoconductive drum  12  charged by the charging device  13 ; a developing device  15  that develops the electrostatic latent image formed on the photoconductive drum  12 ; and a cleaner  16  that cleans the surface of the photoconductive drum  12  after a transfer. 
     The image forming units  11  have almost the same configuration except toner contained in the developing device  15 , and form yellow (Y), magenta (M), cyan (C) and black (K) color toner images, respectively. 
     Further, the image forming portion  10  includes: an intermediate transfer belt  20  onto which multiple layers of color toner images formed on the photoconductive drums  12  of the image forming units  11  are transferred; and primary transfer rolls  21  that sequentially transfer (primarily transfer) color toner images formed in respective image forming units  11  onto the intermediate transfer belt  20 . Furthermore, the image forming portion  10  includes: a secondary transfer roll  22  that collectively transfers (secondarily transfers) the color toner images superimposingly transferred onto the intermediate transfer belt  20  onto a sheet P which is a recording medium (recording sheet); and a fixing unit  60  as an example of a fixing unit (a fixing device) that fixes the color toner images having been secondarily transferred, onto the sheet P. Note that, in the image forming apparatus  1  according to the exemplary embodiments, the intermediate transfer belt  20 , the primary transfer rolls  21  and the secondary transfer roll  22  configure a transfer unit. 
     In the image forming apparatus  1  of the exemplary embodiments, image formation processing using the following processes is performed under operations controlled by the controller  31 . Specifically, image data from the PC  3  or the scanner  4  is received by the communication unit  32 , and after the image data is subjected to certain image processing performed by the image processor  33 , the image data of each color is generated and sent to a corresponding one of the image forming units  11 . Then, in the image forming unit  11 K that forms a black-color (K) toner image, for example, the photoconductive drum  12  is uniformly charged by the charging device  13  at the potential set in advance while rotating in a direction of an arrow A, and then is exposed by the LED print head  14  on the basis of the black color image data transmitted from the image processor  33 . Thereby, an electrostatic latent image for the black-color image is formed on the photoconductive drum  12 . The black-color electrostatic latent image formed on the photoconductive drum  12  is then developed by the developing device  15 . Then, the black-color toner image is formed on the photoconductive drum  12 . In the same manner, yellow (Y), magenta (M) and cyan (C) color toner images are formed in the image forming units  11 Y,  11 M and  11 C, respectively. 
     The color toner images formed on the respective photoconductive drums  12  in the image forming units  11  are electrostatically transferred (primarily transferred), in sequence, onto the intermediate transfer belt  20  that moves in a direction of an arrow B, by the primary transfer rolls  21 . Then, superimposed toner images on which the color toner images are superimposed on one another are formed. Then, the superimposed toner images on the intermediate transfer belt  20  are transported to a region (secondary transfer portion T) at which the secondary transfer roll  22  is arranged, along with the movement of the intermediate transfer belt  20 . The sheet P is supplied from a sheet holding unit  40  to the secondary transfer portion T at a timing when the superimposed toner images being transported arrive at the secondary transfer portion T. Then, the superimposed toner images are collectively and electrostatically transferred (secondarily transferred) onto the transported sheet P by action of a transfer electric field formed at the secondary transfer portion T by the secondary transfer roll  22 . 
     Thereafter, the sheet P onto which the superimposed toner images are electrostatically transferred is transported to the fixing unit  60 . The toner images on the sheet P transported to the fixing unit  60  are heated and pressurized by the fixing unit  60  and thereby are fixed onto the sheet P. Then, the sheet P including the fixed images formed thereon is transported to a sheet output unit  45  provided at an output portion of the image forming apparatus  1 . 
     Meanwhile, the toner (primary-transfer residual toner) attached to the photoconductive drums  12  after the primary transfer and the toner (secondary-transfer residual toner) attached to the intermediate transfer belt  20  after the secondary transfer are removed by the respective cleaners  16  and a belt cleaner  25 . 
     In this way, the image formation processing in the image forming apparatus  1  is repeatedly performed for a designated number of print sheets. 
     &lt;Description of Configuration of Fixing Unit&gt; 
     Next, a description will be given of the fixing unit  60  in the exemplary embodiments. 
       FIGS. 2 and 3  are diagrams showing a configuration of the fixing unit  60  of the exemplary embodiments.  FIG. 2  is a front view of the fixing unit  60 , and  FIG. 3  is a cross sectional view of the fixing unit  60 , taken along the line III-III in  FIG. 2 . 
     Firstly, as shown in  FIG. 3 , which is a cross sectional view, the fixing unit  60  includes: an induction heating (IH) heater  80  as an example of a magnetic field generating member that generates an AC (alternate-current) magnetic field; a fixing belt  61  as an example of a fixing member that is subjected to electromagnetic induction heating by the IH heater  80 , and thereby fixes a toner image; a pressure roll  62  that is arranged so as to face the fixing belt  61 ; and a pressing pad  63  that is pressed by the pressure roll  62  with the fixing belt  61  therebetween. 
     The fixing unit  60  further includes: a holder  65  that supports a constituent member such as the pressing pad  63  and the like; a temperature-sensitive magnetic member  64  that forms a magnetic path by inducing the AC magnetic field generated at the IH heater  80 ; an induction member  66  that induces magnetic field lines passing through the temperature-sensitive magnetic member  64 ; and a peeling assisting member  70  that assists peeling of the sheet P from the fixing belt  61 . 
     &lt;Description of Fixing Belt&gt; 
     The fixing belt  61  is formed of an endless belt member originally formed into a cylindrical shape, and is formed with a diameter of 30 mm and a width-direction length of 370 mm in the original shape (cylindrical shape), for example. In addition, as shown in  FIG. 4  (a configuration diagram showing cross sectional layers of the fixing belt  61 ), the fixing belt  61  is a belt member having a multi-layer structure including: a base layer  611 ; a conductive heat-generating layer  612  that is coated on the base layer  611 ; an elastic layer  613  that improves fixing properties of a toner image; and a surface release layer  614  that is applied as the uppermost layer. 
     The base layer  611  is formed of a heat-resistant sheet-like member that supports the conductive heat-generating layer  612 , which is a thin layer, and that gives a mechanical strength to the entire fixing belt  61 . Moreover, the base layer  611  is formed of a specified material with a specified thickness. The base layer material has properties (relative permeability, specific resistance) that allow a magnetic field to pass therethrough so that the AC magnetic field generated at the IH heater  80  may act on the temperature-sensitive magnetic member  64 . Meanwhile, the base layer  611  itself is formed so as not to generate heat by action of the magnetic field or not to easily generate heat. 
     Specifically, for example, a non-magnetic metal such as a non-magnetic stainless steel having a thickness of 30 to 200 μm (preferably, 50 to 150 μm), or a resin material or the like having a thickness of 60 to 200 μm is used as the base layer  611 . 
     The conductive heat-generating layer  612  is an example of a conductive layer and is an electromagnetic induction heat-generating layer that is self-heated by electromagnetic induction of the AC magnetic field generated at the IH heater  80 . Specifically, the conductive heat-generating layer  612  is a layer that generates an eddy current when the AC magnetic field from the IH heater  80  passes therethrough in the thickness direction. 
     Normally, an inexpensively manufacturable general-purpose power supply is used as the power supply for an excitation circuit  88  that supplies an AC current to the IH heater  80  (also refer to later-described  FIG. 6 ). For this reason, in general, a frequency of the AC magnetic field generated by the IH heater  80  ranges from 20 kHz to 100 kHz by use of the general-purpose power supply. Accordingly, the conductive heat-generating layer  612  is formed to allow the AC magnetic field having a frequency of 20 kHz to 100 kHz to enter and to pass therethrough. 
     A region of the conductive heat-generating layer  612 , where the AC magnetic field is allowed to enter is defined as a skin depth δ representing a region where the AC magnetic field attenuates to 1/e. The skin depth δ is calculated by use of the following formula (1), where f is a frequency of the AC magnetic field (20 kHz, for example), ρ is a specific resistance value (Ω·m), and μ r  is a relative permeability. 
     Accordingly, in order to allow the AC magnetic field having a frequency of 20 kHz to 100 kHz to enter and then to pass through the conductive heat-generating layer  612 , the thickness of the conductive heat-generating layer  612  is formed to be smaller than the skin depth δ of the conductive heat-generating layer  612 , which is defined by the formula (I). In addition, as the material that forms the conductive heat-generating layer  612 , a metal such as Au, Ag, Al, Cu, Zn, Sri, Pb, Bi, Be or Sb, or a metal alloy including at least one of these elements is used, for example. 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     503 
                     ⁢ 
                     
                       
                         ρ 
                         
                           f 
                           · 
                           
                             μ 
                             r 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Specifically, as the conductive heat-generating layer  612 , a non-magnetic metal (paramagnetic material having a relative permeability substantially equal to 1) including Cu or the like, having a thickness of 2 to 20 μm and a specific resistance value not greater than 2.7×10 −8  Ωm is used, for example. 
     In addition, in view of shortening the period of time required for self-heating the fixing belt  61  to reach a fixation setting temperature (hereinafter, referred to as a “warm-up time”) as well, the conductive heat-generating layer  612  may be formed of a thin layer. 
     Next, the elastic layer  613  is formed of a heat-resistant elastic material such as a silicone rubber. The toner image to be held on the sheet P, which is to become the fixation target, is formed of a multi-layer of color toner as powder. For this reason, in order to uniformly supply heat to the entire toner image at a nip portion N, the surface of the fixing belt  61  may particularly be deformed so as to correspond with unevenness of the toner image on the sheet P. In this respect, a silicone rubber having a thickness of 100 to 600 μm and a hardness of 10° to 30° (JIS-A), for example, may be used for the elastic layer  613 . 
