Abstract:
An electromagnetic induced heating roller comprises a core member, an elastic layer, induction heating layer, and a mold release layer in this order from inside to outside. Further, a magnetic shield layer for preventing a magnetic flux from penetrating into the core member is interposed between the induction heating layer and the core member. The production of an alternating magnetic field from a magnetic field generating device, a leakage flux having penetrated through the induction heating layer is trapped by the magnetic shield layer. As a result, most of the impressed alternating magnetic flux is consumed for heating the heating layer, resulting in an improvement in the heating efficiency. A trouble is prevented which is generated by heating the bearing of the core member.

Description:
This application is a Continuation-in-Part of U.S. patent application Ser. No. 10/480,344 filed Dec. 9, 2003 now abandoned, which is a National Stage of PCT/JP02/11328 filed on Oct. 31, 2002. 

   TECHNICAL FIELD 
   The present invention relates to an electromagnetic induction heat generating roller that generates heat by electromagnetic induction heating for heating a sheet-like material to be heated by making continuous contact with the material to be heated. Further, this invention relates to a heating device for fixing a toner image on a recording material by heating in an image forming apparatus that forms an image using toner by an electrophotographic method or another similar method that is used for copiers, printers and the like. Moreover, this invention relates to an image forming apparatus including such a heating device as a fixing device. 
   BACKGROUND ART 
   Description will be made by using as an example a fixing device (heating device) in an image forming apparatus such as an electrophotographic copier, a printer or the like. A fixing device for use in image forming apparatuses is a device that permanently fixes an unfixed toner image on a surface of a recording material by heat. The unfixed toner image has been formed on the recording material using toner formed from a thermally meltable resin or the like. The fixing of the toner image is performed by an appropriate image formation processing method such as electrophotography, electrostatic recording or the like. 
   As a method used most commonly for such fixing devices, a roller fixing method has been known. In the roller fixing method, a recording material is introduced into a nip part formed by a heating roller that is heated and adjusted so that a predetermined fixing temperature is attained and a pressing roller that is opposed to and is in contact under pressure with the heating roller. At the nip part, the recording material is conveyed while being sandwiched between the rollers so that an unfixed toner image is fixed on a surface of the recording material by heating. As a heat source of a heating roller for use in the roller fixing method, a halogen lamp has been in frequent use. 
   Meanwhile, in recent years, in response to the demand for a reduction in power consumption and warm-up time, roller heating type devices employing an electromagnetic induction heating method have been proposed.  FIG. 11  shows an example of a conventional induction heating fixing device including a heat generating roller that is heated by electromagnetic induction (see, for example, JP11(1999)-288190 A). 
   In  FIG. 11 , reference numeral  820  denotes a heat generating roller including a core material  824  made of metal, an elastic layer  823  that is formed from a heat-resistant foam rubber and molded integrally on an outer side of the core material  824 , a heat generating layer  821  formed of a metallic tube, and a mold releasing layer  822  provided on an outer side of the heat generating layer  821 , which are provided outwardly in this order. Reference numeral  827  denotes a pressing roller that is formed from a heat-resistant resin and has the shape of a hollow cylinder. A ferrite core  826  wound with an excitation coil  825  is placed in an inner portion of the pressing roller  827 . The ferrite core  826  applies pressure to the heat generating roller  820  through the pressing roller  827 , and thus a nip part  829  is formed. While the heat generating roller  820  and the pressing roller  827  rotate in the respective directions indicated by arrows, a high-frequency current is fed through the excitation coil  825 . This causes alternating magnetic fields H to be generated, so that the heat generating layer  821  of the heat generating roller  820  is heated rapidly by electromagnetic induction to a predetermined temperature. While predetermined heating is continued in this state, a recording material  840  is inserted into and passed through the nip part  829 . Thus, toner images  842  formed on the recording material  840  are fixed on the recording material  840 . 
   Furthermore, in addition to devices of the above-mentioned roller heating type using the heat generating roller  820  having the induction heat generating layer  821  as shown in  FIG. 11 , devices of a belt heating type using an endless belt including an induction heat generating layer have been proposed.  FIG. 12  shows an example of a conventional induction heating fixing device using an endless belt that is heated by electromagnetic induction (see, for example, JP10(1998)-74007 A). 
   In  FIG. 12 , reference numeral  960  denotes a coil assembly as an excitation unit that generates a high-frequency magnetic field. Reference numeral  910  denotes a metal sleeve (heat generating belt) that generates heat under a high-frequency magnetic field generated by the coil assembly  960 . The metal sleeve  910  is formed by coating a surface of an endless tube made from a thin layer of nickel or stainless steel with a fluorocarbon resin. An inner pressing roller  920  is inserted in an inner portion of the metal sleeve  910 , and an outer pressing roller  930  is placed outside the metal sleeve  910 . The outer pressing roller  930  is pressed against the inner pressing roller  920  such that the metal sleeve  910  is interposed between them, and thus a nip part  950  is formed. While the metal sleeve  910 , the inner pressing roller  920 , and the outer pressing roller  930  rotate in the respective directions indicated by arrows, a high-frequency current is fed through the coil assembly  960 . Thus, the metal sleeve  910  is heated rapidly by electromagnetic induction to a predetermined temperature. While predetermined heating is continued in this state, a recording material  940  is inserted into and passed through the nip part  950 . Thus, a toner image formed on the recording material  940  is fixed on the recording material  940 . 
   In the above-mentioned conventional induction heating fixing device of the roller heating type shown in  FIG. 11 , the following problems have been presented. That is, in the case of using a metallic material such as iron, aluminum, a stainless material or the like that is in common use for the core material  824  of the heat generating roller  820 , the core material  824  itself generates heat by induction heating due to passing of the alternating magnetic fields H, resulting in a loss of power. Further, the heat generation of the core material  824  leads to the occurrence of problems such as, for example, damage to bearings supporting the core material  824  caused due to a high temperature. 
   Similarly, in the conventional induction heating fixing device of the belt heating type shown in  FIG. 12 , the following problems have been presented. That is, in the case where the inner pressing roller  920  is formed of a metallic material such as iron, aluminum, a stainless material or the like, a high-frequency magnetic field generated by the coil assembly  960  reaches the inner pressing roller  920 , so that the inner pressing roller  920  generates heat, resulting in a loss of power. Further, the heat generation of the inner pressing roller  920  leads to the occurrence of problems such as, for example, damage to bearings supporting the inner pressing roller  920  caused due to a high temperature. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the problems with the conventional technique and to provide a heat generating roller for use in an induction heating method that achieves an improvement in heat generation efficiency and allows damage to bearings or the like to be prevented, a heating device using the heat generating roller, and a heating device of the belt heating type also utilizing electromagnetic induction. Further, another object of the present invention is to provide an image forming apparatus that achieves excellent energy efficiency and allows problems occurring in bearings to be reduced. 
   An electromagnetic induction heat generating roller according to the present invention includes a core material, an elastic layer, an induction heat generating layer, and a mold releasing layer, which are provided outwardly in this order. In the electromagnetic induction heat generating roller, a magnetism shielding layer that prevents entry of magnetic flux into the core material is provided between the induction heat generating layer and the core material. 
   Furthermore, a first heating device according to the present invention includes the above-mentioned electromagnetic induction heat generating roller according to the present invention, a pressing roller that is in contact under pressure with the electromagnetic induction heat generating roller so as to form a nip part, and a magnetic field generating unit that applies a magnetic field to the induction heat generating layer of the electromagnetic induction heat generating roller so that the induction heat generating layer generates heat by induction. In the first heating device, a material to be heated introduced into the nip part is conveyed under pressure by the electromagnetic induction heat generating roller and the pressing roller so as to be heated continuously. 
