Patent Publication Number: US-2009238593-A1

Title: Heating apparatus and induction heating control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation-in-Part application of U.S. patent application Ser. No. 12/115,904, filed May 6, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing device which is mounted on an image forming device, a copying machine, a printer or the like to form an image on a transfer material by use of an electrophotographic process and which fixes, to the transfer material, a developer on the transfer material. 
     2. Description of the Related Art 
     In a copying machine or a printer using an electronic process, it is known that a toner image formed on a photosensitive drum is transferred to a transfer member, and thereafter the melted toner image by a fixing device including a heating roller and a pressurizing roller is fixed to the transfer member. 
     Furthermore, an induction heating system is known in which, in the above case, the surface of the heating roller is heated using a plurality of coils. In a case where the plurality of coils are utilized, cost might increase as compared with a case where one coil is utilized. In this case, circuits to drive the plurality of coils must be prepared in accordance with the number of the coils, which leads to the cost increase, and in addition, there rises a problem that the whole device is enlarged. 
     Moreover, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-151470, in a case where a temperature of a conductive member for use in the heating roller exceeds the Curie point, a skin effect deepens, and therefore the conductive member does not generate any heat. This is utilized, and heating of the heating roller is stopped at a time when it is detected that a temperature of the heating roller rises to an abnormal temperature. In this known technology, in a case where the temperature of the whole heating roller exceeds the Curie point, there is not any problem even when power supply is stopped with respect to a coil which supplies a magnetic field to the conductive member of the heating roller. However, in a case where a small-sized sheet continues to be passed, the temperature reaches the Curie point on the only surface of the heating roller in a portion through which any sheet does not pass, and the conductive member of this portion has an increased depth of penetration. Therefore, any heat is not generated from the only heating roller of the portion through which any sheet does not pass. In this case, since the driving circuit for supplying the power to the coil is not matched with the heating roller, it becomes difficult to heat an only area that passes the sheet. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a fixing device of an image forming apparatus, comprising: 
     an endless unit includes a conductive layer which, fixes a toner image on a medium; 
     a first conveying unit which is located inside of and is in contact with the endless unit; 
     a second conveying unit which is located outside of the endless unit and is pressed to the first conveying unit in a predetermined direction for conveying the medium; 
     an induced current generation unit, arranged near the first and second conveying units and outside of the endless unit, which includes a coil having a load resistance R and an inductance which meet L/R=35×10 −6  (H/Ω) with a drive current having a frequency of 40 kHz or higher; and 
     a roller, arranged apart from the first and second conveying units and inside of the endless unit, which has a metal layer with a thermal capacity. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a schematic diagram showing one example of a fixing device to which an embodiment of the present invention is applicable; 
         FIG. 2  is a schematic diagram of the fixing device shown in  FIG. 1  as viewed from a different direction; 
         FIG. 3  is a block diagram showing a control system of the fixing device shown in  FIG. 1 ; 
         FIG. 4  is a flowchart showing one example of a heating apparatus control method which is applicable to the fixing device shown in  FIG. 1 ; 
         FIG. 5  is a schematic diagram showing an example that is different from the fixing device shown in  FIG. 1 ; 
         FIG. 6  is a schematic diagram of the fixing device shown in  FIG. 5  as viewed from a different direction; 
         FIG. 7  is a schematic diagram showing another example that is different from the fixing device shown in  FIG. 1 ; 
         FIGS. 8A and 8B  are schematic diagrams of the fixing device shown in  FIG. 7  as viewed from a different direction; 
         FIG. 9  is a schematic diagram showing still another example that is different from the fixing device shown in  FIG. 1 ; 
         FIG. 10  is a schematic diagram of the fixing device shown in  FIG. 9  as viewed from a different direction; 
         FIG. 11  is a sectional view cut along the arrows E 1  and E 2 , showing a heating belt mounted on the fixing device shown in  FIG. 9 ; 
         FIG. 12  is a schematic diagram showing a further example that is different from the fixing device shown in  FIG. 1 ; 
         FIG. 13  is a schematic diagram of the fixing device shown in  FIG. 12  as viewed from a different direction; 
         FIG. 14  is a schematic diagram of the fixing device shown in  FIG. 12  as viewed from a different direction; 
         FIG. 15  is a flowchart showing one example of a heating apparatus control method applicable to the fixing device shown in  FIG. 12 ; 
         FIG. 16  is a flowchart showing another example of a heating apparatus control method applicable to the fixing device shown in  FIG. 12 ; 
         FIG. 17  is a schematic diagram showing a heating roller and an induction heating unit which are applicable to the above-described fixing device; 
         FIG. 18  is a sectional view cut along the arrows E 3  and E 4  shown in  FIG. 17 ; 
         FIG. 19  is a sectional view cut along the arrows E 5  and E 6  shown in  FIG. 17 ; 
         FIG. 20  is a schematic diagram showing another example that is different from the fixing device shown in  FIG. 1 ; 
         FIG. 21  is a schematic block diagram showing the image forming apparatus of the first embodiment of the present invention; 
         FIG. 22  is a schematic block diagram showing the fixing device of the first embodiment of the present invention; 
         FIG. 23  is a schematic side view showing the fixing device of the first embodiment of the present invention; 
         FIG. 24  is a schematic block diagram showing the heating control system of the heat roller of the first embodiment of the present invention; 
         FIG. 25  is a schematic illustration showing one cycle by a switching element of the inverter circuit of the first embodiment of the present invention; 
         FIG. 26  is a table showing characteristics of the induction heating coil of the first embodiment of the present invention; 
         FIG. 27  is a schematic block diagram showing the heat roller and induction heating coil of the second embodiment of the present invention; 
         FIG. 28  is a table showing characteristics of the induction heating coil of the second embodiment of the present invention; 
         FIG. 29  is a schematic block diagram showing the heating system of the heat roller by a power source of 100 V of the third embodiment of the present invention; 
         FIG. 30  is a schematic block diagram showing the heating system of the heat roller by a power source of 200 V of the third embodiment of the present invention; 
         FIG. 31  is a table showing characteristics of the induction heating coil by a power source of 100 V of the third embodiment of the present invention; and 
         FIG. 32  is a table showing characteristics of the induction heating coil by a power source of 200 V of the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There will be described hereinafter an example of a fixing device to which an embodiment of this invention is applied with reference to the drawings. 
     First Embodiment 
       FIG. 1  shows one example of a fixing device to which an embodiment of this invention is applied.  FIG. 2  is a schematic diagram of the fixing device shown in  FIG. 1  as viewed from a different direction. 
     As shown in  FIG. 1 , a fixing device  1  has a heating member (heating roller)  2 , a pressurizing member (pressurizing roller)  3 , a pressurizing spring  4 , a peeling claw  5 , a cleaning roller  6 , an induction heating unit  7 , a temperature detecting section  8 , and a thermostat  9 . 
     The heating roller  2  includes a rolled conductive layer  2 A constituted by forming a conductive material into a cylindrical shape, and a coating layer (mold-releasing layer)  2 B disposed on an outer peripheral surface of this conductive layer  2 A and made of a fluorine resin such as an ethylene tetrafluoride resin. This heating roller  2  has a 20 μm thick mold-releasing layer formed on the surface of the conductive layer  2 A having a diameter of 40 mm and a thickness of 1 mm. 
     The pressurizing roller  3  is an elastic roller having a diameter of 40 mm. This pressurizing roller  3  is constituted of: a core metal  100  having a thickness of 1.5 mm; a 3 mm thick silicon rubber  101  formed on an outer periphery of this core metal  100 ; and a 30 μm thick PFA tube with which an outer periphery of this silicon rubber  101  is coated. 
     The pressurizing spring  4  comes into contact under a predetermined pressure with an axial line of the heating roller  2 , and a predetermined nip is formed between the heating roller  2  and the pressurizing roller  3 . This pressurizing spring  4  supplies a predetermined pressure from opposite ends of the pressurizing roller  3  via a pressurizing support bracket (not shown) which supports a shaft of the pressurizing roller  3 . 
     The heating roller  2  is rotated in a clockwise direction shown by an arrow CW at a substantially constant speed by a predetermined fixing motor (not shown). When the heating roller  2  is rotated, the pressurizing roller  3  is rotated in a direction opposite to a direction in which the heating roller  2  is rotated in a position where the pressurizing roller comes into contact with the heating roller  2 . 
     The peeling claw  5  peels, from the heating roller  2 , a sheet P disposed in a downstream position of the nip in the heating roller  2  and passed through the nip. It is to be noted that the present invention is not limited to the present embodiment. For example, in a case where there is a large amount of developer to be fixed to the sheet as in color image formation, the sheet is not easily peeled from the heating roller  2 . Therefore, a plurality of peeling claws  5  may be disposed. Alternatively, any peeling claw may not be disposed in a case where the sheet easily peels from the heating roller  2 . 
     The cleaning roller  6  removes a toner offset on the surface of the heating roller  2 , or dust such as waste paper. 
     The induction heating unit  7  is disposed in the heating roller  2 , and includes a heating coil (exciting coil)  71  to which a predetermined power is supplied and which supplies a predetermined magnetic field to the heating roller  2 . As shown in  FIG. 2 , the exciting coil  71  is one coil disposed at a substantially uniform distance from an inner surface of the heating roller  2 , and the coil is constituted of one conductor. This exciting coil  71  generates a predetermined magnetic flux, when a predetermined high-frequency current is supplied to the coil by an induction heating control circuit described later in detail with reference to  FIG. 3 , and the heating roller  2  is induction-heated at a predetermined temperature. 
     As the exciting coil  71 , a litz wire is usable which is constituted by bundling a plurality of copper wires whose surfaces are coated with an insulating material (e.g., heat-resistant polyamide imide). In the present embodiment, the litz wire is used which is constituted by bundling  50  copper wires having a linear diameter of 0.3 mm. In a case where a frequency of the high-frequency current to be supplied to the exciting coil  71  is high, a depth of penetration of an eddy current is further reduced, the eddy current flowing through the conductive layer  2 A of the heating roller  2 . This increases a copper loss. Therefore, when the linear diameter of the copper wire for use in the exciting coil  71  is reduced, the copper loss can be reduced, and an alternating current can be efficiently passed through the exciting coil  71 . 
     The temperature detecting section  8  includes thermistors  81 ,  82  which detect a surface temperature of the heating roller  2  in two portions of the heating roller  2  along a longitudinal direction. The thermistor  81  detects the temperature of each area A 1  described later. The thermistor  82  detects a temperature of an area A 2 . 
     The thermostat  9  detects heat generation abnormality indicating that the surface temperature of the heating roller  2  rises at an abnormal temperature. In a case where the heat generation abnormality is generated, the thermostat is used in order to interrupt a power supplied to the exciting coil  71 . 
     Moreover, along a periphery of the pressurizing roller  3 , there are arranged: a peeling claw  10  which peels the sheet P from the pressurizing roller  3 ; and a cleaning roller  11  which removes a toner attached to a peripheral surface of the pressurizing roller  3  in the same manner as in the heating roller  2 . 
     When the sheet P holding a toner T is passed through a nip portion formed between the heating roller  2  and the pressurizing roller  3 , the melted toner T is attached to the sheet P under pressure, and an image on the sheet P is fixed to the sheet P. 
     Next, the heating roller  2  will be described in more detail with reference to  FIG. 2 . 
     The conductive layer  2 A includes the whole sheet passing area A 3  constituted of the end areas (first areas) A 1  and the central area (second area) A 2 . The central area A 2  is an area through which a small-sized sheet is passed, and each end area A 1  is adjacent to the central area A 2  in the longitudinal direction of the heating roller  2 . The central area A 2  has a length of 180 mm, the whole sheet passing area A 3  has a length of 300 mm, and the heating roller  2  has the whole length of 340 mm. It is to be noted that the whole sheet passing area A 3  is a sheet passing area, and a further outer area of the whole sheet passing area A 3  is referred to as a sheet non-passing area. 
     The central area A 2  has a double-layer structure including a first conductive member  21 A and a second conductive member  22 A. A thickness of the conductive layer  2 A is formed to be uniform in the longitudinal direction. In the second area A 2  of the conductive layer  2 A, the second conductive member  22 A is disposed on a side close to the exciting coil  71  in the laminated first conductive member  21 A and second conductive member  22 A. 
     In the present embodiment, the first conductive member  21 A is made of aluminum, and the second conductive member  22 A is made of iron. A magnetic permeability of the first conductive member  21 A made of aluminum is smaller than that of the second conductive member  22 A made of iron. In other words, the second conductive member  22 A made of iron generates a larger amount of heat by the eddy current as compared with the first conductive member  21 A made of aluminum. Therefore, the second conductive member  22 A made of iron can generate heat in a state in which the frequency of the high-frequency current to be supplied to the exciting coil  71  is low as compared with the first conductive member  21 A made of aluminum. 
     As described above, since the first conductive member  21 A made of aluminum has a magnetic permeability smaller than that of the second conductive member  22 A made of iron, the first conductive member does not easily generate heat in a frequency region (around 20 kHz) where iron generates heat, and can generate sufficient heat in a higher frequency region (around 60 kHz). That is, assuming that a first frequency region F 1  is below 40 kHz, the only second conductive member  22 A made of iron can be induction-heated in this first frequency region F 1 . Assuming that a second frequency region F 2  is not less than 40 kHz, it is possible to induction-heat both of the second conductive member  22 A made of iron and the first conductive member  21 A made of aluminum in this second frequency region F 2 . 
     When the frequency of the high-frequency current to be supplied to the exciting coil  71  is set to be high in this manner, the depth of penetration of the eddy current flowing through the conductive material (metal) can be set to be small (shallow). Therefore, an eddy current&#39;s property of flowing through the surface of a conductor is strengthened, and a current density increases. This increases the amount of heat to be generated. Consequently, the conductive member (aluminum) having a smaller magnetic permeability induction-heats the conductive member (iron) having a larger magnetic permeability. Therefore, when supplying, to the exciting coil  71 , the high-frequency current whose frequency is higher than that of the high-frequency current to be supplied to the exciting coil  71 , heat generation efficiency is improved. 
     It is to be noted that in a case where the alternating current flows through the conductor, the flowing current is not necessarily distributed with a certain density over the whole sectional area. The alternating current flows through a portion having a small impedance, that is, the surface of the conductor in a concentrated manner. A phenomenon in which the current eccentrically flows through the surface, and the current density of the surface increases in this manner is generally referred to as a skin effect. This phenomenon appears with respect to the alternating current. The higher the frequency is, the more remarkably the phenomenon appears. This depth of penetration is generally represented by the following equation, and can indicate a degree of concentration of the current onto this surface. 
     Penetration depth 
     