     The surface release layer  614  directly contacts with an unfixed toner image held on the sheet P. Accordingly, a material with a high releasing property is used. For example, a PFA (a copolymer of tetrafluoroethylene and perfluoroalkylvinylether) layer, a PTFE (polytetrafluoroethylene) layer or a silicone copolymer layer or a composite layer formed of these layers is used. As to the thickness of the surface release layer  614 , if the thickness is too small, no sufficient abrasion resistance is obtained, hence, reducing the life of the fixing belt  61 . On the other hand, if the thickness is too large, the heat capacity of the fixing belt  61  becomes so large that the warm-up time becomes longer. In this respect, the thickness of the surface release layer  614  may be particularly 1 to 50 μm in consideration of the balance between the abrasion resistance and heat capacity. 
     &lt;Description of Pressing Pad&gt; 
     The pressing pad  63  is formed of an elastic material such as a silicone rubber or fluorine-contained rubber, and is supported by the holder  65  at a position facing the pressure roll  62 . Then, the pressing pad  63  is arranged in a state of being pressed by the pressure roll  62  with the fixing belt  61  therebetween, and forms the nip portion N with the pressure roll  62 . 
     In addition, the pressing pad  63  has different nip pressures set for a pre-nip region  63   a  on the sheet entering side of the nip portion N (upstream side in the transport direction of the sheet P) and a peeling nip region  63   b  on the sheet exit side of the nip portion N (downstream side in the transport direction of the sheet P), respectively. Specifically, a surface of the pre-nip region  63   a  at the pressure roll  62  side is formed into a circular arc shape approximately corresponding with the outer circumferential surface of the pressure roll  62 , and the nip portion N, which is uniform and wide, is formed. Moreover, a surface of the peeling nip region  63   b  at the pressure roll  62  side is formed into a shape so as to be locally pressed with a larger nip pressure from the surface of the pressure roll  62  in order that a curvature radius of the fixing belt  61  passing through the peeling nip region  63   b  may be small. Thereby, a curl (down curl) in a direction in which the sheet P is separated from the surface of the fixing belt  61  is formed on the sheet P passing through the peeling nip region  63   b , thereby promoting the peeling of the sheet P from the surface of the fixing belt  61 . 
     Note that, in the exemplary embodiments, the peeling assisting member  70  is arranged at the downstream side of the nip portion N as an assistance unit for the peeling of the sheet P by the pressing pad  63 . In the peeling assisting member  70 , a peeling baffle  71  is supported by a holder  72  in a state of being positioned to be close to the fixing belt  61  in a direction opposite to the rotational moving direction of the fixing belt  61  (so-called counter direction). Then, the peeling baffle  71  supports the curl portion formed on the sheet P at the exit of the pressing pad  63 , thereby preventing the sheet P from moving toward the fixing belt  61 . 
     &lt;Description of Temperature-Sensitive Magnetic Member&gt; 
     Next, the temperature-sensitive magnetic member  64  is formed into a circular arc shape corresponding with an inner circumferential surface of the fixing belt  61  and is arranged to be close to, but not to be in contact with the inner circumferential surface of the fixing belt  61  so as to have a predetermined gap (0.5 to 1.5 mm, for example) with the inner circumferential surface of the fixing belt  61 . The reason for arranging the temperature-sensitive magnetic member  64  so as to be close to the fixing belt  61  is to achieve a configuration in which the temperature of the temperature-sensitive magnetic member  64  changes in accordance with the temperature of the fixing belt  61 , that is, the temperature of the temperature-sensitive magnetic member  64  becomes substantially equal to the temperature of the fixing belt  61 . In addition, the reason for arranging the temperature-sensitive magnetic member  64  so as not to be in contact with the fixing belt  61  is to suppress heat of the fixing belt  61  flowing into the temperature-sensitive magnetic member  64  when the fixing belt  61  is self-heated up to the fixation setting temperature after a main switch of the image forming apparatus  1  is turned on, and thereby to achieve shortening of the warm-up time. 
     Moreover, the temperature-sensitive magnetic member  64  is formed of a material whose “permeability change start temperature” (refer to later part of the description) at which the permeability of the magnetic properties drastically changes is not less than the fixation setting temperature at which each color toner image starts melting, and whose permeability change start temperature is also set within a temperature range lower than the heat-resistant temperatures of the elastic layer  613  and the surface release layer  614  of the fixing belt  61 . Specifically, the temperature-sensitive magnetic member  64  is formed of a material having a property (“temperature-sensitive magnetic property”) that reversibly changes between the ferromagnetic property and the non-magnetic property (paramagnetic property) in a temperature range including the fixation setting temperature. Thus, the temperature-sensitive magnetic member  64  functions as a magnetic path forming member that forms a magnetic path in the temperature-sensitive magnetic member  64  within a temperature range not greater than the permeability change start temperature, where the temperature-sensitive magnetic member  64  has the ferromagnetic property. Further, within the temperature range not greater than the permeability change start temperature, the temperature-sensitive magnetic member  64  induces magnetic field lines generated by the IH heater  80  and going through the fixing belt  61  to the inside thereof, and forms a magnetic path so that the magnetic field lines may pass through the inside of the temperature-sensitive magnetic member  64 . Thereby, the temperature-sensitive magnetic member  64  forms a closed magnetic path that internally wraps the fixing belt  61  and an excitation coil  82  (refer to later-described  FIG. 6 ) of the IH heater  80 . Meanwhile, within a temperature range exceeding the permeability change start temperature, the temperature-sensitive magnetic member  64  causes the magnetic field lines generated by the IH heater  80  and going through the fixing belt  61  to go therethrough so as to run across the temperature-sensitive magnetic member  64  in the thickness direction of the temperature-sensitive magnetic member  64 . Then, the magnetic field lines generated by the IH heater  80  and going through the fixing belt  61  form a magnetic path in which the magnetic field lines go through the temperature-sensitive magnetic member  64 , pass through the inside of the induction member  66 , and then return to the III heater  80 . 
     Note that, the “permeability change start temperature” herein refers to a temperature at which a permeability (permeability measured by JIS C2531, for example) starts decreasing continuously and refers to a temperature point at which the amount of the magnetic flux (the number of magnetic field lines) going through a member such as the temperature-sensitive magnetic member  64  starts to change, for example. Accordingly, the permeability change start temperature is a temperature close to the Curie point, which is a temperature at which the magnetic property is lost, but is a temperature with a concept different from the Curie point. 
     Examples of the material of the temperature-sensitive magnetic member  64  include a binary component Fe—Ni alloy or a ternary component Fe—Ni—Cr alloy such as permalloys, magnetic compensator alloys flux or the like whose permeability change start temperature is set within a range of 140 degrees C. (the fixation setting temperature) to 240 degrees C. For example, the permeability change start temperature may be set around 225 degrees C. by setting the ratios of Fe and Ni at approximately 64% and 36% (atom number ratio), respectively, in a binary magnetic compensator alloys flux of Fe—Ni. The aforementioned metal alloys or the like including the permalloy and the magnetic compensator alloys flux are suitable for the temperature-sensitive magnetic member  64  since they are excellent in formability and workability, and a high heat conductivity as well as less expensive costs. Another example of the material includes a metal alloy made of Fe, Ni, Si, B, Nb, Cu, Zr, Co, Cr, V, Mn, Mo or the like. 
     In addition, the temperature-sensitive magnetic member  64  is formed with a thickness smaller than the skin depth δ (refer to the formula (1) described above) with respect to the AC magnetic field (magnetic field lines) generated by the IH heater  80 . Specifically, a thickness of approximately 50 to 300 μm is set when a Fe—Ni alloy is used as the material, for example. Note that, the configuration and the function of the temperature-sensitive magnetic member  64  will be described later in detail. 
     &lt;Description of Holder&gt; 
     The holder  65  that supports the pressing pad  63  is formed of a material having a high rigidity so that the amount of deflection in a state where the pressing pad  63  receives pressing force from the pressure roll  62  may be a certain amount or less. In this manner, the amount of pressure (nip pressure N) at the nip portion N in the longitudinal direction is kept uniform. Moreover, since the fixing unit  60  of the exemplary embodiments employs a configuration in which the fixing belt  61  is self-heated by use of electromagnetic induction, the holder  65  is made of a material that provides no influence or hardly provides influence on an induction magnetic field, and that is not influenced or is hardly influenced by the induction magnetic field. For example, a heat-resistant resin such as glass mixed PPS (polyphenylene sulfide), or a paramagnetic metal material such as Al, Cu or Ag is used. 
     &lt;Description of Induction Member&gt; 
     The induction member  66  is formed into a circular arc shape corresponding with the inner circumferential surface of the temperature-sensitive magnetic member  64  and is arranged so as not to be in contact with the inner circumferential surface of the temperature-sensitive magnetic member  64 . Here, the induction member  66  has a gap set in advance (1.0 to 5.0 mm, for example) with the inner circumferential surface of the temperature-sensitive magnetic member  64 . The induction member  66  is formed of, for example, a non-magnetic metal such as Ag, Cu and Al having a relatively small specific resistance. When the temperature of temperature-sensitive magnetic member  64  increases to a temperature not less than the permeability change start temperature, the induction member  66  induces an AC magnetic field (magnetic field lines) generated by the IH heater  80  and thereby forms a state where an eddy current I is more easily generated in comparison with the conductive heat generating layer  612  of the fixing belt  61 . For this reason, the thickness of the induction member  66  is formed to be a predetermined thickness (1.0 mm, for example) sufficiently larger than the skin depth δ (refer to the aforementioned formula (I)) so as to allow the eddy current I to easily flow therethrough. 
     &lt;Description of Drive Mechanism of Fixing Belt&gt; 
     Next, a description will be given of a drive mechanism of the fixing belt  61 . 