   Furthermore, a second heating device according to the present invention includes an electromagnetic induction heat generating belt having an induction heat generating layer, a supporting roller that is composed of a core material and a heat insulating layer provided on an outer side of the core material and makes contact internally with the electromagnetic induction heat generating belt so that the electromagnetic induction heat generating belt is supported rotatably, a pressing roller that makes contact externally with the electromagnetic induction heat generating belt so that a nip part is formed between the pressing roller and the electromagnetic induction heat generating belt, and a magnetic field generating unit that is disposed outside the electromagnetic induction heat generating belt and applies a magnetic field to the induction heat generating layer so that the induction heat generating layer generates heat by induction. In the second heating device, a material to be heated introduced into the nip part is conveyed under pressure by the electromagnetic induction heat generating belt and the pressing roller so as to be heated continuously. Further, in the supporting roller, a magnetism shielding layer that prevents entry of magnetic flux into the core material is provided on an outer side of the core material. 
   Moreover, an image forming apparatus according to the present invention includes an image forming unit that forms a toner image on a recording material and the above-mentioned first or second heating device according to the present invention. In the image forming apparatus, the toner image to be fixed formed on the recording material by the image forming unit is fixed on the recording material by the heating device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of a heating device according to Embodiment 1 of the present invention. 
       FIG. 2  is a structural view of a magnetic field generating unit as seen from a direction indicated by an arrow II of  FIG. 1 . 
       FIG. 3  is a cross sectional view taken on line III—III of  FIG. 2  for showing the heating device according to Embodiment 1 of the present invention. 
       FIG. 4A  is a cross sectional view of a heat generating roller according to Embodiment 1 of the present invention that is used in a fixing device shown in  FIG. 1 .  FIG. 4B  is an expanded sectional view of a portion  4 B shown in  FIG. 4A . 
       FIG. 5  is a cross sectional view schematically showing a configuration of an image forming apparatus according to an embodiment of the present invention. 
       FIG. 6A  is a cross sectional view of a heat generating roller according to Embodiment 2 of the present invention that is used in the fixing device shown in  FIG. 1 .  FIG. 6B  is an expanded sectional view of a portion  6 B shown in  FIG. 6A . 
       FIG. 7A  is a schematic perspective view of a core material of an electromagnetic induction heat generating roller according to Embodiment 3 of the present invention that includes a magnetism shielding layer.  FIG. 7B  is a schematic perspective view of a ring forming the magnetism shielding layer of the electromagnetic induction heat generating roller shown in  FIG. 7A .  FIG. 7C  is a schematic perspective view of an arc-shaped member forming the magnetism shielding layer of the electromagnetic induction heat generating roller shown in  FIG. 7A . 
       FIG. 8  is a cross sectional view schematically showing a configuration of an image forming apparatus according to another embodiment of the present invention. 
       FIG. 9  is a cross sectional view of a heating device according to Embodiment 5 of the present invention. 
       FIG. 10  is a cross sectional view of a heating device according to Embodiment 6 of the present invention. 
       FIG. 11  is a cross sectional view schematically showing a configuration of a conventional induction heating fixing device including a heat generating roller that is heated by electromagnetic induction. 
       FIG. 12  is a cross sectional view schematically showing a configuration of a conventional induction heating fixing device including a heat generating belt that is heated by electromagnetic induction. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An electromagnetic induction heat generating roller according to the present invention includes a core material and an induction heat generating layer that is provided on an outer side of the core material. In the electromagnetic induction heat generating roller, a magnetism shielding layer that prevents entry of magnetic flux into the core material is provided between the induction heat generating layer and the core material. 
   Accordingly, the magnetism shielding layer prevents leakage magnetic flux from reaching the core material, which is magnetic flux that has penetrated the induction heat generating layer from the exterior, and thus the heat generation of the core material is suppressed. As a result, a loss of supplied energy is reduced, and the heat generation efficiency of the induction heat generating layer is increased. Further, it is possible to prevent the occurrence of problems such as, for example, damage to bearings supporting the core material caused due to heating of the bearings to a high temperature. 
   Preferably, the magnetism shielding layer has a specific resistance of 10 −3  Ωcm or higher. 
   According to this preferred embodiment, the generation of an eddy current in the magnetism shielding layer can be prevented, and thus the heat generation of the magnetism shielding layer is suppressed. As a result, a loss of supplied energy is reduced, and the heat generation efficiency of the induction heat generating layer is increased. 
   Preferably, the magnetism shielding layer has a relative magnetic permeability of 10 or higher. 
   According to this preferred embodiment, it is possible to prevent magnetic flux from penetrating the magnetism shielding layer and then reaching the core material, and thus the generation of heat in the core material further can be suppressed. 
   Preferably, the magnetism shielding layer has a thickness of not less than 0.2 mm. 
   According to this preferred embodiment, it is possible to prevent magnetic flux from penetrating the magnetism shielding layer and then reaching the core material, and thus the generation of heat in the core material further can be suppressed. 
   Preferably, the magnetism shielding layer is a layer of an insulating magnetic material formed on a surface of the core material. 
   According to this preferred embodiment, a material of the magnetism shielding layer has an insulating property, and thus the generation of an eddy current in the magnetism shielding layer can be prevented. Thus, the generation of heat in the magnetism shielding layer is suppressed. Further, the material of the magnetism shielding layer has magnetism, and thus it is possible to prevent magnetic flux from penetrating the magnetism shielding layer and then reaching the core material. Thus, the generation of heat in the core material further can be prevented. 
   Preferably, the magnetism shielding layer is composed of a plurality of rings or arc-shaped members that are arranged in rows on a surface of the core material. 
   According to this preferred embodiment, it is made easier to form the magnetism shielding layer. 
   The magnetism shielding layer may be the elastic layer in which a magnetic filler is dispersed. 
   According to this preferred embodiment, the elastic layer functions also as a magnetism shielding layer, and thus a simplified layer configuration can be achieved, and the manufacturing of the electromagnetic induction heat generating roller is made easier, thereby contributing to a cost reduction. 
   Preferably, the core material is formed from a non-magnetic metal. 
   According to this preferred embodiment, it is possible to prevent magnetic flux from penetrating the magnetism shielding layer and then entering the core material, and thus the heat generation of the core material further can be suppressed. Further, it is made easier to secure the strength of the core material. 
   Preferably, the induction heat generating layer has a thickness not larger than a skin depth. 
   According to this preferred embodiment, the induction heat generating layer has a small thermal capacity and high pliability, and thus an electromagnetic induction heat generating roller that achieves a reduction in warm-up time and has an excellent fixing property can be obtained. 
   Next, a first heating device according to the present invention is a heating device of the roller heating type and includes the above-mentioned electromagnetic induction heat generating roller according to the present invention, a pressing roller that is in contact under pressure with the electromagnetic induction heat generating roller so as to form a nip part, and a magnetic field generating unit that applies a magnetic field to the induction heat generating layer of the electromagnetic induction heat generating roller so that the induction heat generating layer generates heat by induction. 
   The above-mentioned first heating device includes the electromagnetic induction heat generating roller according to the present invention. Accordingly, the magnetism shielding layer prevents leakage magnetic flux from reaching the core material, which is magnetic flux that has penetrated the induction heat generating layer from the magnetic field generating unit, and thus the heat generation of the core material is suppressed. As a result, a loss of supplied energy is reduced, and the heat generation efficiency of the induction heat generating layer is increased. Further, it is made possible to prevent the occurrence of problems such as, for example, damage to bearings supporting the core material caused due to heating of the bearings to a high temperature. 