       
         
           
             
               δ 
               = 
               
                 503 
                 × 
                 
                   
                     ρ 
                     
                       μ 
                        
                       
                           
                       
                        
                       f 
                     
                   
                 
                  
                 
                   ( 
                   m 
                   ) 
                 
               
             
             , 
           
         
       
     
     wherein ρ: resistivity [Ω·m] of the conductor; 
     μ: relative permeability of the conductor; and 
     f: frequency (Hz) of the high-frequency current flowing through the exciting coil. 
     Moreover, a characteristic indicating heat generation in the high-frequency region can be represented based on a value of a skin resistance Rs represented by the following equation: 
     
       
         
           
               
             
               
                 
                   
                     
                       
                         
                           Rs 
                           = 
                           
                             ρ 
                             δ 
                           
                         
                       
                     
                     
                       
                         
                           = 
                           
                             
                               
                                 4 
                                 × 
                                 
                                   π 
                                   2 
                                 
                                 × 
                                 
                                   10 
                                   
                                     - 
                                     7 
                                   
                                 
                               
                             
                             × 
                             
                               
                                 
                                   f 
                                   · 
                                   μ 
                                   · 
                                   ρ 
                                 
                               
                               . 
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     
                       Equation 
                        
                       
                           
                       
                        
                       1 
                     
                     ) 
                   
                 
               
             
           
         
       