     As shown in  FIG. 2 , which is a front view, end caps  67  are secured to both ends in the axis direction of the holder  65  (refer to  FIG. 3 ), respectively. The end caps  67  rotationally drive the fixing belt  61  in a circumferential direction while keeping cross sectional shapes of both ends of the fixing belt  61  in a circular shape. Then, the fixing belt  61  directly receives rotational drive force via the end caps  67  at the both ends and rotationally moves at, for example, a process speed of 140 minis in a direction of an arrow C in  FIG. 3   
     Here,  FIG. 5A  is a side view of one of the end caps  67 , and  FIG. 5B  is a plain view of the end cap  67  when viewed from a VB direction of  FIG. 5A . As shown in  FIGS. 5A and 5B , the end cap  67  includes: a fixing portion  67   a  that is fitted into the inside of a corresponding one of the ends of the fixing belt  61 ; a flange  67   d  that has an outer diameter larger than that of the fixing portion  67   a  and that is formed so as to project from the fixing belt  61  in the radial direction when attached to the fixing belt  61 ; a gear  67   b  to which the rotational drive force is transmitted; and a bearing unit  67   c  that is rotatably connected to a support member  65   a  formed at a corresponding one of the ends of the holder  65  with a connection member  166  interposed therebetween. Then, as shown in  FIG. 2 , the support members  65   a  at the both ends of the holder  65  (refer to  FIG. 3 ) are secured onto the both ends of a chassis  69  of the fixing unit  60 , respectively, thereby, supporting the end caps  67  so as to be rotatable with the bearing units  67   c  respectively connected to the support members  65   a.    
     As the material of the end caps  67 , so-called engineering plastics having a high mechanical strength or heat-resistant properties is used. For example, a phenol resin, polyimide resin, polyamide resin, polyamide-imide resin, PEEK resin, PES resin, PPS resin, LCP resin or the like is suitable. Then, as shown in  FIG. 2 , in the fixing unit  60 , rotational drive force from a drive motor  90  is transmitted to a shaft  93  via transmission gears  91  and  92 . The rotational drive force is then transmitted from transmission gears  94  and  95  connected to the shaft  93  to the gears  67   b  of the respective end caps  67  (refer to  FIGS. 5A and 5B ). Thereby, the rotational drive force is transmitted from the end caps  67  to the fixing belt  61 , and the end caps  67  and the fixing belt  61  are integrally driven to rotate. 
     As described above, the fixing belt  61  directly receives the drive force at the both ends of the fixing belt  61  to rotate, thereby rotating stably. 
     Here, a torque of approximately 0.1 to 0.5 Nm is generally exerted when the fixing belt  61  directly receives the drive force from the end caps  67  at the both ends thereof and then rotates. However, in the fixing belt  61  of the exemplary embodiments, the base layer  611  is formed of, for example, a non-magnetic stainless steel having a high mechanical strength. Thus, buckling or the like does not easily occur on the fixing belt  61  even when a torsional torque of approximately 0.1 to 0.5 Nm is exerted on the entire fixing belt  61 . 
     In addition, the fixing belt  61  is prevented from inclining or leaning to one direction by the flanges  67   d  of the end caps  67 , but at this time, compressive force of approximately 1 to 5 N is exerted toward the axis direction from the ends (flanges  67   d ) on the fixing belt  61  in general. However, even in a case where the fixing belt  61  receives such compressive force, the occurrence of buckling or the like is prevented since the base layer  611  of the fixing belt  61  is formed of a non-magnetic stainless steel or the like. 
     As described above, the fixing belt  61  of the exemplary embodiments receives the drive force directly at the both ends of the fixing belt  61  to rotate, thereby, rotating stably. In addition, the base layer  611  of the fixing belt  61  is formed of, for example, a non-magnetic stainless steel or the like having a high mechanical strength, hence providing the configuration in which buckling or the like caused by a torsion torque or compressive force does not easily occur in this case. Moreover, the softness and flexibility of the entire faxing belt  61  is obtained by forming the base layer  611  and the conductive heat-generating layer  612  respectively as thin layers, so that the fixing belt  61  is deformed so as to correspond with the nip portion N and recovers to the original shape. 
     With reference back to  FIG. 3 , the pressure roll  62  is arranged to face the fixing belt  61  and rotates at, for example, a process speed of 140 mm/s in the direction of an arrow D in  FIG. 3  while being driven by the fixing belt  61 . Then, the nip portion N is formed in a state where the fixing belt  61  is held between the pressure roll  62  and the pressing pad  63 . Then, while the sheet P holding an unfixed toner image is caused to pass through this nip portion N, heat and pressure are applied to the sheet P, and thereby, the unfixed toner image is fixed onto the sheet P. 
     The pressure roll  62  is formed of a multi-layer configuration including: a solid aluminum core (cylindrical core metal)  621  having a diameter of 18 mm, for example; a heat-resistant elastic layer  622  that covers the outer circumferential surface of the core  621 , and that is made of silicone sponge having a thickness of 5 mm, for example; and a release layer  623  that is formed of a heat-resistant resin such as PFA containing carbon or the like, or a heat-resistant rubber, having a thickness of 50 μm, for example, and that covers the heat-resistant elastic layer  622 . Then, the pressing pad  63  is pressed under a load of 245.166 N (25 kgf) for example, by pressing springs  68  (refer to  FIG. 2 ) with the fixing belt  61  therebetween. 
     &lt;Description of IH Heater&gt; 
     Next, a description will be given of the IH heater  80  that induces the heat generation of the fixing belt  61  by electromagnetic induction by action of an AC magnetic field in the conductive heat-generating layer  612  of the fixing belt  61 . 
       FIG. 6  is a cross sectional view for explaining a configuration of the IH heater  80  of the exemplary embodiments. As shown in  FIG. 6 , the IH heater  80  includes: a support member  81  that is formed of a non-magnetic material such as a heat-resistant resin, for example; and the excitation coil  82  that generates the AC magnetic field. Moreover, the IH heater  80  includes: elastic support members  83  each of which is formed of an elastic material and secures the excitation coil  82  onto the support member  81 ; and a magnetic core  84  that forms a magnetic path of the AC magnetic field generated by the excitation coil  82 . Further, the IH heater  80  includes: a shield  85  that shields a magnetic field; a pressing member  86  that presses the magnetic cores  84  toward the support member  81 ; and an excitation circuit  88  that supplies an AC current to the excitation coil  82 . 
     The support member  81  is formed to have a cross section in a shape curving along the surface shape of the fixing belt  61 , and includes an upper surface (supporting surface)  81   a  that supports the excitation coil  82  and that is formed so as to keep a gap set in advance (for example, 0.5 to 2 mm) with a surface of the fixing belt  61 . As a material of the support member  81 , a non-magnetic material having heat resistance is used, such as heat-resistant glass; heat-resistant resin such as polycarbonate, polyether sulphone and polyphenylene sulfide (PPS); and the aforementioned heat-resistant resin mixed with glass fibers. 
     The excitation coil  82  is formed by winding a litz wire in a closed loop of an oval shape, elliptical shape or rectangular shape having an opening inside, the litz wire being obtained by bundling 90 pieces of mutually isolated copper wires each having a diameter of 0.17 mm, for example. Then, when an AC current having a frequency set in advance is supplied from the excitation circuit  88  to the excitation coil  82 , an AC magnetic field on the litz wire wound in a closed loop shape as the center is generated around the excitation coil  82 . In general, a frequency of 20 kHz to 100 kHz, which is generated by the aforementioned general-purpose power supply, is used for the frequency of the AC current supplied to the excitation coil  82  from the excitation circuit  88 . 
     As the material of the magnetic core  84 , a ferromagnetic material that is formed of an oxide or alloy material with a high permeability, such as a soft ferrite, a ferrite resin, a non-crystalline alloy (amorphous alloy), permalloys or a magnetic compensator alloys flux is used. The magnetic core  84  functions as a magnetic path unit. The magnetic core  84  induces, to the inside thereof, the magnetic field lines (magnetic flux) of the AC magnetic field generated at the excitation coil  82 , and forms a path (magnetic path) of the magnetic field lines in which the magnetic field lines from the magnetic core  84  run across the fixing belt  61  to be directed to the temperature-sensitive magnetic member  64 , then pass through the inside of the temperature-sensitive magnetic member  64 , and return to the magnetic core  84 . Specifically, a configuration in which the AC magnetic field generated at the excitation coil  82  passes through the inside of the magnetic core  84  and the inside of the temperature-sensitive magnetic member  64  is employed, and thereby, a closed magnetic path where the magnetic field lines internally wrap the fixing belt  61  and the excitation coil  82  is formed. Thereby, the magnetic field lines of the AC magnetic field generated at the excitation coil  82  are concentrated at a region of the fixing belt  61 , which faces the magnetic core  84 . 
     Here, the material of the magnetic core  84  may be one that has a small amount of loss due to the forming of the magnetic path. Specifically, the magnetic core  84  may be particularly used in a form that reduces the amount of eddy-current loss (shielding or controlling of the electric current path by having a slit or the like, or bundling of thin plates, or the like). In addition, the magnetic core  84  may be particularly formed of a material having a small hysteresis loss. 
     The length of the magnetic core  84  along the rotational direction of the fixing belt  61  is formed so as to be shorter than the length of the temperature-sensitive magnetic member  64  along the rotational direction of the fixing belt  61 . Thereby, the amount of leakage of the magnetic field lines toward the periphery of the IH heater  80  is reduced, resulting in improvement in the power factor. Moreover, the electromagnetic induction toward the metal materials forming the fixing unit  60  is also suppressed and the heat-generating efficiency at the fixing belt  61  (conductive heat-generating layer  612 ) increases. 
     &lt;Description of Securing Method of Excitation Coil&gt; 
     Next, a description will be given of the securing method of the excitation coil  82  to the support member  81  in the IH heater  80  of the exemplary embodiments. 