   Furthermore, a second heating device according to the present invention is a heating device of the belt heating type and includes an electromagnetic induction heat generating belt having an induction heat generating layer, a supporting roller that is composed of a core material and a heat insulating layer provided on an outer side of the core material and makes contact internally with the electromagnetic induction heat generating belt so that the electromagnetic induction heat generating belt is supported rotatably, a pressing roller that makes contact externally with the electromagnetic induction heat generating belt so that a nip part is formed between the pressing roller and the electromagnetic induction heat generating belt, and a magnetic field generating unit that is disposed outside the electromagnetic induction heat generating belt and applies a magnetic field to the induction heat generating layer so that the induction heat generating layer generates heat by induction. Further, in the supporting roller, a magnetism shielding layer that prevents entry of magnetic flux into the core material is provided on an outer side of the core material. 
   Accordingly, the magnetism shielding layer prevents leakage magnetic flux from reaching the core material, which is magnetic flux that has penetrated the induction heat generating layer from the magnetic field generating unit and then reached the supporting roller, and thus the heat generation of the core material is suppressed. As a result, a loss of supplied energy is reduced, and the heat generation efficiency of the induction heat generating layer is increased. Further, it is made possible to prevent the occurrence of problems such as, for example, damage to bearings supporting the core material caused due to heating of the bearings to a high temperature. 
   Preferably, in the above-mentioned second heating device, the magnetism shielding layer is formed on a surface of the supporting roller. This allows the realization of a simplified layer configuration and a cost reduction of the supporting roller. 
   Next, an image forming apparatus according to the present invention includes an image forming unit that forms a toner image on a recording material and the above-mentioned first or second heating device according to the present invention. 
   Thus, an image forming apparatus can be obtained that achieves a reduction in power consumption and allows problems occurring in bearings can be suppressed. 
   Hereinafter, the present invention will be described in further detail with reference to appended drawings. 
   Embodiment 1 
     FIG. 5  is a cross sectional view of an image forming apparatus using a heating device according to an embodiment of the present invention as a fixing device. The heating device according to this embodiment is an electromagnetic induction heating device of the roller heating type. The following description is directed to the configuration and operation of this device. 
   Reference numeral  1  denotes an electrophotographic photoreceptor (hereinafter, referred to as a “photosensitive drum”). The photosensitive drum  1 , while being driven to rotate at a predetermined peripheral velocity in a direction indicated by an arrow, has its surface charged uniformly to a predetermined potential by a charger  2 . Reference numeral  3  denotes a laser beam scanner that outputs a laser beam modulated in accordance with a time-series electric digital pixel signal of image information input from a host device such as an image reading apparatus, a computer or the like, which is not shown in the figure. The surface of the photosensitive drum  1  charged uniformly as described above is scanned by and exposed to this laser beam selectively. This allows a static latent image corresponding to the image information to be formed on the surface of the photosensitive drum  1 . Next, the static latent image is supplied with powdered toner charged by a developer  4  having a developing roller  4   a  that is driven to rotate, and made manifest as a toner image. 
   Meanwhile, a recording material  11  is fed one at a time from a paper feeding part  10  and passed between a pair of resist rollers  12  and  13 . Then, the recording material  11  is conveyed to a nip part composed of the photosensitive drum  1  and a transferring roller  14  that is in contact with the photosensitive drum  1 , and the timing thereof is appropriate and synchronized with the rotation of the photosensitive drum  1 . By the action of the transferring roller  14  to which a transfer bias is applied, toner images on the photosensitive drum  1  are transferred one after another to the recording material  11 . The recording material  11  that has been passed through the nip part (transferring part) is released from the photosensitive drum  1  and introduced to a fixing device  15  where fixing of the transferred toner image is performed. The recording material  11  on which the image is fixed by the fixing process is output to a paper ejecting tray  16 . The surface of the photosensitive drum  1  from which the recording material has been released is cleaned by removing residual materials such as toner remaining after the transferring process by a cleaning device  17  and used repeatedly for successive image formation. 
   Next, the embodiment of the heating device according to the present invention that can be used as the above-mentioned fixing device  15  will be described in detail by way of an example. 
     FIG. 1  is a cross sectional view of a fixing device as the heating device according to Embodiment 1 of the present invention that is used in the above-mentioned image forming apparatus.  FIG. 2  is a structural view of a magnetic field generating unit as seen from a direction indicated by an arrow II of  FIG. 1 .  FIG. 3  is a perspective sectional view taken on line III—III (a plane including a rotation center axis of a heat generating roller  21  and a winding center axis  36   a  of an excitation coil  36 ) of  FIG. 2 .  FIG. 4A  is a sectional structural view of the heat generating roller  21  according to the present invention that is used in the fixing device shown in  FIG. 1 .  FIG. 4B  is an expanded sectional view of a portion  4 B shown in  FIG. 4A . The following description is directed to the fixing device and the heat generating roller according to this embodiment with reference to  FIGS. 1 to 4B . 
   In  FIGS. 4A and 4B , the heat generating roller  21  is composed of a mold releasing layer  27 , a thin elastic layer (second elastic layer)  26 , an induction heat generating layer (hereinafter, referred to simply as “heat generating layer”)  22  that is formed of a thin conductive material, an elastic layer  23  that has an excellent heat insulating property, a magnetic body layer  19  as a magnetism shielding layer, and a core material  24  that is to function as a rotary shaft, which are provided in this order from a surface side. 
     FIG. 3  is a perspective sectional view taken on line III—III of  FIG. 2  and shows a configuration of the whole fixing device as seen in cross section from a lateral direction. The heat generating roller  21  has an outer diameter of 30 mm and is supported rotatably by side plates  29 ,  29 ′ via bearings  28 ,  28 ′ at both ends of the core material  24  that is the lowest layer thereof. The heat generating roller  21  is driven to rotate by a driving unit of a main body of the apparatus, which is not shown in the figure, through a gear  30  fixed integrally to the core material  24 . 
   Reference numeral  36  denotes the excitation coil as the magnetic field generating unit. The excitation coil  36  is disposed so as to be opposed to a cylindrical face on an outer periphery of the heat generating roller  21 . Further, the excitation coil  36  includes nine turns of a wire bundle composed of 60 wires of a copper wire with its surface insulated, which has an outer diameter of 0.15 mm. 
   The wire bundle of the excitation coil  36  is arranged, at end portions of the cylindrical face of the heat generating roller  21  in a direction of the rotation center axis (not shown in the figure), in the form of an arc along outer peripheral faces of the end portions. The wire bundle is arranged, in a portion other than the end portions, along a direction of a generatrix of the cylindrical face. Further, as shown in  FIG. 1 , which is a cross section orthogonal to the rotation center axis of the heat generating roller  21 , the wire bundle of the excitation coil  36  is arranged tightly without being overlapped (except at the end portions of the heat generating roller) on an assumed cylindrical face formed around the rotation center axis of the heat generating roller  21  so as to cover the cylindrical face of the heat generating roller  21 . Further, as shown in  FIG. 3 , which is a cross section including the rotation center axis of the heat generating roller  21 , in portions opposed to the end portions of the heat generating roller  21 , the wire bundle of the excitation coil  36  is overlapped in two rows and thus forced into bulges. Thus, the whole excitation coil  36  is formed into a saddle-like shape. The winding center axis  36   a  of the excitation coil  36  is a straight line substantially orthogonal to the rotation center axis of the heat generating roller  21 , which passes through substantially a center point of the heat generating roller  21  in the direction of the rotation center axis. The excitation coil  36  is formed so as to be substantially symmetrical with respect to the winding center axis  36   a . The wire bundle is wound so that adjacent turns of the wire bundle are bonded to each other with an adhesive applied to their surface, thereby maintaining a shape shown in the figure. The excitation coil  36  is opposed to the heat generating roller  21  at a distance of about 2 mm from the outer peripheral face of the heat generating roller  21 . In the cross section shown in  FIG. 1 , the excitation coil  36  is opposed to the outer peripheral face of the heat generating roller  21  in a large area defined by an angle of about 180 degrees with respect to the rotation center axis of the heat generating roller  21 . 