     
     It is to be noted that it has been experimentally clarified in the present embodiment that the conductive material having the following value of skin resistance Rs at each frequency (f) is suitable for induction heating: 
         Rs≧ 8.0×10 −5   (Equation 2). 
     For example, in a case where the frequency is 20 kHz, the skin resistance Rs of iron is as follows, and the induction heating is possible: 
         Rs≧ 88×10 −5  (Ω)  (Equation 3). 
     On the other hand, the skin resistance Rs of aluminum at a frequency of 20 kHz is as follows, and the induction heating is difficult: 
         Rs≧ 4.7×10 −5  (Ω)  (Equation 4). 
     That is, at the frequency of 20 kHz, iron sufficiently generates heat by the induction heating, but aluminum does not easily generate heat. That is, aluminum having a magnetic permeability which is lower than that of iron does not easily generate heat in the vicinity of the frequency (20 kHz) in which iron generates heat. It is to be noted that to allow aluminum to generate heat in the vicinity of the above-described frequency (around 20 kHz), a thickness of a film of aluminum has to be set to be considerably small. This requires much manufacturing labor. Since the film thickness is considerably small, durability degrades, and the film might be broken. 
     Therefore, when increasing the frequency of the conductive material whose skin resistance value does not satisfy Equation 2, such as aluminum, the depth of penetration is reduced. Therefore, heat can be generated by the induction heating. Aluminum satisfies Equation 2 described above at a frequency of 60 kHz or more, and generates heat. 
     It is to be noted that in a case where the frequency is 60 kHz, even iron having a magnetic permeability which is larger than that of aluminum can generate heat by the induction heating. Therefore, when the frequency of the high-frequency current to be supplied to the exciting coil  71  is set to 60 kHz or more, heat can be generated from both of aluminum and iron by the induction heating. 
     Next, there will be described a constitution of an induction heating control circuit applicable to the fixing device  1  shown in  FIG. 1 , and a method of operating the fixing device  1 . 
       FIG. 3  is a block diagram showing a control system of the fixing device shown in  FIG. 1 . 
     As shown in  FIG. 3 , this induction heating control circuit includes: a rectifying circuit  21 ; a commercial alternating-current power supply  22 ; an input power detecting section  23 ; a CPU  24 ; a reactor  25 ; a smoothing capacitor  26 ; an IGBT  27 ; an IGBT  28 ; an inverter circuit  29 ; a diode  30 ; a diode  31 ; a resonance capacitor  32 ; an oscillator  33 ; a current transformer (high-frequency current detecting means)  34 ; a current detection circuit (input current value detecting means, regenerative current value detecting means)  35 ; a PWM generation circuit  36 ; a driving circuit  37 ; the exciting coil  71 ; and the temperature detecting section  8 . It is to be noted that the commercial alternating-current power supply  22  supplies a power to operate the fixing device  1 , and the power supply may supply a part of a power to be supplied to the whole copying machine on which the fixing device  1  is to be mounted. 
     The rectifying circuit  21  is connected to the commercial alternating-current power supply  22 , and also connected to the smoothing capacitor  26  via the reactor  25 . The input power detecting section  23  is connected between the rectifying circuit  21  and the commercial alternating-current power supply  22  via a transformer  23 A, and the input power detecting section  23  is connected to the CPU  24 . 
     Arms constituted of the IGBTs  27  and  28  are connected to opposite ends of the smoothing capacitor  26  to constitute the inverter circuit  29  of a half bridge type (current resonance type). The diodes  30  and  31  are connected between collectors and emitters of the IGBTs  27  and  28 , respectively. An output terminal of the inverter circuit  29  is connected to one end of the exciting coil  71  for generating a high-frequency magnetic field, and the other end of the exciting coil  71  is connected to the resonance capacitor  32 . 
     The current detection circuit  35  is connected between the output terminal of the inverter circuit  29  and the exciting coil  71  via the current transformer  34 , and the current detection circuit  35  is connected to the CPU  24 . The CPU  24  is also connected to the temperature detecting section  8 , and the CPU is further connected to the inverter circuit  29  via the PWM generation circuit  36  and the driving circuit  37 . 
     There is supplied, to the inverter circuit  29 , a direct-current power from the commercial alternating-current power supply  22 , the power being smoothed by the rectifying circuit  21 . The input power detecting section  23  detects the whole power consumption to be supplied from the commercial alternating-current power supply  22  to the inverter circuit  29  via the transformer  23 A, and the section outputs, to the CPU  24 , a detected power signal corresponding to the whole power consumption. The current detection circuit  35  detects the high-frequency current supplied from the inverter circuit  29  to the exciting coil  71  via the current transformer  34 , and the circuit outputs, to the CPU  24 , a detected current signal corresponding to this high-frequency current. The temperature detecting section  8  detects a surface temperature of the heating roller  2  induction-heated by the exciting coil  71 , and outputs a detected temperature signal (voltage value). 
     The CPU  24  executes a control based on at least one of the detected power signal output from the input power detecting section  23 , the detected current signal output from the current detection circuit  35 , and the detected temperature signal output from the temperature detecting section  8 , so that the surface temperature of the heating roller  2  becomes uniform in the longitudinal direction. There are simultaneously supplied, to the PWM generation circuit  36 , a control signal from the CPU  24  and an oscillation signal output by the oscillator  33  based on a fixed frequency (driving frequency). The PWM generation circuit controls the driving circuit  37  to drive the inverter circuit  29 . Accordingly, the driving circuit  37  outputs a gate signal (on and off signal) based on a predetermined driving frequency to gates of the IGBTs  27  and  28  of the inverter circuit  29 . The inverter circuit  29  can generate a high-frequency power having a frequency corresponding to the driving frequency. 
     When the high-frequency current is supplied from the inverter circuit  29  to the exciting coil  71 , a magnetic field is generated in accordance with the frequency of the high-frequency current, and the eddy current flows through the conductive layer  2 A of the heating roller  2  to which this magnetic field has been supplied. Accordingly, the Joule heat is generated in the conductive layer  2 A, and the heating roller  2  generates heat. 
     In the present embodiment, the CPU  24  indicates a driving frequency of 60 kHz to the inverter circuit  29 , and supplies, to the exciting coil  71 , the high-frequency current in accordance with this frequency in a case where the fixing device  1  or an image forming device (not shown) on which this fixing device  1  is mounted is started, in a case where a sheet (sheet having an A4 or A3 size) is passed through the whole sheet passing area A 3  of the heating roller  2 , or until a temperature of the heating roller  2  reaches a set temperature (e.g., 180° C.). 
     It is to be noted that in the present embodiment, the induction heating control circuit has a range of 20 to 70 kHz as a driving frequency region to be indicated to the inverter circuit  29 . If the frequency is in this range, the driving frequency of the inverter circuit  29  can be arbitrarily changed. 
     Next, there will be described an induction heating control method based on a temperature detection signal from the temperature detecting section  8  with reference to  FIG. 4 . 
     As described above, the CPU  24  drives the inverter circuit  29  at the driving frequency of 60 kHz. The high-frequency current generated by the inverter circuit  29  is supplied to the exciting coil  71 . Accordingly, the heating roller  2  is induction-heated, and the surface temperature (center) of the heating roller  2  is detected by the thermistor  82 . The temperature detected by this thermistor  82  is compared with a set temperature of 180° C. (S 1 ). When the temperature detected by the thermistor  82  is 180° C. or less (S 1 —YES), the surface temperature (end portion) of the heating roller  2  is detected by the thermistor  81 . The temperature detected by this thermistor  81  is compared with a temperature of, for example, 200° C., which is higher than the set temperature by a predetermined temperature (S 2 ). When the temperature detected by the thermistor  81  is below 200° C. (S 2 —NO), the driving frequency of the inverter circuit  29  is successively controlled into 60 kHz (S 3 ), and the high-frequency current is supplied to the exciting coil  71  in accordance with this driving frequency of 60 kHz (S 4 ). 
     On the other hand, when the temperature detected by the thermistor  81  is 200° C. or more in the step S 2  (S 2 —YES), the driving frequency of the inverter circuit  29  is controlled into 30 kHz (S 5 ), and the high-frequency current is supplied to the exciting coil  71  in accordance with this driving frequency of 30 kHz (S 6 ). 
     It is to be noted that in a case where the temperature detected by the thermistor  82  is higher than 180° C. in the step S 1  (S 1 —NO), the power supply from the commercial alternating-current power supply  22  is interrupted, and the induction heating is stopped (S 7 ). 
     As described above, in the induction heating control method of the present embodiment, when the temperature detected by the thermistor  81  disposed in the end portion of the heating roller  2  in the longitudinal direction is above 200° C., the driving frequency of the inverter circuit  29  is changed from 60 kHz around 30 kHz. Accordingly, the depth of penetration in the conductive layer  2 A of the heating roller  2  increases, and the second conductive member  22 A made of iron generates heat, but the first conductive member  21 A made of aluminum does not generate any heat. Therefore, since the only second conductive member  22 A generates heat, the only vicinity of the center of the heating roller  2  is heated, and it is possible to prevent the temperature of the end portion of the heating roller  2  from being excessively raised. In a case where the temperature detected by the thermistor  81  is below 200° C., the driving frequency of the inverter circuit  29  is set to 60 kHz. 
     As described above, when the driving frequency of the inverter circuit  29  is changed, it is possible to change the frequency of the high-frequency current to be supplied to the exciting coil  71 . Therefore, it is possible to change the depth of penetration of the eddy current flowing through the conductive layer  2 A of the heating roller  2 , and the only conductive member corresponding to this depth of penetration can be induction-heated. Therefore, as in the present embodiment, the driving frequency can be changed to change the heat generating area of the heating roller  2  by use of the conductive member having a different driving frequency region in which heat is generated. 
     Therefore, during continuous printing of, for example, a small-sized sheet, even in a case where the temperature rises in the only end portions of the heating roller  2  that do not pass the small-sized sheet, the induction heating of the only end portions of the heating roller  2  can be stopped, and the induction heating of the center of the heating roller  2  can be continued. 
     The method of the present embodiment controls heat generation of the first conductive member  21 A and the second conductive member  22 A for use in the conductive layer  2 A of the heating roller  2 , based on the detected temperature signal from the temperature detecting section  8 . 
     That is, the driving frequency output from the inverter circuit  29  can be changed to make uniform the surface temperature of the heating roller  2  along the longitudinal direction. 
     Moreover, when a plurality of conductive members are disposed in accordance with the driving frequency even in the fixing device including only one exciting coil as in the present embodiment, heating areas of a plurality of heating rollers  2  can be constituted. Therefore, since the exciting coils or the driving circuits do not have to be increased in accordance with the number of the heating areas, manufacturing costs can be reduced. 
     Furthermore, the induction heating control method usable in the present invention is not limited to the method described with reference to  FIG. 4 , and there may be performed a method of changing the driving frequency of the inverter circuit  29  from 60 kHz around 30 kHz, for example, in a case where a difference between the temperature of the central area A 2  of the heating roller  2  and the temperature of each end area A 1  is in a predetermined defined range (e.g., 20° C.). 
     Second Embodiment 
     Next, there will be another example of the first embodiment with reference to  FIGS. 5 and 6 .  FIG. 5  shows an example of a fixing device to which the present embodiment is applicable.  FIG. 6  shows a schematic diagram of the fixing device shown in  FIG. 5  as viewed from a different direction. It is to be noted that components having the same constitutions and functions as those of components shown in  FIGS. 1 to 4  are denoted with the same reference numerals, and detailed description thereof is omitted. 