     In the IH heater  80  of the exemplary embodiments, the elastic support member  83  as an example of an elastic support member that supports the excitation coil  82  to the support member  81  is formed of an elastic material such as silicone rubber or fluorine-contained rubber. The elastic support member  83  elastically deforms while pressing the excitation coil  82  toward the support member  81 , and thereby supporting the excitation coil  82  to the supporting surface of the support member  81 . In other words, the elastic support member  83  is made of a material having a low Young&#39;s modulus, elastically deforms when the elastic support member  83  having the low Young&#39;s modulus presses the excitation coil  82  toward the support member  81 , and then supports the excitation coil  82  to the support member  81 . 
       FIG. 7  is a diagram for explaining a multi-layer structure of the IH heater  80  in the exemplary embodiments. As shown in  FIG. 7 , the excitation coil  82  is arranged on the supporting surface  81   a  of the support member  81  so that a closed loop hollow  82   a  of the excitation coil  82  may surround a convex portion  81   b  arranged in the center axis in the longitudinal direction of the supporting surface  81   a . The supporting surface  81   a  is formed as a position setting surface whose gap with the fixing belt  61  that is supported by the above-described end caps  67  (refer to  FIG. 5 ) and that rotationally moves in a substantially circular orbit is set at a defined value (design value). The excitation coil  82  is arranged so as to be in close contact with the supporting surface  81   a , and thereby the gap between the excitation coil  82  and the fixing belt  61  is set at the designed value. 
     By this setting, in the IH heater  80  of the exemplary embodiments, the excitation coil  82  arranged on the supporting surface  81   a  of the support member  81  is pressed toward the supporting surface  81   a  by the elastic support members  83 . In other words, the magnetic cores  84  arranged above the excitation coil  82  each have both ends  84   a  attached to supporting rails  81   c  provided at the both ends of the support member  81  (also refer to  FIG. 6 ). Thereby, the elastic support members  83  arranged at the lower side faces of the magnetic cores  84  (side faces on the support member  81  side) are arranged so as to be in contact with the upper surface of the excitation coil  82 . On the other hand, the magnetic cores  84  are pressed toward the support member  81  by the pressing member  86  provided on the lower surface of the shield  85  when the shield  85  is attached to the support member  81 . Thereby, the excitation coil  82  receives elastic force from the elastic support members  83  which have received pressing force from the magnetic cores  84 , and the excitation coil  82  is supported on the supporting surface  81   a  while being pressed toward the supporting surface  81   a  by the elastic support members  83  elastically deformed by the pressing force. Accordingly, the excitation coil  82  is in close contact with the supporting surface  81   a  and the gap between the excitation coil  82  and the fixing belt  61  is set at the designed value. 
     Note that, as the pressing member  86 , an elastic member such as a spring may be used instead of an elastic material such as a silicone rubber or fluorine-contained rubber. 
     In general, when an AC magnetic field is generated by the excitation coil  82 , magnetic force acts between the magnetic cores  84  arranged in the vicinity of the excitation coil  82 , the temperature-sensitive magnetic member  64  arranged on the inner circumferential surface side of the fixing belt  61  and the like, and thereby the excitation coil  82  vibrates itself (exhibits a magnetostrictive property). Thereby, if the excitation coil  82  is secured to the support member  81  by using a so-called rigid body (material having a high Young&#39;s modulus) such as an adhesive agent, peeling easily occurs between the excitation coil  82  and the rigid body such as the adhesive agent due to the vibration of the excitation coil  82  during the accumulated use of the fixing unit  60  for a long period. Then, when the excitation coil  82  is peeled from the rigid body such as the adhesive agent, the excitation coil  82  is displaced on the supporting surface  81   a , or the excitation coil  82  deforms. Thereby, the gap between the excitation coil  82  and the fixing belt  61  is deviated from the originally designed value, and the density of the magnetic field lines (density of magnetic flux) passing through the fixing belt  61  via the magnetic cores  84  partially varies on the surface of the fixing belt  61 . For this reason, the amount of the eddy current I generated at the fixing belt  61  becomes uneven, and the amount of heat generation on the surface of the fixing belt  61  may partially vary in some cases. 
     When the excitation coil  82  is secured to the support member  81  by use of a rigid body such as an adhesive agent, whole surfaces of the excitation coil  82  are necessary to be immobilized so as not to be displaced from the support member  81  until the adhesive agent or the like sets. The excitation coil  82 , however, has a configuration in which litz wires are bundled in a closed loop shape and are adhered to each other, for example. Thus, the excitation coil  82  is easily deformed. Accordingly, it is difficult to immobilize the excitation coil  82  so that the excitation coil  82  is not displaced from the support member  81 , until the adhesive agent or the like sets, and thus a positional accuracy of the excitation coil  82  with respect to the support member  81  is likely to be lowered. If the positional accuracy of the excitation coil  82  with respect to the support member  81  is lowered, a condition in which the heat generating amount of the surface of the fixing belt  61  partially varies is formed, similarly to the above. 
     In the IH heater  80  of the exemplary embodiments, the elastic support members  83  formed of an elastic material such as silicone rubber, fluorine-contained rubber or the like press the excitation coil  82  toward the support member  81 , and thereby a configuration in which the excitation coil  82  is supported by the supporting surface  81   a  of the support member  81  is achieved. The elastic support members  83  formed of an elastic material elastically deform in response to the vibration of the excitation coil  82  while absorbing the vibration of the excitation coil  82 . Thereby, even if the accumulated number of vibrations of the excitation coil  82  is large due to the accumulated use of the fixing unit  60  for a long period, the elastic support members  83  and the excitation coil  82  are not peeled from each other, and the positional relationship between the support member  81  and the excitation coil  82  is maintained to be a default setting one. 
     Moreover, the elastic support member  83  is controlled so as to have the thickness (setting value) within the dimensional precision set in advance at the production. Therefore, pressing force for supporting the excitation coil  82  on the supporting surface  81   a  in the longitudinal direction is set to be approximately uniform. In particular, in the IH heater  80  of the exemplary embodiments, the multiple excitation cores  84  uniformly press the excitation coil  82  in the longitudinal direction. Here, the multiple excitation cores  84  are separately provided in the longitudinal direction of the excitation coil  82 . Thereby, closeness between the excitation coil  82  and the supporting surface  81   a  is increased in the longitudinal direction, and the positions of the excitation coil  82  and the fixing belt  61  are set in the longitudinal direction. 
     At the production of the IH heater  80 , the excitation coil  82  is attached in a short time without time until the adhesive agent or the like sets. 
     &lt;Description of a State in which Fixing Belt Generates Heat&gt; 
     Next, a description will be given of a state in which the fixing belt  61  generates heat by use of the AC magnetic field generated by the IH heater  80 . 
     Firstly, as described above, the permeability change start temperature of the temperature-sensitive magnetic member  64  is set within a temperature range (140 to 240 degrees C., for example) where the temperature is not less than the fixation setting temperature for fixing color toner images and not greater than the heat-resistant temperature of the fixing belt  61 . Then, when the temperature of the fixing belt  61  is not greater than the permeability change start temperature, the temperature of the temperature-sensitive magnetic member  64  near the fixing belt  61  corresponds to the temperature of the fixing belt  61  and then becomes equal to or lower than the permeability change start temperature. For this reason, the temperature-sensitive magnetic member  64  has a ferromagnetic property at this time, and thus, the magnetic field lines H of the AC magnetic field generated by the IH heater  80  form a magnetic path where the magnetic field lines H go through the fixing belt  61  and thereafter, pass through the inside of the temperature-sensitive magnetic member  64  along a spreading direction. Here, the “spreading direction” refers to a direction orthogonal to the thickness direction of the temperature-sensitive magnetic member  64 . 
       FIG. 8  is a diagram for explaining the state of the magnetic field lines H in a case where the temperature of the fixing belt  61  is within the temperature range not greater than the permeability change start temperature. As shown in  FIG. 8 , in the case where the temperature of the fixing belt  61  is within the temperature range not greater than the permeability change start temperature, the magnetic field lines H of the AC magnetic field generated by the IH heater  80  form a magnetic path where the magnetic field lines H go through the fixing belt  61 , and then pass through the inside of the temperature-sensitive magnetic member  64  in the spreading direction (direction orthogonal to the thickness direction). Accordingly, the number of the magnetic field lines H (density of magnetic flux) per unit area in the region where the magnetic field lines H run across the conductive heat-generating layer  612  of the fixing belt  61  becomes large. 
     Specifically, after the magnetic field lines H are radiated from the magnetic cores  84  of the IH heater  80  and pass through regions R 1  and R 2  where the magnetic field lines H run across the conductive heat-generating layer  612  of the fixing belt  61 , the magnetic field lines H are induced to the inside of the temperature-sensitive magnetic member  64 , which is a ferromagnetic member. For this reason, the magnetic field lines H running across the conductive heat-generating layer  612  of the fixing belt  61  in the thickness direction are concentrated so as to enter the inside of the temperature-sensitive magnetic member  64 . Accordingly, the magnetic flux density becomes high in the regions R 1  and R 2 . In addition, in a case where the magnetic field lines H passing through the inside of the temperature-sensitive magnetic member  64  along the spreading direction return to the magnetic core  84 , in a region R 3  where the magnetic field lines H run across the conductive heat-generating layer  612  in the thickness direction, the magnetic field lines H are generated toward the magnetic core  84  in a concentrated manner from a portion, where the magnetic potential is low, of the temperature-sensitive magnetic member  64 . For this reason, the magnetic field lines H running across the conductive heat-generating layer  612  of the fixing belt  61  in the thickness direction head from the temperature-sensitive magnetic member  64  toward the magnetic core  84  in a concentrated manner, so that the magnetic flux density in the region R 3  becomes high as well. 