   Reference numeral  37  denotes a rear core, which together with the excitation coil  36 , constitutes the magnetic field generating unit. The rear core  37  is composed of a bar-like central core  38  and a substantially U-shaped core  39 . The central core  38  passes through the winding center axis  36   a  of the excitation coil  36  and is arranged parallel to the rotation center axis of the heat generating roller  21 . The U-shaped core  39  is arranged at a distance from the excitation coil  36  on a side opposite to that of the heat generating roller  21  with respect to the excitation coil  36 . The central core  38  and the U-shaped core  39  are connected magnetically. As shown in  FIG. 1 , the U-shaped core  39  is of a U shape substantially symmetrical with respect to a plane including the rotation center axis of the heat generating roller  21  and the winding center axis  36   a  of the excitation coil  36 . As shown in  FIGS. 2 and 3 , a plurality of the U-shaped cores  39  described above are arranged at a distance from each other in the direction of the rotation center axis of the heat generating roller  21 . In this example, the width of the U-shaped core  39  in the direction of the rotation center axis of the heat generating roller  21  is 10 mm, and seven such U-shaped cores  39  in total are spaced at a distance of 26 mm from each other. The U-shaped cores  39  capture magnetic flux from the excitation coil  36 , which leaks to the exterior. 
   As shown in  FIG. 1 , both ends of each of the U-shaped cores  39  are extended to areas that are not opposed to the excitation coil  36 , so that opposing portions F are formed, which are opposed to the heat generating roller  21  without the excitation coil  36  interposed between them. Further, the central core  38  is opposed to the heat generating roller  21  without the excitation coil  36  interposed between them and protrudes further than the U-shaped core  39  to a side of the heat generating roller  21  to form an opposing portion N. The opposing portion N of the protruding central core  38  is inserted into a hollow portion of a winding center of the excitation coil  36 . The central core  38  has a cross-sectional area of 4 mm by 10 mm. 
   In this example, the rear core  37  was formed from ferrite. As a material of the rear core  37 , it is desirable to use a material having high magnetic permeability and a high specific resistance such as ferrite, Permalloy or the like. However, a material having somewhat low magnetic permeability can be used as long as the material is a magnetic material. 
   Reference numeral  40  denotes a heat insulating member that has a thickness of 1 mm and is formed from a resin having high heat resistance such as PEEK (polyether ether ketones), PPS (polyphenylene sulfide) or the like. 
   In  FIG. 1 , a pressing roller  31  as a pressing member is formed of a metal shaft  32  whose surface is coated with an elastic layer  33  of a silicone rubber. The elastic layer has a hardness of 50 degrees (JIS-A). The pressing roller  31  is in contact under pressure with the heat generating roller  21  with a force of about 200 N in total, and thus a nip part  34  is formed. The pressing roller  31  has an outer diameter of 30 mm and a length that is substantially the same as that of the heat generating roller  21 , while having an effective length slightly larger than the length of the heat generating layer  22 . 
   At the nip part  34 , the elastic layer  23  of the heat generating roller  21  is deformed by compression, and the heat generating layer  22  is pressed with substantially a uniform pressure in a width direction (the direction of the rotation center axis of the heat generating roller  21 ). The nip part  34  has a width W along a moving direction C of the recording material  11  of about 5.5 mm. Although an extremely large force is applied to the heat generating roller  21  and the heat generating layer  22  on a surface of the heat generating roller  21  is thin, the nip part  34  is formed such that the width W is substantially uniform in the direction of the rotation center axis. This can be achieved because the solid core material  24  bears the pressure through the elastic layer  23 , and thus distortion with respect to the rotation center axis is suppressed to a minimal amount. Moreover, at the nip part  34 , the heat generating layer  22  and the elastic layer  23  are deformed into the shape of a concave along an outer peripheral face of the pressing roller  31 . Therefore, when the recording material  11  comes out of the nip part  34  after passing therethrough, a traveling direction of the recording material  11  is on an increased angle with respect to an outer surface of the heat generating roller  21 , thereby achieving an excellent peeling property for the recording material  11 . 
   The pressing roller  31  in this state is supported rotatably by follower bearings  35 ,  35 ′ at both ends of the metal shaft  32 . As a material of the elastic layer  33  of the pressing roller  31 , as well as the above-mentioned silicone rubber, heat-resistant resin and heat-resistant rubber such as fluorocarbon rubber, fluorocarbon resin and the like may be used. Further, in order to obtain improved abrasion resistance and mold releasability, a surface of the pressing roller  31  may be coated with a single material or a combination of materials selected from resin and rubber such as PFA (tetrafluoroethylene-perfluoroalkylvinyl ether copolymer), PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene hexafluoropropylene copolymer) and the like. In order to prevent heat dissipation, it is desirable that the pressing roller  31  be formed of a material having low thermal conductivity. 
   In  FIG. 1 , reference numeral  41  denotes a temperature detecting sensor that is in contact with the surface of the heat generating roller  21  so as to detect the temperature of the surface of the heat generating roller  21  at a portion right before entering the nip part  34 , and feeds back a result of the detection to a controlling circuit that is not shown in the figure. During operation, this function is used to regulate an excitation power of an excitation circuit  42 , and thus a temperature of the surface of the heat generating roller  21  at a portion right before entering the nip part  34  is controlled so as to be 170 degrees centigrade. In this example, in order to achieve the object of reducing a warm-up time, the elastic layer  26  and the mold releasing layer  27  that are provided on an outer side of the heat generating layer  22  as well as the heat generating layer  22  are set so as to have an extremely small thermal capacity. 
   Using the above-mentioned configuration, while the heat generating roller  21  and the pressing roller  31  are rotated, a high-frequency current at 20 to 50 kHz is fed to the excitation coil  36  by the excitation circuit  42 . This causes alternating magnetic flux to flow via the central core  38  and the U-shaped cores  39  that surround the excitation coil  36  and the heat generating layer  22  of the heat generating roller  21  that is opposed to the excitation coil  36 . Due to this alternating magnetic flux, an eddy current is generated in the heat generating layer  22 , so that a surface temperature of the heat generating roller  21  begins to increase rapidly. The surface temperature of the heat generating roller  21  is detected by the temperature detecting sensor  41  and adjusted to a predetermined temperature of 170° C. Then, the recording material  11  carrying unfixed toner images  9  is inserted into the nip part  34  where the toner images  9  and the recording material  11  are heated successively, so that the toner images  9  are fixed on the recording material  11 . 
   Next, the configuration of the heat generating roller  21  will be described in detail. 
   In this example, the core material  24  is formed of a non-magnetic stainless material (SUS304) having a diameter of 20 mm. A surface of the core material  24  is coated with the insulating magnetic body layer  19  of about 500 μm thickness as a magnetism shielding layer. The magnetic body layer  19  is formed of a base material of a silicone rubber in which a ferrite powder is dispersed. The material of the core material  24  is not limited to a stainless material, and aluminum and the like also can be used. Further, a magnetic powder to be contained in the magnetic body layer  19  is not limited to a ferrite powder, and a Sendust powder and the like also can be used. 