     As shown in  FIG. 5 , a fixing device  100  has a heating roller  200 , an induction heating unit  700 , a pressurizing roller  3 , a pressurizing spring  4 , a peeling claw  5 , a cleaning roller  6 , a temperature detecting section  8 , and a thermostat  9 . 
     The heating roller  200  has: a shaft  200   a  made of a material having a rigidity (hardness) such that the material does not deform under a predetermined pressure; an elastic layer (a foam rubber layer, a sponge layer, and a silicon rubber layer)  200   b  disposed around this shaft  200   a ; a conductive layer  200   c ; and a mold-releasing layer  200   d.    
     As shown in  FIG. 6 , the conductive layer  200   c  includes: a second area A 2  through which a small-sized sheet is passed; first areas A 1  disposed adjacent to opposite ends of the second area A 2  in a longitudinal direction of the heating roller  200 ; and the whole sheet passing area A 3  including the first areas A 1  and the second area A 2 . 
     The conductive layer  200   c  includes: first conductive members  201   c  positioned in the first areas A 1 ; and a second conductive member  202   c  positioned in the second area A 2 . In the present embodiment, the conductive layer  200   c  is made of the same conductive material in a thickness direction, and made of different conductive materials in the longitudinal direction. That is, different conductive materials are utilized in the conductive members disposed in the first areas A 1  and the second area A 2 , and portions which connect the first conductive members  201   c  to the second conductive member  202   c  are disposed in the vicinity of boundaries between the first areas A 1  and the second area A 2 . For example, the first conductive members  201   c  are made of aluminum, and the second conductive member  202   c  is made of iron. The mold-releasing layer  200   d  is a thin film layer made of, for example, a heat-resistant silicon rubber, and a length of the heating roller  200  along the longitudinal direction is 330 mm. 
     The induction heating unit  700  is disposed externally along the heating roller  200 , and connected to the induction heating control circuit described above with reference to  FIG. 3 . The induction heating unit includes: an exciting coil  71  to which a predetermined power is supplied and which supplies a predetermined magnetic field to the heating roller  220 ; and a magnetic core  72 . It is to be noted that as the exciting coil  71 , a litz wire is usable which is constituted by bundling a plurality of copper wires having surfaces coated with an insulating material as described above. The magnetic core  72  can generate a magnetic flux in a concentrated manner. Consequently, the number of windings (turns) of the exciting coil  71  can be reduced, and the induction heating unit  700  can efficiently and locally heat a predetermined area of the heating roller  200 . 
     The fixing device  100  constituted in such manner is controlled by the induction heating control circuit shown in  FIG. 3  in the same manner as in the first embodiment. It is possible to apply an induction heating control method based on a temperature detection signal as shown in  FIG. 4 . Therefore, a driving frequency can be changed to thereby select the conductive member to be induction-heated in the same manner as in the first embodiment. Therefore, when the driving frequency is set around 20 kHz, the only second conductive member  202   c  made of iron can be induction-heated to generate heat. When the driving frequency is set to 60 kHz or more, it is possible to induction-heat both of the second conductive member  202   c  made of iron and the first conductive members  201   c  made of aluminum to thereby generate heat. 
     Therefore, during continuous printing of, for example, a small-sized sheet, even in a case where the temperature rises in the only end portions of the heating roller  200  that do not pass this small-sized sheet, the induction heating of the only end portions of the heating roller  200  can be stopped, and the induction heating of the center of the heating roller  200  can be continued. Accordingly, based on a detected temperature signal from the temperature detecting section  8 , the method of the present embodiment controls heat generation of the first conductive members  201   c  and the second conductive member  202   c  for use in the conductive layer  200   c  of the heating roller  200 , so that the surface temperature of the heating roller  200  along a longitudinal direction can be set to be uniform. 
     It is to be noted that in the present embodiment, a distance between the exciting coil  71  and an outer peripheral surface of the heating roller  200  is set to approximately 3 mm. 
     Third Embodiment 
     Next, there will be described another example of a first embodiment with reference to  FIGS. 7 ,  8 A, and  8 B.  FIG. 7  shows an example of a fixing device to which the present embodiment is applicable.  FIGS. 8A and 8B  show schematic diagrams of a heating roller  220  which is applicable to the fixing device shown in  FIG. 7 . 
     As shown in  FIG. 7 , a fixing device  120  includes: a fixing belt  12 ; the heating roller  220 ; a pressurizing roller  321 ; a fixing roller  322 ; and an induction heating unit  720 . 
     The induction heating unit  720  is disposed externally along the heating roller  220 , and the fixing belt  12  is sandwiched between the induction heating unit and the heating roller  220 . The induction heating unit is connected to an induction heating control circuit described above with reference to  FIG. 3 , and includes: an exciting coil  721  to which a predetermined power is supplied and which supplies a predetermined magnetic field to the heating roller  220 ; and a magnetic core  722 . 
     The fixing belt  12  is an endless member extended externally between the heating roller  220  and the fixing roller  322  while keeping its predetermined tensile force. The fixing belt  12  includes: a base member  121  made of a resin or the like having a resistance to thermal stress; and an elastic layer  122  and a mold-releasing layer  123  disposed in order externally along the base material  121 , that is, the heating roller  220 . In the present embodiment, the base member  121  is made of a polyimide resin having a thickness of 40 μm, the elastic layer  122  is made of a silicon rubber having a thickness of 300 μm, and the mold-releasing layer  123  is made of a fluorine resin having a thickness of 30 μm. In the present embodiment, a peripheral length of the fixing belt  12  is set so that the belt has a diameter of 70 mm. 
     The pressurizing roller  321  is constituted of: a shaft made of a material having a rigidity (hardness) such that the material does not deform under a predetermined pressure; and an elastic layer (fluorine rubber layer, silicon rubber layer) disposed around this shaft, and the pressurizing roller supplies the predetermined pressure to the fixing roller  322 . 
     The fixing roller  322  retains the fixing belt  12  together with the heating roller  220  while applying a predetermined tension to the fixing belt  12 , and is given the predetermined pressure from the pressurizing roller  321 . In the present embodiment, the fixing roller  322  is made of foam silicon sponge whose surface has low hardness and elasticity. 
     Accordingly, a nip having a predetermined width is formed between the fixing roller  322  and the pressurizing roller  321 . 
     The fixing roller  322  is rotated in a direction shown by an arrow CW at an approximately constant speed by a predetermined fixing motor (not shown). The pressurizing roller  321  is brought into contact with the fixing roller  322  under a predetermined pressure by a predetermined pressurizing mechanism (not shown). Therefore, when the fixing roller  322  is rotated, the pressurizing roller  321  is rotated in a counterclockwise direction shown by an arrow CCW, the direction being opposite to a direction in which the fixing roller  322  is rotated, in a position where the pressurizing roller comes into contact with the fixing roller  322 . The fixing belt  12  is moved with the rotation of this fixing roller  322 , and the heating roller  220  is rotated with the movement of this fixing belt  12 . 
     When a high-frequency current having a predetermined frequency is supplied to the exciting coil  721  connected to the induction heating control circuit shown in  FIG. 3 , a magnetic field is generated from the exciting coil  721  in accordance with the frequency of the high-frequency current, and an eddy current flows through a conductive layer  220 A of the heating roller  220  to which this magnetic field has been supplied. Accordingly, the Joule heat is generated in the conductive layer  220 A, and the heating roller  220  generates heat. Moreover, the fixing belt  12  brought into contact with the heating roller  220  which has generated heat is warmed by conduction of heat. A toner T on a sheet P passes through a nip formed between the pressurizing roller  321  and the fixing roller  322 , and is accordingly melted by this warmed fixing belt  12 . The melted toner T is attached to the sheet P under pressure, and an image on the sheet P is fixed to the sheet P. 
     Moreover, in the fixing belt  12 , a temperature detecting section  801  is disposed which detects a temperature of the surface of the fixing belt  12 . The temperature detecting section  801  includes: a first thermistor (not shown) which detects a surface temperature of each end area of the fixing belt  12  facing each end area A 1  of the heating roller  220 ; and a second thermistor (not shown) which detects a surface temperature of a central area of the fixing belt  12  facing a central area A 2  of the heating roller  220 . The present invention is not limited to this embodiment, and the temperature detecting section may include, for example, a third thermistor (not shown) which detects a surface temperature of a sheet non-passing area of the fixing belt  12 . 
     The heating roller  220  will be described in more detail. As shown in  FIG. 8A , the heating roller  220  includes: the central area A 2  through which a small-sized sheet is passed; the end areas A 1  adjacent to opposite ends of the central area A 2  in a longitudinal direction of the heating roller  2 ; and the whole sheet passing area A 3  including the end areas A 1  and the second area A 2 . The heating roller  220  includes the conductive layer  220 A constituted of a first conductive member  221 A positioned in at least the end area A 1  and a second conductive member  222 A positioned in the central area A 2 . For example, this first conductive member  221 A is positioned in the whole sheet passing area A 3  including the end areas A 1  and the central area A 2 , and the second conductive member  222 A is positioned in the only central area A 2 . That is, the central area A 2  has a double-layer structure of the first conductive member  221 A and the second conductive member  222 A. It is to be noted that the conductive layer  220 A has a thickness of, for example, 0.5 mm, and the thickness is formed to be approximately uniform. In the second area A 2  of the conductive layer  220 A, the second conductive member  222 A is disposed on a side close to the exciting coil  721  in the laminated first conductive member  221 A and second conductive member  222 A. 
     That is, in the central area A 2  of this conductive layer  220 A having a laminated structure, the second conductive member  222 A is disposed on the side close to the exciting coil  721 . Here, unlike the fixing device  1  shown in  FIG. 2 , the fixing device  120  has a constitution in which the induction heating unit  720  is disposed externally along the heating roller  220 . Therefore, as shown in  FIG. 8A , the second conductive member  222 A is disposed in an outer part of the conductive layer  220 A in the central area A 2  of the conductive layer  220 A. 
     In the fixing device  120  constituted in this manner, the first thermistor is regarded as the thermistor  81  shown in  FIG. 1 , the second thermistor is regarded as the thermistor  82  shown in  FIG. 1 , and it is possible to apply an induction heating control method based on a temperature detection signal as shown in  FIG. 4 . That is, a driving frequency can be changed to thereby select the conductive member to be induction-heated in the same manner as in the first embodiment. 
     Therefore, when the driving frequency is set around 20 kHz, the only second conductive member  222 A made of iron can be induction-heated to thereby generate heat. When the driving frequency is set to 60 kHz or more, it is possible to induction-heat both of the second conductive member  222 A made of iron and the first conductive members  221 A made of aluminum to thereby generate heat. 
     Therefore, during continuous printing of, for example, a small-sized sheet, even in a case where the temperature rises in the only end portions of the heating roller  220  that do not pass this small-sized sheet, the induction heating of the only end portions of the heating roller  220  can be stopped, and the induction heating of the center of the heating roller  220  can be continued. Accordingly, based on a detected temperature signal from the temperature detecting section  801 , the method of the present embodiment controls heat generation of the first conductive member  221 A and the second conductive member  222 A for use in the conductive layer  220 A of the heating roller  220 , so that the surface temperature of the heating roller  220  along a longitudinal direction can be set to be uniform. In consequence, the temperature of the fixing belt  12  can be set to be uniform in the longitudinal direction. 