     In the conductive heat-generating layer  612  of the fixing belt  61  which the magnetic field lines H run across in the thickness direction, the eddy current I proportional to the amount of change in the number of the magnetic field lines H per unit area (magnetic flux density) is generated. Thereby, as shown in  FIG. 8 , a larger eddy current I is generated in the regions R 1 , R 2  and R 3  where a large amount of change in the magnetic flux density occurs. The eddy current I generated in the conductive heat-generating layer  612  generates a Joule heat W (W=I 2 R), which is multiplication of the specific resistant value R and the square of the eddy current I of the conductive heat-generating layer  612 . Accordingly, a large Joule heat W is generated in the conductive heat-generating layer  612  where the larger eddy current I is generated. 
     As described above, in a case where the temperature of the fixing belt  61  is within the temperature range not greater than the permeability change start temperature, a large amount of heat is generated in the regions R 1 , R 2  and R 3  where the magnetic field lines H run across the conductive heat-generating layer  612 , and thereby the fixing belt  61  is heated. Incidentally, in the fixing unit  60  of the exemplary embodiments, the temperature-sensitive magnetic member  64  is arranged so as to be close to the inner circumferential surface of the fixing belt  61 , thereby, providing the configuration in which the magnetic cores  84  inducing the magnetic field lines H generated at the excitation coil  82  to the inside thereof, and the temperature-sensitive magnetic member  64  inducing, to the inside thereof, the magnetic field lines H running across and going through the fixing belt  61  in the thickness direction are arranged to be close to each other. For this reason, the AC magnetic field generated by the IH heater  80  (excitation coil  82 ) forms a loop of a short magnetic path, so that the magnetic flux density and the degree of magnetic coupling in the magnetic path increase. Thereby, heat is more efficiently generated in the fixing belt  61  in a case where the temperature of the fixing belt  61  is within the temperature range not greater than the permeability change start temperature. 
     &lt;Description of Function for Suppressing Increase in Temperature of Non-Sheet Passing Portion of Fixing Belt&gt; 
     Next, a description will be given of a function for suppressing an increase in the temperature of a non-sheet passing portion of the fixing belt  61 . 
     Firstly, a description will be given herein of a case where sheets P of a small size (small size sheets P 1 ) are successively inserted into the fixing unit  60 .  FIG. 9  is a diagram showing a summary of a temperature distribution in the width direction of the fixing belt  61  when the small size sheets P 1  are successively inserted into the fixing unit  60 . In  FIG. 9 , Ff denotes a maximum sheet passing region, which is the width (A 3  long side, for example) of the maximum size of a sheet P used in the image forming apparatus  1 , Fs denotes a region through which the small size sheet P 1  (A4 longitudinal feed, for example) having a smaller horizontal width than that of a maximum size sheet P passes, and Fb denotes a non-sheet passing region through which no small size sheet P 1  passes. Note that, sheets are inserted into the image forming apparatus  1  with the center position thereof as the reference point. 
     As shown in  FIG. 9 , when the small size sheets P 1  are successively inserted into the fixing unit  60 , the heat for fixing is consumed at the small size sheet passing region Fs where each of the small size sheets P 1  passes. For this reason, the controller  31  (refer to  FIG. 1 ) performs a temperature adjustment control with a fixation setting temperature, so that the temperature of the fixing belt  61  at the small size sheet passing region Fs is maintained within a range near the fixation setting temperature. Meanwhile, at the non-sheet passing regions Fb as well, the same temperature adjustment control as that performed for the small size sheet passing region Fs is performed. However, the heat for fixing is not consumed at the non-sheet passing regions Fb. For this reason, the temperature of the non-sheet passing regions Fb easily increases to a temperature higher than the fixation setting temperature. Then, when the small size sheets P 1  are successively inserted into the fixing unit  60  in this state, the temperature of the non-sheet passing regions Fb increases to a temperature higher than the heat-resistant temperature of, for example, the elastic layer  613  or the surface release layer  614  of the fixing belt  61 , hence deteriorating the fixing belt  61  in some cases. 
     In this respect, as described above, in the fixing unit  60  of the exemplary embodiments, the temperature-sensitive magnetic member  64  is formed of, for example, a Fe—Ni alloy or the like whose permeability change start temperature is set within a temperature range not less than the fixation setting temperature and not greater than the heat-resistant temperature of the elastic layer  613  or the surface release layer  614  of the fixing belt  61 . Specifically, as shown in  FIG. 9 , a permeability change start temperature Tcu of the temperature-sensitive magnetic member  64  is set within a temperature range not less than a fixation setting temperature Tf and not greater than a heat-resistant temperature Tlim of, for example, the elastic layer  613  or the surface release layer  614 . 
     Thus, when the small size sheets P 1  are successively inserted into the fixing unit  60 , the temperature of the non-sheet passing regions Fb of the fixing belt  61  exceeds the permeability change start temperature of the temperature-sensitive magnetic member  64 . Accordingly, the temperature of the temperature-sensitive magnetic member  64  near the fixing belt  61  at the non-sheet passing regions Fb also exceeds the permeability change start temperature in response to the temperature of the fixing belt  61  as in the case of the fixing belt  61 . For this reason, the relative permeability of the temperature-sensitive magnetic member  64  at the non-sheet passing regions Fb becomes close to 1, so that the temperature-sensitive magnetic member  64  at the non-sheet passing regions Fb loses the ferromagnetic properties. Since the relative permeability of the temperature-sensitive magnetic member  64  decreases and becomes closer to 1, the magnetic field lines H at the non-sheet passing regions Fb are no longer induced to the inside of the temperature-sensitive magnetic member  64 , and start going through the temperature-sensitive magnetic member  64 . For this reason, in the fixing belt  61  at the non-sheet passing regions Fb, the magnetic field lines H spread after passing through the conductive heat-generating layer  612 , hence leading to a decrease in the density of magnetic flux of the magnetic field lines H running across the conductive heat-generating layer  612 . Thereby, the amount of an eddy current I generated at the conductive heat-generating layer  612  decreases, and then, the amount of heat (Joule heat W) generated at the fixing belt  61  decreases. As a result, an excessive increase in the temperature at the non-sheet passing regions Fb is suppressed, and the fixing belt  61  is prevented from being damaged. 
     As described above, the temperature-sensitive magnetic member  64  functions as a detector that detects the temperature of the fixing belt  61  and also functions as a temperature increase suppresser that suppresses an excessive increase in the temperature of the fixing belt  61  in accordance with the detected temperature of the fixing belt  61 , at a time. 
     The magnetic field lines H passing through the temperature-sensitive magnetic member  64  arrive at the induction member  66  (refer to  FIG. 3 ) and then are induced to the inside thereof. When the magnetic flux arrives at the induction member  66  and then is induced to the inside thereof, a large amount of the eddy current I flows into the induction member  66 , into which the eddy current I flows more easily than into the heat conducive layer  612 . Thus, the amount of eddy current flowing into the conductive layer  612  is further suppressed, so that an increase in the temperature at the non-sheet passing regions Fb is suppressed. 
     At this time, the thickness, material and shape of the induction member  66  are selected in order that the induction member  66  may induce most of the magnetic field lines H from the excitation coil  82  and the magnetic field lines H may be prevented from leaking from the fixing unit  60 . Specifically, the induction member  66  is formed of a material having a sufficiently large thickness of the skin depth δ. Thereby, even when the eddy current I flows into the induction member  66 , the amount of heat to be generated is extremely small. In the exemplary embodiments, the induction member  66  is formed of Al (aluminum), with a thickness of 1 mm, of a substantially circular arc shape along the temperature-sensitive magnetic member  64 . The induction member  66  is also arranged so as not to be in contact with the temperature-sensitive magnetic member  64  (average distance therebetween is 4 mm, for example). As another example of the material, Ag or Cu may be particularly used. 
     Incidentally, when the temperature of the fixing belt  61  at the non-sheet passing regions Fb becomes lower than the permeability change start temperature of the temperature-sensitive magnetic member  64 , the temperature of the temperature-sensitive magnetic member  64  at the non-sheet passing regions Fb also becomes lower than the permeability change start temperature thereof. For this reason, the temperature-sensitive magnetic member  64  becomes ferromagnetic again, and the magnetic field lines H are induced to the inside of the temperature-sensitive magnetic member  64 . Thus, a large amount of the eddy current I flows into the conductive heat-generating layer  612 . For this reason, the fixing belt  61  is again heated. 
       FIG. 10  is a diagram for explaining a state of the magnetic field lines H when the temperature of the fixing belt  61  at the non-sheet passing regions Fb is within the temperature range exceeding the permeability change start temperature. As shown in  FIG. 10 , when the temperature of the fixing belt  61  at the non-sheet passing regions Fb is within the temperature range exceeding the permeability change start temperature, the relative permeability of the temperature-sensitive magnetic member  64  at the non-sheet passing regions Fb decreases. For this reason, the magnetic field lines H of the AC magnetic field generated by the IH heater  80  changes so as to easily go through the temperature-sensitive magnetic member  64 . Thereby, the magnetic field lines H of the AC magnetic field generated by the IH heater  80  (excitation coil  82 ) are radiated from the magnetic cores  84  so as to spread toward the fixing belt  61  and arrive at the induction member  66 . 
     Specifically, at the regions R 1  and R 2  where the magnetic field lines H are radiated from the magnetic cores  84  of the IH heater  80  and then run across the conductive heat-generating layer  612  of the fixing belt  61 , since the magnetic field lines H are not easily induced to the temperature-sensitive magnetic member  64 , the magnetic field lines H radially spread. Accordingly, the density of the magnetic flux (the number of the magnetic field lines H per unit area) of the magnetic field lines H running across the conductive heat-generating layer  612  of the fixing belt  61  in the thickness direction decreases. In addition, at the region R 3  where the magnetic field lines H run across the conductive heat-generating layer  612  in the thickness direction when returning to the magnetic cores  84  again, the magnetic field lines H return to the magnetic cores  84  from the wide region where the magnetic field lines H spread, so that the density of the magnetic flux of the magnetic field lines H running across the conductive heat-generating layer  612  of the fixing belt  61  in the thickness direction decreases. 