   The elastic layer  23  is formed of a foam body of a silicone rubber having low thermal conductivity. In the example, the elastic layer  23  is set to have a thickness of 5 mm and a hardness of 45 degrees (ASKER-C). Although the material of the elastic layer  23  is not limited to a foamed silicone rubber, it is desirable to use a material having a hardness of 20 to 55 degrees (ASKER-C) so that the width W of the nip part  34  is secured with moderate elasticity and that heat diffusion from the heat generating layer  22  is reduced. Further, in the case of not using a foam body, it is desirable, in terms of heat resistance and pliability, to use a material of a silicone rubber having a hardness of not more than 50 degrees (JIS-A). 
   The heat generating layer  22  of this example is formed on the elastic layer  23  as a coating of 60 μm thickness formed of a base material of a silicone rubber in which scale-like pieces of nickel are dispersed. Alternating magnetic flux generated by the excitation coil  36  passes through the heat generating layer  22  by way of the nickel pieces in the heat generating layer  22 . This causes an eddy current to be generated in the nickel pieces, so that the heat generating layer  22  is heated rapidly. In this example, the base material of the heat generating layer  22  was formed from a silicone rubber. However, in place of a silicone rubber, heat-resistant resin or heat-resistant rubber that has pliability such as polyimide resin, fluorocarbon resin, fluorocarbon rubber or the like also can be used. Further, a filler to be dispersed in the base material is not limited to the above-mentioned nickel pieces, and a magnetic metal powder and a non-magnetic metal powder also may be used in the form of a mixture or a laminate of these powders so as to be dispersed in the base material. Particles of such powders may have any of the shapes of a fiber, a sphere, a scale and the like. Needless to say, a filler to be dispersed is required only to be formed of a conductive material through which an eddy current flows due to alternating magnetic flux. In this example, however, particularly, a magnetic metal of nickel was used as a filler. Thus, heating can be performed efficiently because: alternating magnetic flux generated by the excitation coil  36  can be led into the heat generating layer  22 ; a magnetic resistance of a magnetic circuit formed by a magnetic flux flow around the excitation coil  36  can be reduced; and magnetic flux (leakage magnetic flux) penetrating the heat generating layer  22  and then leaking to another layer can be decreased. It is preferable that the heat generating layer  22  has a thickness of 10 to 200 μm. 
   The elastic layer (second elastic layer)  26  is provided so as to improve adhesion to the recording material  11 . In this example, the elastic layer  26  is formed of a silicone rubber layer having a thickness of 200 μm and a hardness of 20 degrees (JIS-A). The thickness of the elastic layer  26  is not limited to 200 μm, and it is desirable to set the thickness to be in a range of 50 to 500 μm. In the case where the thickness of the elastic layer  26  is too large, due to the thermal capacity that is too large, a longer warm-up time is required. In the case where the thickness of the elastic layer  26  is too small, the effect of providing adhesion to the recording material  11  is deteriorated. The material of the elastic layer  26  is not limited to a silicone rubber, and other types of heat-resistant rubber and hear-resistant resin also can be used. Although the elastic layer  26  is not necessarily provided and a configuration without the elastic layer  26  poses no problem, it is desirable to provide the elastic layer  26  in the case of obtaining a toner image as a color image. 
   The mold releasing layer  27  can be formed from a fluorocarbon resin such as PTFE (polytetrafluoroethylene), PFA (tetrafluoroethylene-perfluoroalkylvinyl ether copolymer), FEP (tetrafluoroethylene hexafluoropropylene copolymer) or the like. In this example, the mold releasing layer  27  was set to have a thickness of 30 μm. 
   The heat generating roller  21  used in this example is formed by the following manufacturing method. That is, after the elastic layer  23  is formed by foam-molding (it is preferable that the elastic layer  23  has a skin layer on its surface), on the elastic layer  23 , a coating of an undiluted liquid of a silicone rubber in which a conductive filler is dispersed is applied in a predetermined thickness by a spray method, a dipping method or the like. Then, the coating is subjected to vulcanization, and thus the heat generating layer  22  is formed on the elastic layer  23 . In this case, the core material  24  with the magnetic body layer  19  may be bonded fixedly to the elastic layer  23  before the formation of the heat generating layer  22 . Further, the core material  24  with the magnetic body layer  19  may be inserted into and bonded to an inner portion of the elastic layer  23  after the formation of the heat generating layer  22 . Further, it also is possible to form the elastic layer  23  by molding directly on the magnetic body layer  19  of the core material  24 . Further, the heat generating layer  22  may be formed of a plurality of coatings. After the heat generating layer  22  is formed on the elastic layer  23 , in the same manner that the heat generating layer  22  is formed, coatings of a silicone rubber that is used for the elastic layer (second elastic layer)  26  are applied on the heat generating layer  22 . Then, the coatings are subjected to vulcanization. After that, the mold releasing layer  27  is formed by, for example, the following method. That is, a PFA tube is fitted around the elastic layer  26  and then is bonded thereto through a primer layer, or the elastic layer  26  is coated with PTFE and then a body thus obtained is subjected to sintering. Between the layers in each pair of the adjacent layers, a primer layer selected so as to correspond to the materials of the layers may be interposed. Further, as in the above-mentioned case, also in the case of using a polyimide resin for the base material of the heat generating layer  22 , the heat generating layer  22  is formed by applying a coating of a polyimide varnish on the elastic layer  23 . 
   The description is directed to the operation of the above-mentioned heating device according to Embodiment 1. During the operation of the excitation coil  36 , alternating magnetic flux D is generated in such a manner as to flow around the excitation coil  36 . Most of the alternating magnetic flux D passes through the heat generating layer  22  as indicated by a broken line H in  FIG. 4B . The remaining portion of the alternating magnetic flux D penetrates the heat generating layer  22  as indicated by a broken line E. Due to magnetic flux H passing though the heat generating layer  22 , an eddy current is generated in the heat generating layer  22 , so that the heat generating layer  22  generates heat. On the other hand, leakage magnetic flux E that has penetrated the heat generating layer  22  heads toward the core material  24 . However, since the surface of the core material  24  is coated with the insulating magnetic body layer  19  of about 500 μm thickness that has ferrite, the leakage magnetic flux E is captured by the magnetic body layer  19 . Therefore, an amount of magnetic flux entering the core material  24  is decreased considerably. Further, since the magnetic body layer  19  has an insulating property, heat generation due to magnetic flux F passing though the magnetic body layer  19  is not caused in the magnetic body layer  19 . Thus, most of the applied alternating magnetic flux D is consumed for the heat generation of the heat generating layer  22 , thereby allowing the heat generation efficiency to be increased. Further, the phenomenon in which the core material  24  generates heat due to an eddy current generated therein is prevented. Thus, it also is possible to eliminate problems and the like including damage to the bearings of the core material  24  caused due to heating of the bearings. 
   It is preferable that the magnetic body layer  19  as the magnetism shielding layer has as high a relative magnetic permeability as possible with respect to the relative magnetic permeability of the core material  24 . In this example in which the core material  24  was formed from a non-magnetic metal, with the magnetic body layer  19  having a relative magnetic permeability of about 20 or higher and a thickness of not less than 0.3 mm, a magnetism shielding effect could be attained sufficiently. Generally, the magnetic body layer  19  has a relative magnetic permeability of, preferably 10 or higher, and more preferably 15 or higher. Further, the magnetic body layer  19  has a thickness of, preferably not less than 0.2 mm, and more preferably not less than 0.5 mm. 