     It is to be noted that in the present embodiment, the first conductive member  221 A of the conductive layer  220 A is made of aluminum, and the second conductive member  222 A is made of iron. The heating roller  220  is formed into a diameter of 20 mm, the fixing roller  322  is formed into a diameter of 30 mm, the whole length of the heating roller  220  in the longitudinal direction is set to 330 mm, and a length of the central area A 2  in the longitudinal direction is set to 180 mm. Furthermore, a distance between the exciting coil  721  and an outer peripheral surface of the heating roller  220  is set to approximately 2 mm. 
     Moreover, the heating roller  220  shown in  FIG. 7  may include a conductive layer  220 C shown in  FIG. 8B , 
     The conductive layer  220 C includes first conductive members  221 C positioned in the end areas A 1  and a second conductive member  222 C positioned in the central area A 2  in the same manner as in the conductive layer  200   c  shown in  FIG. 6 . As shown in  FIG. 8B , the conductive layer  220 C includes the same conductive material in a thickness direction, and includes different conductive materials in a longitudinal direction. The first conductive members  221 C are made of aluminum, and the second conductive member  222 C is made of iron. In the heating roller  220  having the conductive layer  220 C constituted in this manner, there is applicable an induction heating control method based on a temperature detection signal shown in  FIG. 4  in the same manner as in the heating roller  220  having the conductive layer  220 A. Therefore, the driving frequency can be changed to thereby select the conductive member to be induction-heated. 
     Fourth Embodiment 
     Next, there will be another example of the first embodiment with reference to  FIGS. 9 ,  10 , and  11 .  FIG. 9  shows an example of a fixing device to which the present embodiment is applicable.  FIG. 10  shows a schematic diagram of the fixing device shown in  FIG. 9  as viewed from a different direction.  FIG. 11  is a sectional view cut along the arrows E 1  and E 2 , showing a heating belt mounted on the fixing device shown in  FIG. 9 . 
     As shown in  FIG. 9 , a fixing device  130  includes: a heating belt  13 ; a pressurizing roller  331 ; a first fixing roller  332 ; a second fixing roller  333 ; an induction heating unit  730 ; and a temperature detecting section  831 . 
     The induction heating unit  730  is disposed externally along the heating belt  13 , and connected to an induction heating control circuit described above with reference to  FIG. 3 . The induction heating unit  730  includes: exciting coils  731  to which a predetermined power is supplied and which supplies a predetermined magnetic field to the heating belt  13 ; and a magnetic core  732 . The exciting coils  731  are arranged at an equal distance from the heating belt  13 . 
     The heating belt  13  is an endless member extended externally between the first fixing roller  332  and the second fixing roller  333  while keeping its predetermined tensile force. The heating belt  13  includes: a conductive layer  131 ; and an elastic layer  132  and a mold-releasing layer  133  disposed in order externally along this conductive layer  131 . 
     The pressurizing roller  331  is constituted of: a shaft made of a material having a rigidity (hardness) such that the material does not deform under a predetermined pressure; and an elastic layer (a fluorine rubber layer, a silicon rubber layer) disposed around this shaft. The pressurizing roller  331  applies a predetermined pressure to the first fixing roller  332 . 
     The first fixing roller  332  retains the heating belt  13  together with the second fixing roller  333  while applying a predetermined tension to the heating belt  13 , and is given the predetermined pressure from the pressurizing roller  331 . 
     The second fixing roller  333  is a cylindrical ceramic product (ceramics) formed into a diameter of, for example, 20 mm, and a thickness of 0.5 mm. However, the present invention is not limited to this embodiment, and the second fixing roller  333  may be made of, for example, iron, SUS430, SUS304, aluminum or the like. 
     Accordingly, a nip having a predetermined width is formed between the pressurizing roller  331  and the first fixing roller  332 . 
     The first fixing roller  332  is rotated in a direction shown by an arrow CW at an approximately constant speed by a predetermined fixing motor (not shown). The pressurizing roller  331  is brought into contact with the first fixing roller  332  under a predetermined pressure by a predetermined pressurizing mechanism (not shown). Therefore, when the first fixing roller  332  is rotated, the pressurizing roller  331  is rotated in a direction (arrow CCW direction) opposite to a direction in which the first fixing roller  332  is rotated in a position where the pressurizing roller comes into contact with the first fixing roller  332 . The heating belt  13  is moved with the rotation of this first fixing roller  332 , and the second fixing roller  333  is rotated with the movement of this heating belt  13 . 
     When a high-frequency current having a predetermined frequency is supplied to the exciting coils  731  connected to the induction heating control circuit shown in  FIG. 4 , a magnetic field is generated from the exciting coils  731  in accordance with the frequency of the high-frequency current, and an eddy current flows through a conductive layer  131  of the heating belt  13  to which this magnetic field has been supplied. Accordingly, the Joule heat is generated in the conductive layer  131 , and the heating belt  13  generates heat. A toner T on a sheet P is melted by the heating belt  13 . When the sheet passes through the nip formed between the pressurizing roller  331  and the first fixing roller  332 , the melted toner T is attached to the sheet P under pressure, and an image on the sheet P is fixed to the sheet P. 
     Moreover, in the heating belt  13 , the temperature detecting section  831  which detects a surface temperature of the heating belt  13  is disposed in a position facing the induction heating unit  730 . As shown in  FIG. 10 , the temperature detecting section  831  includes: a first thermistor  831  which detects a surface temperature of each first conductive member  1311  of the heating belt  13  facing each end area A 1 ; and a second thermistor  832  which detects a surface temperature of a second conductive member  1312  of the heating belt  13  facing a central area A 2 . The present invention is not limited to this embodiment, and the temperature detecting section may include, for example, a third thermistor (not shown) which detects a surface temperature of a sheet non-passing area of the heating belt  13 . 
     The conductive layer  131  will be described in more detail. As shown in  FIGS. 10 and 11 , the conductive layer  131  includes: the central area A 2  through which a small-sized sheet is passed; the end areas A 1  adjacent to opposite ends of the central area A 2  in a direction Y (hereinafter referred to as “longitudinal direction”) crossing a moving direction X of the heating belt  13  at right angles; and the whole sheet passing area A 3  including the end areas A 1  and the central area A 2 . 
     As shown in  FIG. 11 , the heating belt  13  includes the conductive layer  131  constituted of the first conductive members  1311  positioned in the end areas A 1  and the second conductive member  1312  positioned in the central area A 2 . The first conductive member  1311  is made of stainless steel (SUS303), and the second conductive member  1312  is made of nickel. These first conductive member  1311  and second conductive member  1312  are bonded to an elastic layer  132 . 
     Furthermore, nickel can generate heat in a frequency region (around 20 kHz) in which iron generates heat. That is, the second conductive member  1312  made of nickel has a frequency region of 20 kHz or more. On the other hand, since nonmagnetic stainless steel has a low magnetic permeability, a heating efficiency is low with a high-frequency current of about 30 kHz, an amount of heat to be generated is small, and heat can be generated at 60 kHz or more. That is, the first conductive members  1311  made of nonmagnetic stainless steel does not easily generate heat in a frequency region (around 20 kHz) in which nickel generates heat, and the members can sufficiently generate heat in a higher frequency region (around 60 kHz). That is, when a first frequency region F 1  is below 40 kHz, the only second conductive member  1312  made of nickel can be induction-heated in this first frequency region F 1 . When a second frequency region F 2  is 40 kHz or more, it is possible to induction-heat both of the second conductive member  1312  made of nickel and the first conductive members  1311  made of nonmagnetic stainless steel in this second frequency region F 2 . 
     In the fixing device  130  constituted in this manner, the first thermistor  831  is regarded as the thermistor  81  shown in  FIG. 1 , the second thermistor  832  is regarded as the thermistor  82  shown in  FIG. 1 , and it is possible to apply an induction heating control method based on a temperature detection signal as shown in  FIG. 4 . That is, a driving frequency can be changed to thereby select the conductive member to be induction-heated in the same manner as in the first embodiment. 
     That is, when the driving frequency is set around 20 kHz, the only second conductive member  1312  made of nickel can be induction-heated to thereby generate heat. When the driving frequency is set to 60 kHz or more, it is possible to induction-heat both of the second conductive member  1312  made of nickel and the first conductive members  1311  made of nonmagnetic stainless steel to thereby generate heat. 
     Therefore, during continuous printing of, for example, a small-sized sheet, even in a case where the temperature rises in the only end portions of the heating belt  13  that do not pass this small-sized sheet, the induction heating of the only end portions of this heating belt  13  can be stopped, and the induction heating of the center of the heating belt  13  can be continued. Accordingly, based on a detected temperature signal from the temperature detecting section  831 , the method of the present embodiment controls heat generation of the first conductive members  1311  and the second conductive member  1312  for use in the conductive layer  131  of the heating belt  13 , so that the surface temperature of the heating belt  13  along a longitudinal direction can be set to be uniform. 
     Moreover, the present invention is not limited to this embodiment, and the central area A 2  may have a constitution in which the first conductive member  1311  and the second conductive member  1312  are laminated as described above with reference to, for example,  FIG. 2 . 
     In the present embodiment, the conductive layer  131  is formed into a thickness of 40 μm, the elastic layer  132  is made of a silicon rubber having a thickness of 300 μm, and the mold-releasing layer  123  is made of a fluorine resin having a thickness of 30 μm. As stainless steel for use in the first conductive members  1311 , a nonmagnetic material is used. 
     Fifth Embodiment 
     Next, there will be described another example of the first embodiment with reference to  FIGS. 12 ,  13 , and  14 .  FIG. 12  shows an example of a fixing device to which the present embodiment is applicable.  FIGS. 13 ,  14  show schematic diagrams of the fixing device shown in  FIG. 12  as viewed from a different direction. It is to be noted that components having the same constitutions and functions as those of components shown in  FIGS. 1 to 4  are denoted with the same reference numerals, and detailed description is omitted. 
     As shown in  FIG. 12 , a fixing device  140  includes: a pressurizing roller  3 ; a pressurizing spring  4 ; a peeling claw  5 ; a cleaning roller  6 ; an induction heating unit  7 ; a temperature detecting section  8 ; a thermostat  9 ; and a heating roller  230 . 
     The heating roller  230  includes: a rolled conductive layer  231  constituted by forming an adjusted magnetism alloy into a cylindrical shape; and a mold-releasing layer  232  disposed on an outer peripheral surface of this conductive layer  231  and made of a fluorine resin such as a ethylene tetrafluoride resin. It is to be noted that the adjusted magnetism alloy is an alloy having a characteristic that the alloy loses its magnetism at a raised temperature, and a temperature at which the alloy loses its magnetism is the Curie temperature (magnetism transition point). 
     The adjusted magnetic alloy for use in the conductive layer  231  is made of a composite alloy of nickel and iron, having the Curie temperature in the vicinity of a set temperature (e.g., 180° C.) of the fixing device  140 . The adjusted magnetism alloy for use in this conductive layer  231  has a magnetic characteristic adjusted so that the magnetic characteristic (magnetic permeability) rapidly degrades at the Curie temperature. When the magnetic permeability degrades, the depth of penetration of an eddy current flowing through the conductive layer  231  increases (deepens), and a magnetic flux penetrates the pressurizing roller  321 . Therefore, an electric resistance of the conductive layer  231  is reduced, generation of the Joule heat by the eddy current is reduced, and an amount of heat to be generated is also reduced. 