     For this reason, when the temperature of the fixing belt  61  is within the temperature range exceeding the permeability change start temperature, the density of the magnetic flux of the magnetic field lines H running across the conductive heat-generating layer  612  in the thickness direction at the regions R 1 , R 2  and R 3  decreases. Accordingly, the amount of the eddy current I generated in the conductive heat-generating layer  612  where the magnetic field lines H run across in the thickness direction decreases, and the Joule heat W generated at the fixing belt  61  decreases. Therefore, the temperature of the fixing belt  61  decreases. 
     As described above, when the temperature of the fixing belt  61  at the non-sheet passing regions Fb is within a temperature range not less than the permeability change start temperature, the magnetic field lines H are not easily induced to the inside of the temperature-sensitive magnetic member  64  at the non-sheet passing regions Fb. Thus, the magnetic field lines H of the AC magnetic field generated by the excitation coil  82  spread and run across the conductive heat-generating layer  612  of the fixing belt  61  in the thickness direction. Accordingly, the magnetic path of the AC magnetic field generated by the excitation coil  82  forms a long loop, so that the density of magnetic flux in the magnetic path in which the magnetic field lines H pass through the conductive heat-generating layer  612  of the fixing belt  61  decreases. 
     Thereby, at the non-sheet passing regions Fb where the temperature thereof increases, for example, when the small size sheets P 1  are successively inserted into the fixing unit  60 , the amount of the eddy current I generated at the conductive heat-generating layer  612  of the fixing belt  61  decreases, and the amount of heat (Joule heat W) generated at the non-sheet passing regions Fb of the fixing belt  61  decreases. As a result, an excessive increase in the temperature of the non-sheet passing regions Fb is suppressed. 
     &lt;Description of Configuration for Suppressing Increase in Temperature of Temperature-Sensitive Magnetic Member&gt; 
     In order for the temperature-sensitive magnetic member  64  to satisfy the aforementioned function to suppress an excessive increase in the temperature at the non-sheet passing regions Fb, the temperature of each region of the temperature-sensitive magnetic member  64  in the longitudinal direction needs to change in accordance with the temperature of each region of the fixing belt  61  in the longitudinal direction, which faces each region of the temperature-sensitive magnetic member  64  in the longitudinal direction, to satisfy the aforementioned function as a detector that detects the temperature of the fixing belt  61 . 
     For this reason, as the configuration of the temperature-sensitive magnetic member  64 , a configuration in which the temperature-sensitive magnetic member  64  is not easily subjected to induction heating by the magnetic field lines H is employed. Specifically, even when the temperature-sensitive magnetic member  64  is in a state of being ferromagnetic since the temperature of the fixing belt  61  is not greater than the permeability change start temperature, some of the magnetic field lines H that run across the temperature-sensitive magnetic member  64  in the thickness direction still exist in the magnetic field lines H from the IH heater  80 . Thus, a weak eddy current I is generated inside the temperature-sensitive magnetic member  64 , so that a small amount of heat is generated in the temperature-sensitive magnetic member  64  as well. For this reason, for example, in a case where a large amount of image formation is successively performed, the heat generated by the temperature-sensitive magnetic member  64  is accumulated in itself, and the temperature of the temperature-sensitive magnetic member  64  at the sheet passing region (refer to  FIG. 9 ) tends to increase. When the amount of the self-heating due to the eddy current loss in this manner is large, the temperature of the temperature-sensitive magnetic member  64  increases, and unintentionally reaches the permeability change start temperature. As a result, the magnetic characteristic difference between the sheet-passing region and the non-sheet passing regions no longer exists, and thus, the effect of suppressing a temperature increase becomes no longer effective. In this respect, in order to maintain the correspondence relationship between the respective temperatures of the temperature-sensitive magnetic member  64  and the fixing belt  61  and in order for the temperature-sensitive magnetic member  64  to function as the detector that detects the temperature of the fixing belt  61  with high accuracy, Joule heat W to be generated in the temperature-sensitive magnetic member  64  needs to be suppressed. 
     With this respect, firstly, a material having properties (specific resistance and permeability) not easily subjected to induction heating by the magnetic field lines H is selected as the material of the temperature-sensitive magnetic member  64  for the purpose of reducing an eddy current loss or hysteresis loss in the temperature-sensitive magnetic member  64 . 
     Secondly, the thickness of the temperature-sensitive magnetic member  64  is formed to be larger than the skin depth δ in the state where the temperature-sensitive magnetic member  64  is ferromagnetic, in order that the magnetic field lines H may not easily run across the temperature-sensitive magnetic member  64  in the thickness direction when the temperature of the temperature-sensitive magnetic member  64  is at least within the temperature range not greater than the permeability change start temperature. 
     Thirdly, multiple slits  64   s  (refer to  FIG. 11  described later) controlling the flow of an eddy current I generated by the magnetic field lines H are formed in the temperature-sensitive magnetic member  64 . Even when the material and the thickness of the temperature-sensitive magnetic member  64  are selected so as not to be easily subjected to induction heating, it is difficult to make the eddy current I generated inside the temperature-sensitive magnetic member  64  be zero (0). In this respect, the amount of eddy current I is decreased by controlling the flow of the eddy current I generated in the temperature-sensitive magnetic member  64  with the multiple slits  64   s . Thereby, Joule heat W generated in the temperature-sensitive magnetic member  64  is suppressed to be low. 
       FIGS. 11A and 11B  are diagrams showing slits formed in the temperature-sensitive magnetic member  64 .  FIG. 11A  is a side view showing a state where the temperature-sensitive magnetic member  64  is mounted on the holder  65 .  FIG. 11B  is a plain view showing a state when  FIG. 11A  is viewed from above (XIB direction). As shown in  FIGS. 11A and 11B , the multiple slits  64   s  are formed in a direction orthogonal to the direction of the flow of the eddy current I generated by the magnetic field lines H, in the temperature-sensitive magnetic member  64 . Thereby, the eddy current I (shown by broken lines in  FIG. 11B ), which flows in the entire temperature-sensitive magnetic member  64  in the longitudinal direction while forming a large swirl in a case of forming no slits  64   s , is controlled by the slits  64   s . Accordingly, in a case where the slits  64   s  are formed, the eddy current I (shown by a solid line in  FIG. 11B ) that flows in the temperature-sensitive magnetic member  64  becomes small swirls each being in a region formed between adjacent two of the slits  64   s , hence reducing the entire amount of the eddy current I. As a result, the amount of heat (Joule heat W) generated in the temperature-sensitive magnetic member  64  decreases. Thereby, the configuration in which heat is not easily generated is achieved. Accordingly, each of the multiple slits  64   s  functions as an eddy current controlling unit that controls the eddy current I. 
     Note that, the slits  64   s  are formed in the direction orthogonal to the direction of the flow of the eddy current I in the temperature-sensitive magnetic member  64  exemplified in  FIGS. 11A and 11B . However, as long as the configuration allows the slits  64   s  to control the flow of the eddy current I, slits inclined with respect to the direction of the flow of the eddy current I may be formed, for example. Moreover, other than the configuration as shown in  FIGS. 11A and 11B  in which the slits  64   s  are formed over the entire region in the width direction of the temperature-sensitive magnetic member  64 , slits may be partially formed in the width direction of the temperature-sensitive magnetic member  64 . Furthermore, the number of, the position of or the inclination angle of slits  64   s  may be configured in accordance with the amount of heat to be generated in the temperature-sensitive magnetic member  64 . 
     In addition, the slits  64   s  may be formed in the temperature-sensitive magnetic member  64  in a way that the temperature-sensitive magnetic member  64  is divided into a group of small pieces by the slits  64   s  with an inclination angle of each slit Ms being the maximum. The effects of the present invention may be obtained in this configuration as well. 
     Fourthly, the temperature-sensitive magnetic member  64  is provided with a heat radiation path formed thereon. Here, the heat radiation path is an example of a heat transfer unit that radiates (transfers) heat generated in the temperature-sensitive magnetic member  64  in an inner direction of the temperature-sensitive magnetic member  64  (direction toward the induction member  66 ). In this case, it is desirable to maintain the temperature of the temperature-sensitive magnetic member  64  so that it is substantially the same as the temperature of the fixing belt  61 , from a viewpoint of the aforementioned function of the temperature-sensitive magnetic member  64 . Accordingly, the heat radiation path is configured so that the temperature-sensitive magnetic member  64  and the other members arranged inside the temperature-sensitive magnetic member  64  (for example, the induction member  66 ) keep the non-contact state. Specifically, by existence of air space as a part of the heat radiation path, heat from the temperature-sensitive magnetic member  64  via the heat radiation path is prevented from excessively flowing out. Thereby, in a case where, for example, heat generated in the temperature-sensitive magnetic member  64  is accumulated such as a case where a large amount of image formation is successively performed, the heat radiation path functions as the one in order to easily radiate, from the temperature-sensitive magnetic member  64 , the amount of heat corresponding to the heat generation by increase in the temperature exceeding the temperature of the fixing belt  61 . 
     First Exemplary Embodiment 
     A description will be given of the first exemplary embodiment of the heat radiation path that radiates heat generated in the temperature-sensitive magnetic member  64  toward the inner direction of the temperature-sensitive magnetic member  64 . 