   Furthermore, the magnetic body layer  19  is required not to generate heat due to an eddy current generated when the alternating magnetic flux F passes therethrough. For this reason, it is preferable that the magnetic body layer  19  has an insulating property. However, as long as the magnetic body layer  19  has a specific resistance value of 10 −3  Ωcm or higher, i.e. a value exceeding the range of specific resistance values defining a conductor, practically, the magnetic body layer  19  generates almost no heat and thus is effective. 
   Furthermore, even in the case where the magnetic body layer  19  is provided, if the core material  24  is formed from a magnetic metal, a portion of the leakage magnetic flux E is likely to enter the core material  24 . Therefore, in order to prevent this, it is preferable that the material of the core material  24  is not a magnetic metal such as iron or the like but a non-magnetic metal. Possible examples of a non-magnetic metal include a stainless material, brass, aluminum and the like. Among these materials, particularly, a stainless material is preferred in terms of its strength. 
   In the above-mentioned embodiment, the heat generating roller  21  has a layer configuration in which the magnetic body layer  19 , the elastic layer  23 , the heat generating layer  22 , the second elastic layer  26 , and the mold releasing layer  27  are provided in this order on the core material  24 . However, the present invention is not necessarily limited to this layer configuration. The following configurations also are allowable. That is, the respective layers may have a multi-layer configuration. Further, between the respective layers in each pair of the adjacent layers, an adhesive layer may be provided or an auxiliary layer may be formed. 
   Embodiment 2 
   The only difference between Embodiment 2 and Embodiment 1 lies in the configuration of the electromagnetic induction heat generating roller  21 .  FIG. 6A  is a cross sectional view of an electromagnetic induction heat generating roller according to Embodiment 2 of the present invention that is used in the image forming apparatus shown in  FIG. 5 .  FIG. 6B  is an expanded sectional view of a portion  6 B shown in  FIG. 6A . In  FIGS. 6A and 6B , like reference characters indicate like members that have the same functions as those described with regard to Embodiment 1, for which detailed descriptions are omitted. 
   A heat generating roller  21  according to this embodiment includes a core material  24 , an elastic layer  23 , a heat generating layer  22 , a second elastic layer  26 , and a mold releasing layer  27 , which are provided outwardly in this order. As in the case of Embodiment 1, the core material  24  is formed of a non-magnetic stainless material. Unlike the case of Embodiment 1, the heat generating layer  22  is formed of a base material of a silicone rubber in which scale-like particles of a silver powder as a conductive filler are dispersed. Further, the elastic layer  23  is formed from a foamed silicone rubber in which a magnetic powder of ferrite is dispersed. The configuration according to this embodiment does not include the magnetic body layer  19  used in Embodiment 1. 
   Alternating magnetic flux D generated by an excitation coil  36  penetrates the heat generating layer  22  and then enters the elastic layer  23 . Since the elastic layer  23  contains the magnetic powder of ferrite, after passing through the elastic layer  23 , the alternating magnetic flux D flows back to a U-shaped core  39  and a central core  38 . The alternating magnetic flux D flows around the excitation coil  36  in this manner. Due to the alternating magnetic flux D, an eddy current is generated in the heat generating layer  22 , so that the heat generating layer  22  generates heat. Although the core material  24  is formed of a conductive material, since the magnetic flux D is captured by the magnetic powder in the elastic layer  23 , only a trace amount of magnetic flux passes through the core material  24 . Thus, the core material  24  generates almost no heat. Further, the magnetic powder in the elastic layer  23  has an insulating property, and thus no heat is generated. 
   As described above, in this embodiment, the elastic layer  23  in which the magnetic powder is dispersed functions as a magnetism shielding layer. This eliminates the need for the magnetic body layer  19  used in Embodiment 1. 
   Embodiment 3 
   The only difference between Embodiment 3 and Embodiment 1 lies in the configuration of the magnetism shielding layer of the electromagnetic induction heat generating roller  21 .  FIG. 7A  is a schematic perspective view of a core material  24  of an electromagnetic induction heat generating roller according to Embodiment 3 of the present invention. The core material  24  includes a magnetism shielding layer. 
   In this embodiment, the magnetism shielding layer is formed by using a ring (hollow cylindrical member)  51  shown in  FIG. 7B  and is composed of a plurality of the rings  51 . The plurality of the rings  51  are fitted externally around the core material  24  and fixed thereto. The ring  51  contains a magnetic material such as ferrite. It is preferable that the adjacent rings  51  are joined to each other. However, the adjacent rings  51  also may be spaced slightly from each other. 
   In place of the ring  51 , an arc-shaped member  52  shown in  FIG. 7C  may be attached to an outer surface of the core material  24 . The arc-shaped member  52  contains a magnetic material. The member  52  has a shape obtained by dividing the ring  51  into a plurality of pieces in a circumferential direction. 
   The above-mentioned ring  51  and the arc-shaped member  52  can be manufactured by, for example, a method in which a material containing a magnetic powder is molded into a predetermined shape, and a molded body thus obtained is subjected to sintering. 
   Furthermore, in place of the ring  51  and the arc-shaped member  52 , a sheet-like material containing a magnetic material may be wrapped around the core material  24 , or a tube of a pliable magnetic material may be formed and fitted around the core material  24 . The above-mentioned flexible sheet or tube can be obtained by dispersing a powder of a magnetic material in a base material of resin or rubber. 
   As in the case of Embodiment 1, an elastic layer  23 , a heat generating layer  22 , a second elastic layer  26 , and a mold releasing layer  27  are formed on an outer side of the above-mentioned magnetism shielding layer, and thus the heat generating roller  21  according to this embodiment can be obtained. 
   According to this embodiment, in addition to the effect attained by Embodiment 1, the effect of allowing a magnetism shielding layer to be manufactured more easily can be attained. 
   Embodiment 4 
   The only difference between Embodiment 4 and Embodiment 1 lies in the configuration of the heat generating layer  22  of the electromagnetic induction heat generating roller  21 . As disclosed in, for example, JP11(1999)-288190 A, a heat generating layer  22  according to this embodiment is formed from a metal such as Ni, Fe, Co, Cu, Cr, stainless steel or the like. Such a metallic material is formed in the shape of an endless belt (tube) of a small thickness (of, for example, 40 μm) and fitted around an outer periphery of an elastic layer  23 . In this case, the heat generating layer  22  may be bonded to the elastic layer  23  or also may be just fitted on the elastic layer  23 . 
   Alternating magnetic flux D from a magnetic field generating unit causes an eddy current to be generated in the heat generating layer  22  as in the case of Embodiment 1 and thus allows heat generation to be caused as in the case of Embodiment 1. 
   According to this embodiment, in addition to the effect attained by Embodiment 1, since the thickness of the heat generating layer  22  can be reduced with relative ease, the effect of allowing a warm-up time to be reduced by decreasing the thermal capacity of the heat generating layer  22  can be attained. 