     In the present embodiment, the conductive layer  231  is made of the adjusted magnetism alloy whose Curie temperature has been adjusted into 200° C. As shown in  FIGS. 13 ,  14 , the conductive layer  231  includes a central area A 2  through which a small-sized sheet is passed, and end areas A 1  adjacent to the central area A 2  in a longitudinal direction of the heating roller  2 . 
     The induction heating unit  7  is connected to an induction heating control circuit shown in  FIG. 3  as described above, and includes an exciting coil  71  to which a predetermined power is supplied and which supplies a predetermined magnetic field to the heating roller  230 . Accordingly, a CPU  24  drives an inverter circuit  29  at a predetermined driving frequency, and a high-frequency current is generated from the inverter circuit  29  and supplied to the exciting coil  71 , thereby induction-heating the conductive layer  231  of the heating roller  230 . 
     As shown in  FIGS. 13 ,  14 , the temperature detecting section  8  includes a thermistor  81  which detects a surface temperature of each first area A 1  which is an end portion of the heating roller  230 , and a thermistor  82  which detects a surface temperature of the second area A 2  which is the center of the heating roller  2 . 
     As shown in  FIG. 3 , a current detection circuit  35  detects the high-frequency current supplied from the inverter circuit  29  to the exciting coil  71  via a current transformer  34 , and outputs a detected current signal corresponding to this high-frequency current to the CPU  24 . The CPU  24  can detect a change of an electric resistance of the conductive layer  231  by use of this current detection circuit  35 . This will be described hereinafter. 
     When the conductive layer  231  reaches the Curie temperature as described above, the electric resistance of the conductive layer  231  is reduced. This weakens magnetic bonding between the conductive layer  231  and the exciting coil  71 , and a load resistance of the exciting coil  71  is reduced. Therefore, the current flowing through the exciting coil  71  increases. When the current detection circuit  35  detects that the current flowing through this exciting coil  71  exceeds a defined range, the CPU  24  can detect that the electric resistance of the conductive layer  231  has changed. 
     When the temperature of the conductive layer  231  is lower than the Curie temperature, as shown in  FIG. 13 , the eddy current flowing through the conductive layer  231  flows through both of each end area A 1  and the central area A 2  of the conductive layer  231 , and the whole layer is substantially uniformly heated. For example, at a warming-up time when the surface temperature of the heating roller  230  is heated at the set temperature, or in a case where an image is fixed to an A3 or A4 lateral size sheet passed through the whole sheet passing area including the end areas A 1  and the central area A 2 , as shown in  FIG. 13 , the eddy current is flowed through the conductive layer  231 , and the whole conductive layer  231  is substantially uniformly heated. 
     On the other hand, during continuous printing of a small-sized sheet (vertical A 4 , B 5  or the like), even in a case where the temperature rises in the only end portions of the heating roller  230  that do not pass this small-sized sheet, and the temperature of each end area A 1  of the heating roller  230  is above the Curie temperature of 200° C., the magnetic permeability of the end area A 1  of the conductive layer  231  degrades. This increases the depth of penetration of the eddy current flowing through the end portions of the conductive layer  231 . As shown in  FIG. 14 , any eddy current is not generated in the end areas A 1  of the conductive layer  231 , and the eddy current flows through the central area A 2  of the conductive layer  231 . Therefore, since the heating roller  230  is not heated at 200° C. or more, a temperature difference of the heating roller  230  in the longitudinal direction can be inhibited from being enlarged. 
     Next, there will be described an induction heating control method based on the change of the electric resistance of the conductive layer  231  detected from the detected current supplied to the exciting coil  71  with reference to  FIG. 15 . This method is applicable to the fixing device  140  described above with reference to  FIGS. 12 to 14 . 
     As described above, the CPU  24  drives the inverter circuit  29  at the predetermined driving frequency (20 kHz in the present embodiment), the high-frequency current generated by the inverter circuit  29  is supplied to the exciting coil  71 , and the conductive layer  231  of the heating roller  230  is induction-heated. In a case where each end area A 1  of the heating roller  230  exceeds the Curie temperature of 200° C., the electric resistance of each end area A 1  of the heating roller  230  drops, the magnetic bonding between the conductive layer  231  and the exciting coil  71  weakens, and the load resistance of the exciting coil  71  is reduced. This increases the current flowing through the exciting coil  71 . 
     The current supplied to the exciting coil  71  and detected by the current detection circuit  35  via the current transformer  34  is compared with the defined range of the value of the current flowing through the conductive layer  231  whose temperature does not reach the Curie temperature (S 11 ). When the current detected by the current detection circuit  35  falls in the defined range (S 11 —YES), it is judged that the conductive layer  231  does not reach the Curie temperature. Moreover, the inverter circuit  29  is controlled at a driving frequency of 20 kHz as such (S 12 ), and the high-frequency current corresponding to this driving frequency of 20 kHz is supplied to the exciting coil  71 . 
     On the other hand, in a case where the current detected by the current detection circuit  35  exceeds the defined range in the step S 11  (S 11 —NO), it is judged that the conductive layer  231  has exceeded the Curie temperature. Moreover, the inverter circuit  29  is controlled at a driving frequency of 50 kHz (S 13 ), and a high-frequency current corresponding to this driving frequency of 50 kHz is supplied to the exciting coil  71 . 
     Moreover, the control method in the fixing device  140  of the present embodiment is not limited to this example, and there may be performed, for example, an induction heating control method based on the change of the electric resistance of the conductive layer  231  detected using the temperature detecting section  8  which detects the temperature of the heating roller  230 . There will be described the induction heating control method based on the change of the electric resistance of the conductive layer  231  detected from the temperature detected by the temperature detecting section  8  described above with reference to  FIG. 16 . 
     As described above, the CPU  24  drives the inverter circuit  29  at a driving frequency of, for example, 20 kHz, the high-frequency current is generated by the inverter circuit  29  and supplied to the exciting coil  71 , and the conductive layer  231  of the heating roller  230  is thus induction-heated. The thermistor  81  detects the temperature of each end area A 1  of the heating roller  230  induction-heated in this manner. Moreover, the temperature detected by the thermistor  81  is compared with the Curie temperature of the adjusted magnetism alloy for use in the conductive layer  231  at 200° C. (S 21 ). In a case where the temperature detected by the thermistor  81  is not more than 200° C. (S 21 —YES), the inverter circuit  29  is controlled at the driving frequency of 20 kHz as such (S 22 ), and the high-frequency current corresponding to this driving frequency of 20 kHz is supplied to the exciting coil  71 . 
     On the other hand, in a case where the temperature detected by the thermistor  81  is above 200° C. in the step S 21  (S 21 —NO), the inverter circuit  29  is controlled at a driving frequency of 50 kHz (S 23 ), and the high-frequency current corresponding to this driving frequency of 50 kHz is supplied to the exciting coil  71 . 
     As described above, in the induction heating control method of the present embodiment, (1) the driving frequency of the inverter circuit  29  is changed from 20 kHz to 50 kHz in a case where the current detected by the current detection circuit  35  exceeds the defined range. Moreover, (2) in a case where the temperature detected by the thermistor  81  exceeds the Curie temperature (200° C.), the thermistor being disposed in the end portion of the heating roller  230  in the longitudinal direction, the driving frequency of the inverter circuit  29  is changed from 20 kHz to 50 kHz. 
     As described above, when the temperature of the heating roller  231  is below the Curie temperature, the depth of penetration in the conductive layer  231  is small, and an apparent load resistance of the heating roller  230  is large. Therefore, as described above, the load resistance in a case where the only central area A 2  of the heating roller  230  is heated is set to be substantially equal to that in a case where the whole sheet passing area including the end areas A 1  and the central area A 2  of the heating roller  230  is heated at the driving frequency of 20 kHz. Therefore, the only central area A 2  of the heating roller  230  can be induction-heated without largely charging the current. In a case where the current detected by the current detection circuit  35  falls in the defined range, or the temperature detected by the thermistor  81  is not more than 200° C., the driving frequency of the inverter circuit  29  is 20 kHz. In consequence, the whole heating roller  230  can be heated. 
     Therefore, during continuous printing of, for example, a small-sized sheet, even in a case where the temperature rises in the only end portions of the heating roller  230  that do not pass this small-sized sheet, the end areas A 1  of the heating roller  230  made of the adjusted magnetism alloy does not generate any heat at the Curie temperature, and the only central area A 2  of the heating roller  230  can be heated. In consequence, the surface temperature of the heating roller  230  in the longitudinal direction can be uniform. 
     In the present embodiment, the conductive layer  231  of the heating roller  230  is formed into a thickness of 1 mm and a diameter of 40 mm. It has been described in the present embodiment that the driving frequency at which the whole heating roller  230  is induction-heated is 20 kHz, but the present invention is not limited to this embodiment, and the driving frequency may be changed in accordance with a material, positional relation, and the like of the exciting coil  71  or the conductive layer  230 . It is to be noted that the driving frequency to induction-heat the whole heating roller  230  is in a range of preferably 20 to 40 kHz, more preferably 20 to 30 kHz. The driving frequency to induction-heat the only central area A 2  of the heating roller  230  is in a range of preferably 40 kHz to 60 kHz. 
     The present invention is not limited to the above embodiments as such, and constituting elements can be modified and embodied in an implementation stage without departing from the scope. An appropriate combination of a plurality of constituting elements disclosed in the above embodiments can form various inventions. For example, several constituting elements may be removed from all of the constituting elements described in the embodiments. Furthermore, the constituting elements of different embodiments may be appropriately combined. 
     For example, as described in the above embodiments, iron has a high magnetic permeability and generates a large amount of heat as compared with aluminum. Therefore, as shown in  FIGS. 17 to 19 , a magnetic core  741  facing a conductive layer  241  made of aluminum may have a configuration which is different from that of a magnetic core  742  facing a conductive layer  242  made of iron. It is to be noted that  FIG. 17  shows a schematic diagram of a heating roller and an induction heating unit which are applicable to the present invention.  FIG. 18  shows a sectional view cut along the arrows E 3  and E 4  shown in  FIG. 17 .  FIG. 19  is a sectional view cut along the arrows E 5  and E 6  shown in  FIG. 17 . 
     This example will be described in more detail. As shown in  FIG. 17 , a heating roller  240  includes the conductive layers  241  corresponding to end areas A 1  and made of aluminum, and the conductive layer  242  corresponding to a central area A 2  and made of iron. An induction heating unit  740  includes the magnetic cores  741  disposed in the end areas A 1 , and the magnetic cores  742  disposed in the central area A 2 . 
     As shown in  FIG. 18 , the magnetic core  742  holds an exciting coil  744 , and this exciting coil  744  has a spiral shape around the axial center which is a virtual line N intersecting with an axis M of the heating roller  240 . This magnetic core  742  is disposed on a side opposite to that on which the exciting coil  744  faces the conductive layer  242 , and in the center of the exciting coil  744 . On the other hand, as shown in  FIG. 19 , the magnetic core  741  holds an exciting coil  745 , and this exciting coil  745  also has a spiral shape around the axial center which is a virtual line N in the same manner as in the exciting coil  744 . The magnetic core  741  is disposed on a side opposite to that on which the exciting coil  745  faces the conductive layer  241 , in the center of the exciting coil  745 , and externally along the exciting coil. That is, the magnetic core  741  is formed into a shape to surround the exciting coil  745 , and disposed closer to the heating roller  240 . 