       FIGS. 12A to 12C  are views for explaining the heat radiation path in the first exemplary embodiment.  FIG. 12A  is a perspective view in a state where the temperature-sensitive magnetic member  64  and the induction member  66  are arranged on the holder  65 ,  FIG. 12B  is a cross sectional view of an x-y plane at a coordinate point z 1  in the z axis direction in  FIG. 12A , and  FIG. 12C  is a view showing a modified example of the heat radiation path in the first exemplary embodiment. 
     Note that, in  FIGS. 12A to 12C , the z axis direction denotes the longitudinal direction of the holder  65 , and the x-y plane denotes a plane orthogonal to the z axis direction. The same is true in the following  FIGS. 13A to 17C . 
     As shown in  FIGS. 12A and 12B , a heat radiation member  64   a  is arranged on the inner circumferential surface of the temperature-sensitive magnetic member  64  toward the induction member  66 . Here, the heat radiation member  64   a  is formed of, for example, a metal material, a resin material having metallic particles dispersed therein, or the like, which has excellence in a heat transfer property. 
     The heat radiation member  64   a  is formed into a convex shape projecting from the inner circumferential surface of the temperature-sensitive magnetic member  64 , and, as shown in  FIG. 12A , the heat radiation member  64   a  is arranged over the entire region of the temperature-sensitive magnetic member  64  in the longitudinal direction (z direction). Further, as shown in  FIG. 12B , the heat radiation member  64   a  is formed so as not to be in contact with the induction member  66 , and air space g is interposed between the heat radiation member  64   a  and the induction member  66 . Note that, the heat radiation member  64   a  may be integrally formed with the temperature-sensitive magnetic member  64  or independently formed. 
     As described above, since the heat radiation member  64   a  formed into the convex shape and the induction member  66  are close to each other, the heat of the temperature-sensitive magnetic member  64  easily flows from the heat radiation member  64   a  to the induction member  66 . On the other hand, heat transfer rate of the (static) air space g is 0.024 W/mK, and this value is extremely smaller than that of a metal (having several tens of W/mk to several hundreds of W/mK) or the like. Thereby, since the air space g is interposed therebetween, the heat of the temperature-sensitive magnetic member  64  is not easily transferred to the induction member  66 . 
     In this respect, the length of the heat radiation member  64   a  in the width direction (x direction) and a gap of the air space g are set so as to correspond to the configuration of the fixing unit  60 , and thereby the heat radiation path that causes the temperature-sensitive magnetic member  64  to radiate the amount of heat corresponding to the increase in temperature exceeding the temperature of the fixing belt  61  is formed in a case where heat is accumulated in the temperature-sensitive magnetic member  64  such as a case where a large amount of image formation is successively performed. 
     In other words, the length of the heat radiation member  64   a  in the width direction (x direction) and the gap of the air space g are set so that the amount of heat radiation from the temperature-sensitive magnetic member  64  toward the induction member  66  is balanced with the amount of heat (Joule heat) generated in the temperature-sensitive magnetic member  64 . 
     In this case, as shown in  FIG. 12C , at a position of the induction member  66 , which faces the heat radiation member  64   a , a heat induction member  66   a  formed into a convex shape projecting from the outer circumferential surface of the induction member  66  may be provided. The heat induction member  66   a  is also arranged over the entire region of the induction member  66  in the longitudinal direction (z direction). By arranging the heat induction member  66   a  on the induction member  66  side, the surface area on the induction member  66  side, which faces the heat radiation member  64   a , increases, and thus heat radiated from the heat radiation member  64   a  and transferred to the air space g is easily absorbed on the induction member  66  side. Accordingly, the heat from the temperature-sensitive magnetic member  64  to the induction member  66  through the air space g more smoothly flows, and heat, which corresponds to the increase in temperature exceeding the temperature of the fixing belt  61 , is promptly transferred from the temperature-sensitive magnetic member  64 . 
     Note that, the heat induction member  66   a  may be integrally formed with the induction member  66 , or independently formed. 
     Incidentally, on the inner circumferential surface side of the temperature-sensitive magnetic member  64 , the holder  65  having a large heat capacity is also arranged. Thus, even if the amount of heat from the temperature-sensitive magnetic member  64 , which corresponds to self-heating of the temperature-sensitive magnetic member  64 , is transferred to the induction member  66 , the heat of the induction member  66  is further transferred to the holder  65  having the large heat capacity. Therefore, the temperature of the induction member  66  hardly changes. Accordingly, heat flows stably from the heat radiation member  64   a  to the induction member  66 . 
     Second Exemplary Embodiment 
     A description will be given of the second exemplary embodiment of the heat radiation path for radiating heat generated in the temperature-sensitive magnetic member  64  toward the inner direction of the temperature-sensitive magnetic member  64 . 
       FIGS. 13A to 13C  are diagrams for explaining the heat radiation path in the second exemplary embodiment.  FIG. 13A  is a perspective view in a state where the temperature-sensitive magnetic member  64  and the induction member  66  are arranged on the holder  65 ,  FIG. 13B  is a cross sectional view of an x-y plane at a coordinate point z 1  in the z axis direction in  FIG. 13A , and  FIG. 13C  is a view showing a modified example of the heat radiation path in the second exemplary embodiment. 
     As shown in  FIGS. 13A and 13B , on the outer circumferential surface of the induction member  66 , a heat induction member  66   b  forming as a part of the induction member  66  is arranged toward the temperature-sensitive magnetic member  64 . Here, the induction member  66  is made of a non-magnetic metal such as Ag, Cu or Al. 
     The heat induction member  66   b  is formed into a convex shape projecting from the outer circumferential surface of the induction member  66 , and is arranged over the entire region of the induction member  66  in the longitudinal direction (z direction), as shown in  FIG. 13A . Additionally, as shown in  FIG. 13B , the heat induction member  66   b  is configured so as not to be in contact with the temperature-sensitive magnetic member  64 , and air space g is interposed between the heat induction member  66   b  and the temperature-sensitive magnetic member  64 . 
     As described above, since the heat induction member  66   b  formed into the convex shape and the temperature-sensitive magnetic member  64  are close to each other, heat of the temperature-sensitive magnetic member  64  easily flows from the surface of the temperature-sensitive magnetic member  64  toward the heat induction member  66   b . On the other hand, the air space g having an extremely small heat transfer rate is interposed therebetween, and thereby the heat of the temperature-sensitive magnetic member  64  is difficult to be transferred to the heat induction member  66   b.    
     In this respect, the length of the heat induction member  66   b  in the width direction (x direction) and a gap of the air space g are set so as to correspond to the configuration of the fixing unit  60 , and thereby a heat radiation path that causes the temperature-sensitive magnetic member  64  to radiate the amount of heat corresponding to increase in temperature exceeding the temperature of the fixing belt  61  is formed in a case where heat is accumulated in the temperature-sensitive magnetic member  64  such as a case where a large amount of image formation is successively performed. 
     In other words, the length of the heat radiation member  66   b  in the width direction (x direction) and the gap of the air space g are set so that the amount of heat radiation from the temperature-sensitive magnetic member  64  toward the induction member  66  is balanced with the amount of heat (Joule heat) generated in the temperature-sensitive magnetic member  64 . 
     In this case, similarly to the aforementioned heat radiation path in the first exemplary embodiment, a heat radiation member  64   b  may be arranged at a position of the inner circumferential surface of the temperature-sensitive magnetic member  64 , which faces the heat induction member  66   b , as shown in  FIG. 13C . Here, the heat radiation member  64   b  is made of a metal material, a resin material having metallic particles dispersed therein or the like, which has a heat transfer property. 
     Third Exemplary Embodiment 
     A description will be given of the third exemplary embodiment of the heat radiation path for radiating heat generated in the temperature-sensitive magnetic member  64  toward the inner direction of the temperature-sensitive magnetic member  64 . 
       FIGS. 14A to 14C  are diagrams for explaining the heat radiation path in the third exemplary embodiment.  FIG. 14A  is a perspective view in a state where the temperature-sensitive magnetic member  64  and the induction member  66  are arranged on the holder  65 ,  FIG. 14B  is a cross sectional view of an x-y plane at a coordinate point z 1  in the z axis direction in  FIG. 14A , and  FIG. 14C  is a view showing a modified example of the heat radiation path in the third exemplary embodiment. 
     As shown in  FIGS. 14A and 14B , multiple heat radiation fins  64   c  are arranged on the inner circumferential surface of the temperature-sensitive magnetic member  64  toward the induction member  66 . Here, the heat radiation fins  64   c  are formed of, for example, a metal material, a resin material having metallic particles dispersed therein, or the like, which has a heat transfer property. 
     The heat radiation fins  64   c  are each formed as a board projecting from the inner circumferential surface of the temperature-sensitive magnetic member  64 , and, as shown in  FIG. 14A , the heat radiation fins  64   c  are arranged over the entire region of the temperature-sensitive member  64  in the longitudinal direction (z direction). Further, the multiple heat radiation fins  64   c  (for example, five heat radiation fins  64   c ) are arranged in the width direction (x direction) of the temperature-sensitive magnetic member  64 . Furthermore, as shown in  FIG. 14B , each of the heat radiation fins  64   c  is formed so as not to be in contact with the induction member  66 , and air space g is interposed between each of the heat radiation fins  64   c  and the induction member  66 . Note that, the heat radiation fins  64   c  may be integrally formed with the temperature-sensitive magnetic member  64  or independently formed. 
     As described above, since the heat radiation fins  64   c  each formed as the board and the induction member  66  are close to each other, the heat of the temperature-sensitive magnetic member  64  easily flows from the heat radiation fins  64   c  to the induction member  66 . On the other hand, since the air space g having an extremely small heat transfer rate is interposed therebetween, the heat of the temperature-sensitive magnetic member  64  is not easily transferred to the induction member  66 . 