   As in this embodiment, in the case where the heat generating layer  22  is formed of an endless belt of metal, it is made relatively easier to reduce a warm-up time by decreasing the thickness of the heat generating layer  22 . In this case, however, if the heat generating layer  22  has a thickness not larger than a skin depth, the amount ratio of leakage magnetic flux E penetrating the heat generating layer  22  to the alternating magnetic flux D applied by the magnetic field generating unit is increased in particular. Thus, in the case where a magnetic body layer  19  is not provided, a core material  24  generates heat, and thus the heat generation efficiency of the heat generating layer  22  is decreased considerably. On the other hand, in the case where the magnetic body layer  19  as a magnetism shielding layer is provided between the heat generating layer  22  and the core material  24 , a decrease in the heat generation efficiency can be prevented effectively. As described above, the magnetism shielding layer according to the present invention exerts a considerable effect particularly in the case where the heat generating layer  22  has a thickness of not larger than a skin depth. A skin depth (δ) of the heat generating layer  22  is a value that is determined based on a specific resistance (ρ), a magnetic permeability (μ), and a driving frequency (f) and is expressed by δ=1/(πfμρ) 1/2 . 
   The application of the heat generating layer of a metallic material described with regard to this embodiment is not limited to the application to Embodiment 1 described with regard to the above-mentioned example. The heat generating layer also is applicable to Embodiments 2 and 3, and the same effect as that described above can be attained. 
   Embodiment 5 
     FIG. 8  is a cross sectional view of an image forming apparatus using a heating device according to an embodiment of the present invention as a fixing device. The heating device according to this embodiment is an electromagnetic induction heating device of the belt heating type. The following description is directed to the configuration and operation of this device. 
   In  FIG. 8 , reference numeral  115  denotes an electrophotographic photoreceptor (hereinafter, referred to as a “photosensitive drum”). The photosensitive drum  115 , while being driven to rotate at a predetermined peripheral velocity in a direction indicated by an arrow, has its surface charged uniformly to a negative dark potential V 0  by a charger  116 . Further, reference numeral  117  denotes a laser beam scanner that outputs a laser beam  118  corresponding to a signal of image information. The charged surface of the photosensitive drum  115  is scanned by and exposed to the laser beam  118 . Thus, in an exposed portion of the photosensitive drum  115 , an absolute potential value is decreased to a light potential VL, and a static latent image is formed. The latent image is developed with negatively charged toner of a developer  119  and made manifest. 
   The developer  119  includes a developing roller  120  that is driven to rotate. The developing roller  120  with a thin toner layer formed on its outer peripheral face is opposed to the photosensitive drum  115 . A developing bias voltage, whose absolute value is lower than the dark potential V 0  of the photosensitive drum  115  and higher than the light potential VL, is applied to the developing roller  120 . 
   Meanwhile, a recording material  11  is fed one at a time from a paper feeding part  121  and passed between a pair of resist rollers  122 . Then, the recording material  11  is conveyed to a nip part composed of the photosensitive drum  115  and a transferring roller  123 , and the timing thereof is appropriate and synchronized with the rotation of the photosensitive drum  115 . Toner images on the photosensitive drum  115  are transferred one after another to the recording material  11  by the transferring roller  123  to which a transfer bias voltage is applied. After the recording material  11  is released from the photosensitive drum  115 , an outer peripheral face of the photosensitive drum  115  is cleaned by removing residual materials such as toner remaining after the transferring process by a cleaning device  124  and used repeatedly for successive image formation. 
   Reference numeral  125  denotes a fixing guide that guides the recording material  11  on which the image has been transferred to a fixing device  126 . The recording material  11  is released from the photosensitive drum  115  and conveyed to the fixing device  126  where fixing of the transferred toner image is performed. Further, reference numeral  127  denotes a paper ejecting guide that guides the recording material  11 , which has passed through the fixing device  126 , to the exterior of the apparatus. The fixing guide  125  and the paper ejecting guide  127  that guide the recording material  11  are formed from a resin such as ABS or a non-magnetic metallic material such as aluminum. The recording material  11  on which the image has been fixed by the fixing process is ejected to a paper ejecting tray  128 . 
   Reference numerals  129 ,  130 , and  131  denote a bottom plate of a main body of the apparatus, a top plate of the main body, and a body chassis, which constitute a unit determining the strength of the main body of the apparatus. These strength members are formed of a material that uses a magnetic material of steel as a base material and is plated with zinc. 
   Reference numeral  132  denotes a cooling fan that generates airflow in the apparatus. Further, reference numeral  133  denotes a coil cover formed of a non-magnetic material such as aluminum, which is configured so as to cover an excitation coil  36  and a rear core  37  that constitute the fixing device  126 . 
   Next, the heating device according to Embodiment 5 of the present invention that is used as the above-mentioned fixing device  126  will be described in detail by way of an example. 
     FIG. 9  is a cross sectional view of a fixing device as the heating device according to Embodiment 5 that is used in the above-mentioned image forming apparatus. In this embodiment, like reference characters indicate like members that have the same functions as those of the heating device according to Embodiment 1, for which duplicate descriptions are omitted. In this embodiment, the respective configurations of a pressing roller  31  and a magnetic field generating unit including and an excitation coil  36 , a rear core  37 , and a heat insulating member  40  are the same as those described with regard to Embodiment 1. 
   In  FIG. 9 , a thin electromagnetic induction heat generating belt (hereinafter, referred to simply as a “heat generating belt”)  140  is an endless belt including an induction heat generating layer (hereinafter, referred to simply as a “heat generating layer”) of 40 μm thickness that is formed from Ni by electroforming so as to have a belt-like shape. In order to obtain mold releasability, an outer-side surface of the heat generating belt is coated with a 20-μm thick mold releasing layer (not shown) of a fluorocarbon resin. The mold releasing layer also may be formed of a single material or a combination of materials selected from resin and rubber that have excellent mold releasability such as PTFE, PFA, FEP, silicone rubber, fluorocarbon rubber and the like. In the case where the heat generating belt  140  is used for fixing a monochromatic image, it is only required to secure mold releasability. However, in the case where the heat generating belt  140  is used for fixing of a color image, it is desirable to obtain elasticity. In this case, it is preferable that a thick elastic layer further is formed between the heat generating layer and the mold releasing layer. 
   Reference numerals  150  and  160  denote a supporting roller of 20 mm in diameter and a fixing roller of 20 mm in diameter having low thermal conductivity, respectively. A surface of the fixing roller  160  is coated with a silicone rubber that is an elastic foam body having a low hardness (JIS-A 30 degrees). The heat generating belt  140  is suspended with a predetermined tensile force between the supporting roller  150  and the fixing roller  160 . The heat generating belt  140  is allowed to rotate in a direction indicated by an arrow  140   a . Ribs (not shown) for preventing the heat generating belt  140  from meandering are provided at both ends of the supporting roller  150 . 
   A pressing roller  31  as a pressing member is in contact under pressure with the fixing roller  160  through the heat generating belt  140 , so that a nip part  34  is formed between the heat generating belt  140  and the pressing roller  31 . 
   The supporting roller  150  is composed of an elastic layer (heat insulating layer)  153 , a magnetic body layer  152 , and a core material  151 , which are provided inwardly in this order. The core material  151  is formed of a non-magnetic stainless material. The magnetic body layer  152  as a magnetism shielding layer is an insulating layer of about 500 μm thickness coated with a silicone rubber in which a ferrite powder is dispersed. The elastic layer  153  is formed of a foam body of a silicone rubber having low thermal conductivity. In this example, the elastic layer  153  is set to have a thickness of 2 mm and a hardness of 45 degrees (ASKER-C). In order to reduce heat diffusion from the heat generating layer of the heat generating belt  140 , it also is effective to form a surface of the elastic layer  153  as an uneven surface so as to reduce a contact area between the elastic layer  153  and the heat generating belt  140 . 
   According to this embodiment, alternating magnetic flux from the magnetic field generating unit causes an eddy current to be generated in the heat generating layer of the heat generating belt  140 , so that the heat generating layer generates heat by induction. The heat generating belt  140 , which has generated heat, heats the recording material  11  and a toner image  9  formed on the recording material  11  at the nip part  34 , so that the toner image  9  is fixed on the recording material  11 . 