     As described above, the magnetic cores  741  have many portions disposed close to the exciting coil  745  and the heating roller  240  as compared with the magnetic cores  742 , and a magnetic flux from the exciting coil  745  can be concentrated more intensely. Therefore, it is possible to increase an amount of heat to be generated by the conductive layer  241  of each end area A 1  opposed to the magnetic cores  741 , that is, the conductive layer  241  made of aluminum having a smaller amount of heat to be generated as compared with iron. Therefore, it is possible to reduce a difference of the amount of heat to be generated between the conductive layer  241  made of aluminum and the conductive layer  242  made of iron. 
     Moreover, there is not any restriction on the IGBTs  27  and  28  shown in  FIG. 3  as long as they are switching elements, and in the present embodiments, they are preferably switching elements for use under large pressure and current, such as the IGBTs or MOS-FET. 
       FIG. 20  shows an example of a fixing device to which the present embodiment is applicable. 
     As shown in  FIG. 20 , a fixing device  901  includes: a fixing belt (an endless unit)  910 ; a satellite roller (a roller)  920 ; a pressurizing roller (a second conveying unit)  321 ; a fixing roller (a first conveying unit)  322 ; and an induction heating unit  940 . 
     The induction heating unit  940  is disposed externally along the fixing roller  322 , and the fixing belt  910  is sandwiched between the induction heating unit  940  and the fixing roller  322 . The induction heating unit  940  is connected to an induction heating control circuit described above with reference to  FIG. 3 , and includes: an exciting coil  941  to which a predetermined power is supplied and which supplies a predetermined magnetic field to the satellite roller  920 ; and a magnetic core  942 . 
     The fixing belt  910  is an endless member extended externally between the satellite roller  920  and the fixing roller  322  while keeping its predetermined tensile force. The fixing belt  910  includes: a base member  911  made of a resin or the like having a resistance to thermal stress; and an elastic layer  912  and a mold-releasing layer  913  disposed in order externally along the base member  911 , that is, the satellite roller  920 . In the present embodiment, the base member  911  is made of a polyimide resin having a thickness of 40 μm, the elastic layer  912  is made of a silicon rubber having a thickness of 300 μm, and the mold-releasing layer  913  is made of a fluorine resin having a thickness of 30 μm. In the present embodiment, a peripheral length of the fixing belt  910  is set so that the belt has a diameter of 70 mm. 
     The pressurizing roller  321  is constituted of: a shaft made of a material having a rigidity (hardness) such that the material does not deform under a predetermined pressure; and an elastic layer (fluorine rubber layer, silicon rubber layer) disposed around this shaft, and the pressurizing roller supplies the predetermined pressure to the fixing roller  322 . 
     The fixing roller  322  retains the fixing belt  910  together with the satellite roller  920  while applying a predetermined tension to the fixing belt  910 , and is given the predetermined pressure from the pressurizing roller  321 . In the present embodiment, the fixing roller  322  is made of foam silicon sponge whose surface has low hardness and elasticity. 
     Accordingly, a nip having a predetermined width is formed between the fixing roller  322  and the pressurizing roller  321 . 
     The fixing roller  322  is rotated in a direction shown by an arrow CW at an approximately constant speed by a predetermined fixing motor (not shown). The pressurizing roller  321  is brought into contact with the fixing roller  322  under a predetermined pressure by a predetermined pressurizing mechanism (not shown). Therefore, when the fixing roller  322  is rotated, the pressurizing roller  321  is rotated in a counterclockwise direction shown by an arrow CCW, the direction being opposite to a direction in which the fixing roller  322  is rotated, in a position where the pressurizing roller comes into contact with the fixing roller  322 . The fixing belt  910  is moved with the rotation of this fixing roller  322 , and the satellite roller  920  is rotated with the movement of this fixing belt  910 . 
     When a high-frequency current having a predetermined frequency is supplied to the exciting coil  941  connected to the induction heating control circuit shown in  FIG. 3 , a magnetic field is generated from the exciting coil  941  in accordance with the frequency of the high-frequency current. Moreover, the fixing belt  910  brought into contact with the satellite roller  920  which has generated heat is warmed by conduction of heat. A toner T on a sheet P passes through a nip formed between the pressurizing roller  321  and the fixing roller  322 , and is accordingly melted by this warmed fixing belt  910 . The melted toner T is attached to the sheet P under pressure, and an image on the sheet P is fixed to the sheet P. 
     Moreover, in the fixing belt  910 , a temperature detecting section  801  is disposed which detects a temperature of the surface of the fixing belt  910 . The temperature detecting section  801  includes: a first thermistor (not shown) which detects a surface temperature of each end area of the fixing belt  910  facing each end area of the satellite roller  920 ; and a second thermistor (not shown) which detects a surface temperature of a central area of the fixing belt  910  facing a central area of the satellite roller  920 . The present invention is not limited to this embodiment, and the temperature detecting section may include, for example, a third thermistor (not shown) which detects a surface temperature of a sheet non-passing area of the fixing belt  910 . 
     The satellite roller  920  is made from at least one of an aluminum, a stainless steel (a stainless iron), an iron, a copper, and a silver materials. The satellite roller  920  is formed to be shaped in a cylinder having a diameter of 15 mm to 25 mm and a thickness of 0.3 mm to 5 mm, and have the same length as the fixing roller  322 . 
     The electric power consumed by the fixing device  901  is 80% of the electric power consumed by an image forming apparatus (Multi-Functional Peripheral, so called an MFP). In addition, most of the electric power is consumed at the standby (non-image output) and warm-up. Therefore, reducing the electric power consumed at the warm-up is greatly required. In other words, if a time necessary for the warm-up can be reduced, much electric power does not need to be consumed at the standby and the warm-up can be performed in a short time. 
     On the other hand, “gloss” is very important in color use. For a user who prints out photographic images, “gloss” is significantly important. The “gloss” depends largely on a provided temperature. Therefore, if the temperature is varied, “uneven gloss” becomes noticeable. The “uneven gloss” is associated with the rotation cycle of the fixing belt  910 . In this background, the fixing belt  910  is supported by the satellite roller  920  and the thermal use efficiency at the warm-up is enhanced by the satellite roller  920 . By optimizing the thermal capacity of the satellite roller  920 , the thermal use efficiency at the warm-up can be easily enhanced. 
       FIG. 21  is a schematic block diagram showing image forming apparatus  1001  loading fixing device  1026  of the embodiments of the present invention. Image forming apparatus  1001  has cassette mechanism  1003  for feeding sheets of paper P, which are media to be fixed, to image forming unit  1002  and has scanner section  1006  for reading documents D fed by automatic document feeder  1004  on the top thereof. On conveyor path  1007  from cassette mechanism  1003  to image forming unit  1002 , register rollers  1008  are installed. 
     Image forming unit  1002  includes, around photosensitive drum  1011 , charger  1012  for uniformly charging photosensitive drum  1011  sequentially according to the rotational direction of arrow q of photosensitive drum  1011 , laser exposure apparatus  1013  for forming latent images on charged photosensitive drum  1011  on the basis of image data from scanner  1006 , developing apparatus  1014 , transfer charger  1016 , separation charger  1017 , cleaner  1018 , and discharging LED  1020 . Image forming unit  1002  forms toner images on photosensitive drum  1011  by the known image forming process by the electro-photographic method and transfers them onto sheets of paper P. 
     On the downstream side of image forming unit  1002  in the conveying direction of sheets of paper P, ejection paper conveyor path  1022  for conveying sheets of paper P on which toner images are transferred toward paper ejection section  1021  is installed. On ejection paper conveyor path  1022 , conveyor belt  1023  for conveying sheets of paper P separated from photosensitive drum  1011  to fixing device  1026  and paper ejection rollers  24  for ejecting sheets of paper P after passing fixing device  1026  to paper ejection section  1021  are installed. 
     Next, fixing device  1026  will be described.  FIG. 22  is a schematic block diagram showing fixing device  1026 , and  FIG. 23  is a schematic side view showing fixing device  1026 , and  FIG. 24  is a block diagram showing control system  1100  for heating heat roller  1027 . Fixing apparatus  1026  has heat roller  1027  which is an endless member and pressure roller  1028  which is a pressure member pressed to heat roller  1027 . Furthermore, fixing device  1026  has induction heating coils  1030 ,  1040 , and  1050  which are an induced current generation means for a 100 V power source for heating heat roller  1027  via a gap of about 3 mm on the outer periphery of heat roller  1027 . Induction heating coils  1030 ,  1040 , and  1050  are in an almost coaxial shape with heat roller  1027 . 
     Furthermore, on the outer periphery of heat roller  1027 , separation pawl  1031  for preventing sheets of paper P after fixing from wrapping, thermistors  1032   a  and  1032   b  for detecting the surface temperature of heat roller  1027 , thermostat  1033  for detecting an abnormal surface temperature of heat roller  1027  and interrupting heating, and a cleaning roller  1034  are installed. In heat roller  1027 , around core bar  1027   a , expanded rubber  1027   b  with a thickness of 5 mm, metallic conductive layer  1027   c , made of nickel (Ni), with a thickness of 40 μm, solid rubber layer  1027   d  with a thickness of 200 μm, and release layer  1027   e  with a thickness of 30 μm are sequentially formed to a diameter of 40 mm. Solid rubber layer  1027   d  and release layer  1027   e  form a protective layer. 
     Pressure roller  1028  is composed of core bar  1028   a  around which surface layer  1028   b  such as silicone rubber or fluorine rubber is coated in a diameter of 40 mm. Pressure roller  1028 , since shaft  1028   c  is pressed by pressure spring  1036 , is pressed to heat roller  1027 . By doing this, between heat roller  1027  and pressure roller  1028 , a fixed nipping width is formed. Further, around pressure roller  1028 , cleaning roller  1037  is installed. 
     Induction heating coils  1030 ,  1040 , and  1050  are respectively supplied with a driving current, generate a magnetic field, generate an eddy current in metallic conductive layer  1027   c  by this magnetic field, and heat metallic conductive layer  1027   c . Induction heating coils  1030 ,  1040 , and  1050  respectively heat areas A, B, and C of hear roller  1027  in the longitudinal direction. Induction heating coils  1030 ,  1040 , and  1050  have the same structure though they are different in length. Induction heating coils  1030 ,  1040 , and  1050  are composed of magnetic material cores  30   a ,  40   a , and  50   a  around which electric wires  1030   b ,  1040   b , and  1050   b  are wound  11  turns. Electric wires  1030   b ,  1040   b , and  1050   b  using heat resistant polyamide-imide copper wires are composed of a litz wire of 1050 bundled copper wires with a wire diameter of 0.3 mm. Electric wires  1030   b ,  1040   b , and  1050   b  are formed as a litz wire, so that an AC current can flow effectively. Namely, the copper loss of electric wires  1030   b ,  1040   b , and  1050   b  can be suppressed. 
     Induction heating coils  1040  and  1050  for heating areas B and C on both sides of heat roller  1027  are connected in series and are driven under the same control. Depending on a case of fixing large sheets of paper such as horizontal size A4 or A3 or a case of fixing vertical size A4 or other sheets of paper of small size, the driving ratio of induction heating coils  1030 ,  1040 , and  1050  is controlled, thus the temperature distribution of heat roller  1027  in the longitudinal direction is made uniform. 
     Next, control system  100  for heating heat roller  1027  will be described. As shown in the block diagram in  FIG. 24 , control system  100  for heating heat roller  1027  has inverter circuit  1060  for supplying a driving current to induction heating coils  1030 ,  1040 , and  1050 , rectifier circuit  1070  for supplying a DC supply voltage of 100 V to inverter circuit  1060 , and CPU  1080  for controlling whole the entire image forming apparatus  1001  and controlling inverter circuit  1060  according to detection results of thermistors  1032   a  and  1032   b . CPU  1080 , according to the detection results of thermistors  1032   a  and  1032   b , may drive so as to output induction heating coil  1030  or only either of induction heating coils  1040  and  1050  and may drive simultaneously induction heating coil  1030  and both induction heating coils  1040  and  1050 . 