     In this respect, the number of the heat radiation fins  64   c , an interval between the adjacent two heat radiation fins  64   c  and a gap of the air space g are set so as to correspond to the configuration of the fixing unit  60 , and thereby the heat radiation path that causes the temperature-sensitive magnetic member  64  to radiate the amount of heat corresponding to the increase in temperature exceeding the temperature of the fixing belt  61  is formed in a case where heat is accumulated in the temperature-sensitive magnetic member  64  such as a case where a large amount of image formation is successively performed. 
     In other words, the number of the heat radiation fins  64   c , the interval between the adjacent two heat radiation fins  64   c  and the gap of the air space g are set so that the amount of heat radiation from the temperature-sensitive magnetic member  64  toward the induction member  66  is balanced with the amount of heat (Joule heat) generated in the temperature-sensitive magnetic member  64 . 
     As described above, by providing the heat radiation fins  64   c , an airflow in the longitudinal direction (z direction) of the temperature-sensitive magnetic member  64  is formed on the inner side of the temperature-sensitive magnetic member  64 , in addition to the heat radiation from the temperature-sensitive magnetic member  64  to the induction member  66 . Thereby, this configuration also functions so that the temperature distribution in the longitudinal direction (z direction) of the temperature-sensitive magnetic member  64  becomes uniform. 
     In this case, similarly to the aforementioned heat radiation path in the first exemplary embodiment, multiple heat induction fins  66   c  each formed as a board and formed as a part of the induction member  66  may be arranged on the outer surface of the induction member  66  so as to alternately arranged with the heat radiation fins  64   c  provided to the temperature-sensitive magnetic member  64 , as shown in  FIG. 14C . Here, the induction member  66  is made of a non-magnetic metal such as Ag, Cu or Al. 
     Fourth Exemplary Embodiment 
     A description will be given of the fourth exemplary embodiment of the heat radiation path for radiating heat generated in the temperature-sensitive magnetic member  64  toward the inner direction of the temperature-sensitive magnetic member  64 . 
       FIGS. 15A to 15C  are diagrams for explaining the heat radiation path in the fourth exemplary embodiment.  FIG. 15A  is a perspective view in a state where the temperature-sensitive magnetic member  64  and the induction member  66  are arranged on the holder  65 ,  FIG. 15B  is a cross sectional view of an x-y plane at a coordinate point A in the z axis direction in  FIG. 15A , and  FIG. 15C  is a cross sectional view of the x-y plane at each of coordinate points z 2  and z 3  in the z axis direction in  FIG. 15A . 
     As shown in  FIG. 15A , in the fourth exemplary embodiment, the aforementioned heat radiation path in the third exemplary embodiment is arranged on a part corresponding to, for example, a region (small size sheet passing region Fs) where a small size sheet P 1  having a smaller width than the maximum size sheet P shown in  FIG. 9  passes (for example, A 4  longitudinal feed) ( FIG. 15B ), and is not arranged on a part corresponding to the non-sheet passing regions Fb where the small size sheet P 1  does not pass ( FIG. 15C ). 
     Even in a case where any size sheet P is used in the fixing unit  60 , the small size sheet passing region Fs where the sheet P passes is a region having a high frequency of sequential sheet passage. Therefore, the small size sheet passing region Fs has a higher possibility that the temperature of the temperature-sensitive magnetic member  64  exceeds the permeability change start temperature whereas the temperature of the fixing belt  61  does not exceed the permeability change start temperature, than the other regions. Accordingly, the heat radiation path in the third exemplary embodiment is arranged on a part corresponding to the small size sheet passing region Fs in order to suppress increase in the temperature of the temperature-sensitive magnetic member  64  especially at the small size sheet passing region Fs. 
     Fifth Exemplary Embodiment 
     A description will be given of the fifth exemplary embodiment of the heat radiation path for radiating heat generated in the temperature-sensitive magnetic member  64  toward the inner direction of the temperature-sensitive magnetic member  64 . 
       FIGS. 16A and 16B  are diagrams for explaining the heat radiation path in the fifth exemplary embodiment.  FIG. 16A  is a perspective view in a state where the temperature-sensitive magnetic member  64  and the induction member  66  are arranged on the holder  65 , and  FIG. 16B  is a cross sectional view of an x-y plane at a coordinate point z 1  in the z axis direction in  FIG. 16A . 
     As shown in  FIGS. 16A and 16B , multiple heat radiation fins  66   d  are arranged on the induction member  66  arranged on the inner circumferential surface side of the temperature-sensitive magnetic member  64 , toward the temperature-sensitive magnetic member  64 . Here, the heat radiation fins  66   d  are made of, for example, a metal material, a resin material having metallic particles dispersed therein or the like, which has heat transfer property. 
     The heat radiation fins  66   d  are boards projecting from the outer circumferential surface of the induction member  66 , and are arranged over the induction member  66  in the longitudinal direction (z direction), as shown in  FIG. 16A . Moreover, the multiple heat radiation fins  66   d . (for example, five heat radiation fins  66   d ) are arranged in the width direction (x direction) of the induction member  66   d . In addition, as shown in  FIG. 16B , each of the heat radiation fins  66   d  is configured so as not to be in contact with the temperature-sensitive magnetic member  64 , and air space g is interposed between each of the heat radiation fins  66   d  and the temperature-sensitive magnetic member  64 . Note that, the heat radiation fins  66   d  may be integrally formed with the induction member  66 , or may be independently formed. 
     As described above, since each of the heat radiation fins  66   d  formed as the board and the temperature-sensitive magnetic member  64  are close to each other, heat of the temperature-sensitive magnetic member  64  easily flows toward the induction member  66  via the heat radiation fins  66   d . On the other hand, since the air space g having the extremely small heat transfer rate is interposed therebetween, and thus the heat of the temperature-sensitive magnetic member  64  is not easily transferred to the induction member  66 . 
     In this respect, the number of the heat radiation fins  66   d , an interval between the adjacent two heat radiation fins  66   c   1 , and a gap of the air space g are set so as to correspond to the configuration of the fixing unit  60 , and thereby a heat radiation path that causes the temperature-sensitive magnetic member  64  to radiate the amount of heat corresponding to increase in temperature exceeding the temperature of the fixing belt  61  is formed in a case where heat is accumulated in the temperature-sensitive magnetic member  64  such as a case where a large amount of image formation is successively performed. 
     In other words, the number of the heat radiation fins  66   d , the interval between the adjacent two heat radiation fins  66   d , and the gap of the air space g are set so that the amount of heat radiation from the temperature-sensitive magnetic member  64  toward the induction member  66  is balanced with the amount of heat (Joule heat) generated in the temperature-sensitive magnetic member  64 . 
     As described above, by providing the heat radiation fins  66   d  to the induction member  66 , airflow in the longitudinal direction (z direction) of the temperature-sensitive magnetic member  64  is formed on the inner side of the temperature-sensitive magnetic member  64 , in addition to the heat radiation from the temperature-sensitive magnetic member  64  toward the induction member  66 . Thereby, the heat radiation fins  66   d  also functions so that the temperature distribution in the longitudinal direction (z direction) of the temperature-sensitive magnetic member  64  becomes uniform. 
     Sixth Exemplary Embodiment 
     A description will be given of the sixth exemplary embodiment of the heat radiation path for radiating heat generated in the temperature-sensitive magnetic member  64  toward the inner direction of the temperature-sensitive magnetic member  64 . 
       FIGS. 17A to 17C  are diagrams for explaining the heat radiation path in the sixth exemplary embodiment.  FIG. 17A  is a perspective view in a state where the temperature-sensitive magnetic member  64  and the induction member  66  are arranged on the holder  65 ,  FIG. 17B  is a cross sectional view of an x-y plane at a coordinate point z 1  in the z axis direction in  FIG. 17A , and  FIG. 17C  is a cross sectional view of the x-y plane at each of coordinate points z 2  and z 3  in the z axis direction in  FIG. 17A . 
     As shown in  FIG. 17A , in the sixth exemplary embodiment, the aforementioned heat radiation path in the fifth exemplary embodiment is arranged on a part corresponding to, for example, a region (small size sheet passing region Fs) where a small size sheet P 1  having a smaller width than the maximum size sheet P shown in  FIG. 9  passes (for example, A4 longitudinal feed) ( FIG. 17B ), and is not arranged on a part corresponding to the non-sheet passing regions Fb where the small size sheet P 1  does not pass ( FIG. 17C ). 
     Even in a case where any size sheet P is used in the fixing unit  60 , the small size sheet passing region Fs where the sheet P passes is a region having a high frequency of sequential sheet passage. Therefore, the small size sheet passing region Fs has a higher possibility that the temperature of the temperature-sensitive magnetic member  64  exceeds the permeability change start temperature whereas the temperature of the fixing belt  61  does not exceed the permeability change start temperature, than the other regions. Accordingly, the heat radiation path in the fifth exemplary embodiment is arranged on a part corresponding to the small size sheet passing region Fs in order to suppress increase in the temperature of the temperature-sensitive magnetic member  64  especially at the small size sheet passing region Fs. 
     As described above, in the fixing unit  60  provided to the image forming apparatus  1  in these exemplary embodiments, the temperature-sensitive magnetic member  64  is arranged so as to be close to the inner circumferential surface of the fixing belt  61 . Moreover, the heat radiation path for radiating heat generated in the temperature-sensitive magnetic member  64  in the inner direction of the temperature-sensitive magnetic member  64 . By this configuration, the temperature of the non-sheet passing region Fb is suppressed to excessively increase. In addition, the temperature of the temperature-sensitive magnetic member  64  is suppressed to exceed the permeability change start temperature in a state where the temperature of the fixing belt  61  does not exceed the permeability change start temperature, and a state where the fixing belt  61  is sufficiently heated up to the fixation setting temperature at the sheet passing region is kept. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.