   Most of the leakage magnetic flux, i.e. a portion of alternating magnetic flux from the magnetic field generating unit that has penetrated the heat generating layer of the heat generating belt  140  and then entered the supporting roller  150 , is captured by the magnetic body layer  152  formed on an outer surface of the core material  151 . Therefore, an amount of magnetic flux entering the core material  151  is decreased considerably. Further, heat generation due to magnetic flux passing through the magnetic body layer  152  is not caused in the magnetic body layer  152 . Thus, most of the alternating magnetic flux applied by the magnetic field generating unit is consumed for the heat generation of the heat generating layer, thereby allowing the heat generation efficiency to be increased. Further, it also is possible to eliminate problems and the like including damage to bearings of the core material  151  caused due to heating of the bearings. 
   As for the heat generating layer of the heat generating belt  140  according to this embodiment, the configurations of the heat generating layer  22  of the heat generating roller  21  described above with regard to Embodiments 1 to 4 can be applied thereto, and the same effects as those described with regard to Embodiments 1 to 4 thus can be attained, respectively. 
   Further, as for the core material  151 , the magnetism shielding layer and the elastic layer  153  of the supporting roller  150  according to this embodiment, the respective configurations of the core material  24 , the magnetism shielding layer, and the elastic layer  23  of the heat generating roller  21  described above with regard to Embodiments 1 to 4 can be applied thereto, and the same effects as those described with regard to Embodiments 1 to 4 thus can be attained, respectively. 
   Moreover, this embodiment described a configuration in which the heat generating layer was provided in the heat generating belt  140 , and only the heat generating belt  140  was caused to generate heat by induction. However, the same effect can be attained by a configuration in which both of the heat generating belt  140  and the supporting roller  150  are caused to generate heat by induction. That is, an induction heat generating layer is provided as a surface layer of the supporting roller  150  or provided in the vicinity of the surface layer, and a magnetism shielding layer is formed between the induction heat generating layer and the core material  151 . For example, if the induction heat generating layer of the supporting roller  150  is formed of a thin pipe formed from an iron alloy such as carbon steel or the like, both of the heat generating belt  140  and the supporting roller  150  are caused to generate heat by induction. In this case, while a warm-up time is increased slightly due to the thermal capacity of the supporting roller  150 , the following can be achieved. That is, in the case where the recording materials  11  having a width smaller than a width of the heat generating belt  140  are passed continuously, heat is removed from only a portion of the heat generating belt  140  by the recording materials  11 , thereby causing temperature variations in a width direction of the heat generating belt  140 . Such temperature variations are reduced by heat transmission in the width direction through the supporting roller  150 . Also in this case, since the magnetism shielding layer is provided between the induction heat generating layer and the core material of the supporting roller  150 , the heat generation of the core material is prevented. 
   Furthermore, in this embodiment, the supporting roller  150  does not contribute to the formation of the nip part  34 . Therefore, a configuration without the elastic layer  153  is possible. That is, the magnetic body layer  152  can be provided on a surface of the supporting roller  150 . This allows the realization of a simplified layer configuration and a cost reduction of the supporting roller  150 . 
   Embodiment 6 
   A heating device according to Embodiment 6 of the present invention that is used as the fixing device  126  of the image forming apparatus shown in  FIG. 8  will be described in detail by way of an example. 
     FIG. 10  is a cross sectional view of a fixing device as the heating device according to Embodiment 6. In this embodiment, like reference characters indicate like members that have the same functions as those of the heating device according to Embodiment 1, for which duplicate descriptions are omitted. In this embodiment, the respective configurations of a pressing roller  31  and a magnetic field generating unit including an excitation coil  36 , a rear core  37  and a heat insulating member  40  are the same as those described with regard to Embodiment 1. Further, an electromagnetic induction heat generating belt (hereinafter, referred to simply as “heat generating belt”)  140  and a supporting roller  150  are the same as those described with regard to Embodiment 5. 
   This embodiment is different from Embodiment 5 in that the heat generating belt  140  is suspended rotatably between the supporting roller  150  and a belt guide  170 , and that the supporting roller  150  is in contact under pressure with the pressing roller  31  through the heat generating belt  140 . The belt guide  170  is formed of, for example, a resin material having an excellent sliding property. 
   According to Embodiment 6, as in the case of Embodiment 5, alternating magnetic flux from the magnetic field generating unit causes an eddy current to be generated in a heat generating layer of the heat generating belt  140  so as to cause the heat generating layer to generate heat by induction. The heat generating belt  140 , which has generated heat, heats a recording material  11  and a toner image  9  formed on the recording material  11  at a nip part  34 , so that the toner image  9  is fixed on the recording material  11 . 
   Leakage magnetic flux, i.e. a portion of alternating magnetic flux from the magnetic field generating unit that has penetrated the heat generating layer of the heat generating belt  140 , penetrates the belt guide  170  and then reaches the supporting roller  150 . However, most of the leakage magnetic flux that has entered the supporting roller  150  is captured by a magnetic body layer  152  formed on an outer surface of a core material  151 . Therefore, an amount of magnetic flux entering the core material  151  is decreased considerably. Further, heat generation due to magnetic flux passing through the magnetic body layer  152  is not caused in the magnetic body layer  152 . Thus, most of the alternating magnetic flux applied by the magnetic field generating unit is consumed for the heat generation of the heat generating layer, thereby allowing the heat generation efficiency to be increased. Further, it also is possible to eliminate problems and the like including damage to bearings of the core material  151  caused due to heating of the bearings. 
   As for the heat generating layer of the heat generating belt  140  according to this embodiment, the configurations of the heat generating layer  22  of the heat generating roller  21  described above with regard to Embodiments 1 to 4 can be applied thereto, and the same effects as those described with regard to Embodiments 1 to 4 thus can be attained, respectively. 
   Further, as for the core material  151 , a magnetism shielding layer, and an elastic layer  153  of the supporting roller  150  according to this embodiment, the respective configurations of the core material  24 , the magnetism shielding layer, and the elastic layer  23  of the heat generating roller  21  described above with regard to Embodiments 1 to 4 can be applied thereto, and the same effects as those described with regard to Embodiments 1 to 4 thus can be attained, respectively. 
   This embodiment described the case of the heating device including the magnetic field generating unit disposed outside a loop of the heat generating belt  140  and the supporting roller  150  disposed inside the loop of the heat generating belt  140 , in which the core material of the supporting roller  150  was a metallic member. The present invention, however, is not limited to this configuration but can be applied broadly to any configuration in which a magnetic field generating unit is disposed outside a loop of a heat generating belt and a metallic member is disposed inside the loop of the heat generating belt. For example, as a member for supporting a heat generating belt, a supporting member that includes a metallic member and is not rotatable also can be used in place of the supporting roller. Preferably, the supporting member integrally includes the metallic member, a magnetism shielding member that covers the metallic member so as to prevent entry of magnetic flux into the metallic member, and a heat insulating member that is provided between the metallic member and the heat generating belt. The supporting member is disposed inside a loop of the heat generating belt and presses the heat generating belt against a pressing roller. When the heat generating belt turns in association with the rotation of the pressing roller, the heat generating belt slides on the supporting member. 
   The embodiments disclosed in this application are intended to illustrate the technical aspects of the invention and not to limit the invention thereto. The invention may be embodied in other forms without departing from the spirit and the scope of the invention as indicated by the appended claims and is to be broadly construed.