     Rectifier circuit  1070  is for 100 V and rectifies a current from commercial AC power source  1071  to a direct current at 100 V and supplies it to inverter circuit  1060 . Between rectifier circuit  1070  and commercial AC power source  1071 , power monitor  1072  is connected, detects power supplied from commercial AC power source  1071 , and feeds it back to CPU  1080 . 
     Inverter circuit  1060  uses a self excitation type semi-E class circuit. To induction heating coil  1030  of inverter circuit  1060 , first capacitor  1061   a  for resonance is connected in parallel to form first resonance circuit  1061  and to induction heating coils  1040  and  1050  connected in series, second capacitor  1062   a  for resonance is connected in parallel to form second resonance circuit  1062 . To first resonance circuit  1061 , first switching element  1063   a  is connected in series to form first inverter circuit  1063  and to second resonance circuit  1062 , second switching element  1064   a  is connected in series to form second inverter circuit  1064 . Switching elements  1063   a  and  1064   a  use an IGBT usable at a high breakdown voltage and a large current. Switching elements  1063   a  and  1064   a  may be a MOS-FET. 
     To the control terminals of switching elements  1063   a  and  1064   a , IGBT driving circuits  1066  and  1067  for turning on switching elements  1063   a  and  1064   a  are respectively connected. CPU  1080  controls the application timing of IGBT driving circuits  1066  and  1067 . Inverter circuit  1060  controls the ON time of switching elements  1063   a  and  1064   a  by CPU  1080 , thereby converts the frequency to 40 to 70 kHz. Induction heating coils  1030 ,  1040 , and  1050 , by supply of a drive current at a frequency of 40 to 70 kHz, generate a predetermined magnetic field. 
     One cycle of the frequency by inverter circuit  1060 , as shown in  FIG. 25 , is the time of the ON time of switching elements  1063   a  and  1064   a  plus the OFF time thereof. The ON time (O′-P′ shown in  FIG. 25 ) of switching elements  1063   a  and  1064   a  is controlled by CPU  1080  and the OFF time (P′-S′ shown in  FIG. 25 ) is the time until first capacitor  1061   a  or second capacitor  1062   a  is discharged. Namely, the OFF time of switching elements  1063   a  and  1064   a  varies with the temperature conditions of heat roller  1026  and induction heating coils  1030 ,  1040 , and  1050 . Therefore, the frequency by inverter circuit  1060  varies with the shape of induction heating coils  1030 ,  1040 , and  1050  and the values of capacitors  1061   a  and  1062   a.    
     Therefore, to drive induction heating coils  1030 ,  1040 , and  1050  at a frequency of 40 kHz or higher, the shape of induction heating coils  1030 ,  1040 , and  1050  must be changed from that of the coils driven at a frequency of 20 to 40 kHz. 
     Next, the electric characteristics of induction heating coils  1030 ,  1040 , and  1050  will be considered. Firstly, generally, in the induction heating method, a transformer model in which the induction heating coil is assumed as primary side coil L 1 , and the loss part thereof is assumed as resistance Rc, and the heat roller is assumed as secondary side coil L 2  and load resistance R is shown in  FIG. 26 . Firstly, load resistance R is greatly changed depending on the magnetic coupling intensity between the induction heating coil and the heat roller, and to instantaneously heat the heat roller by an eddy current generated on secondary side coil L 2  by the magnetic field of primary side coil L 1 , load resistance R is preferably larger. Namely, when the ratio of load resistance R of secondary coil L 2  which is a heat roller to inductance L of primary side coil L 1  which is an induction heating coil is large, large output can be obtained by a small current. 
     Secondly, load resistance R varies with the frequency of the induction heating coil. When the frequency is increased, the penetration depth of the eddy current in the heat roller becomes shallow and the eddy current easily flows on the surface of the heat roller. Generally, when a current flows through a conductor, it is not distributed at a fixed density overall the section. The current is apt to flow through a part of secondary side coil L 2 , which is a heat roller, whose impedance is small. Generally, this current polarization is called a skin effect. The skin effect can be obtained remarkably as the frequency increases. The eddy current generated in the conductor flows on the surface of the conductor due to the skin effect, and when the frequency of the induction heating coil is increased due to changes in the penetration depth of the eddy current, load resistance R has a tendency to increase. 
     The degree of concentration of the current onto the surface is expressed by the depth of penetration of the current and Formula 1 is held. 
       Depth of penetration=503×√{square root over (((ρ/(μ f )))}(cm)  (Formula 1) where 
     ρ: resistivity of conductor (Ω/cm), 
     μ: relative permeability of conductor, and 
     f: frequency (Hz). 
     When the depth of penetration expressed by Formula 1 becomes smaller (shallow), the current flows more only on the surface of the conductor, and the current density is increased, and the heat value is also increased. In this embodiment, the frequency is increased, thus by the skin effect, efficient heat generation of metallic conductive layer  1027   c  with a thickness of 40 μm is realized. For example, instead of the conventional frequency 20 kHz, induction heating coils  1030 ,  1040 , and  1050  are driven at 40 kHz, the depth of penetration becomes 1/√{square root over (2)} times of the conventional one. Therefore, when the frequency is increased, load resistance R is increased and the leakage rate of the magnetic flux is reduced. However, when the ratio of load resistance R to inductance L of the induction heating coil is small, to obtain the same output, the current may be increased. However, the current supplied to the induction heating coil is controlled and restricted according to the withstand current of the switching element such as an IGBT. Therefore, as long as the current supplied to the induction heating coil does not exceed the withstand current of the switching element, an experiment of obtaining a condition for obtaining a high heating efficiency by the heat roller is conducted and it is found that a ratio of L/R (H/Ω) of inductance L of the induction heating coil to load resistance R of the heat roller may conform to L/R&lt;35×10 −6  (H/Ω). 
     Further, even if the condition of the induction heating coil and heat roller conforms to L/R&lt;35×10 −6  (H/Ω), when the respective values of inductance L of the induction heating coil and load resistance R of the heat roller are too large and impedance Z is 10Ω or more, at a frequency of 40 kHz or higher, the heat roller cannot obtain a desired quantity of heat. 
     Therefore, the condition of the induction heating coil at a frequency of 40 kHz or higher and at a voltage of 100 V is that coil impedance ZΩ is Z&lt;10Ω and the ratio of L/R (H/Ω) of inductance L of the induction heating coil to load resistance R of the heat roller is L/R&lt;35×10 −6  (H/Ω). 
     Induction heating coils  1030 ,  1040 , and  1050  of this embodiment conform to the aforementioned condition. In induction heating coils  1030 ,  1040 , and  1050 , when the frequency is increased, the impedance is increased, so that within the conventional range from 20 to 40 kHz, although the number of turns of the coils is 14 turns, it can be reduced to 11 turns and the structure can be miniaturized. 
     When induction heating coils  1030 ,  1040 , and  1050  of this embodiment are driven at a frequency of 60 kHz or 40 kHz, inductance L and load resistance R show the results shown in  FIG. 26 . When the coils are driven at a frequency of 60 kHz (Experiment 1), inductance L is 16 (μH), and load resistance R is 1Ω, and L/R=16×10 −6  (H/Ω) is held, and when the coils are driven at a frequency of 40 kHz (Experiment 2), inductance L is 17 (μH), and load resistance R is 0.8Ω, and L/R=21×10 −6  (H/Ω) is held, and both cases conform to L/R&lt;35×10 −6  (H/Ω). On the other hand, when the frequency is set to 25 kHz (Comparison example 1), inductance L is 18 (μH), and load resistance R is 0.43Ω, and L/R=42×10 −6  (H/Ω) is held. The values of inductance L and load resistance R are values measured by an LCR meter by changing the frequency. 
     As a result, the time required from supply of the drive current to induction heating coils  1030 ,  1040 , and  1050  by inverter circuit  60  to arrival of the surface temperature of heat roller  1027  at 180 degree. C. is 40 seconds in Comparison example 1, while it is 32 seconds in Experiment 1 and 35 seconds in Experiment 2, thus a high heating efficiency is obtained. 
     Next, the operation of the invention will be described. When the image forming process starts, in image forming unit  2 , photosensitive drum  11  rotating in the direction of arrow q is uniformly charged by charger  1012  and is irradiated with a laser beam according to document information by laser exposure apparatus  1013 , thus an electrostatic latent image is formed. Next, the electrostatic latent image is developed by developing apparatus  1014  and a toner image is formed on photosensitive drum  1011 . 
     The toner image on photosensitive drum  1011  is transferred onto the sheet of paper P by transfer charger  1016 . Next, the sheet of paper P is separated from photosensitive drum  1011  and then is inserted between heat roller  1027  rotating in the direction of arrow r of fixing device  1026  and pressure roller  1028  rotating in the direction of arrow s to heat, pressurize, and fix the toner image. In fixing device  1026 , according to detection results of the surface temperature of heat roller  1027  by thermistors  1032   a  and  1032   b , when necessary, first inverter circuit  1063  or second inverter circuit  1064  is driven by CPU  80  and a drive current, for example, at 60 kHz is supplied to induction heating coils  1030 ,  1040 , and  1050 . 
     By doing this, the frequency of the drive current by the first or second inverter circuit  1063  or  1064  is high, so that by the skin effect of an eddy current generated by the magnetic field of induction heating coils  1030 ,  1040 , and  1050 , a current is concentrated upon metallic conductive layer  1027   c  of heat roller  1027 . Therefore, heat roller  1027  reaches a desired fixable temperature at a high speed of about 32 seconds and thereafter, the fixable temperature can be easily maintained and controlled under the ON-OFF control of inverter circuit  1060 . 
     According to this embodiment, the ratio of L/R (H/Ω) of inductance L of induction heating coils  1030 ,  1040 , and  1050  for a power source of 100 V to load resistance R of heat roller  1027  is L/R&lt;35×10 −6  (H/Ω), and coil impedance ZΩ is set to Z&lt;10Ω, and a drive current at a high frequency of 40 to 70 kHz is supplied. Therefore, even if metallic conductive layer  1027   c  is formed thinly such as 40 μm, the eddy current generated by induction heating coils  1030 ,  1040 , and  1050  is concentrated upon metallic conductive layer  27   c  by the skin effect, and the leakage of the magnetic flux is reduced, and the heat generation efficiency of heat roller  27  can be improved. By doing this, rapid fixing, energy conservation, and precise temperature control can be realized easily. 
     Furthermore, when the frequency of the drive current of induction heating coils  1030 ,  1040 , and  1050  is increased, the impedance of induction heating coils  1030 ,  1040 , and  1050  can be increased. Therefore, compared with a case of using a drive current at a low frequency, the number of turns of electric wires  1030   b ,  1040   b , and  1050   b  for obtaining the same output can be reduced. As a result, miniaturization and lightweight of induction heating coils  1030 ,  1040 , and  1050  are realized and the degree of freedom of design of fixing device  1026  can be improved. 
     Next, the second embodiment of the present invention will be explained. In the second embodiment, the electric characteristics of the induction heating coils are set to those for a power source of 200 V, thus an inverter circuit for 200 V is used, and the other is the same as that of the first embodiment. Therefore, in the second embodiment, to the same components as those of the first embodiment, the same numerals are assigned and the detailed explanation will be omitted. 
     Although induction heating coils  1130 ,  1140 , and  1150  shown in  FIG. 27  of the second embodiment have the electric characteristics for the power source of 200 V, the ratio L/R (H/Ω) of inductance L of induction heating coils  1030 ,  1040 , and  1050  for obtaining a high heating efficiency by metallic conductive layer  1027   c  of heat roller  1027  to load resistance R of heat roller  1027 , as described in the first embodiment, may conform to L/R&lt;35×10 −6  (H/Ω). However, the supply voltage is two times of that of the first embodiment such as 200 V, so that coil impedance ZΩ of induction heating coils  1130 ,  1140 , and  1150  is required to conform to z&lt;20Ω. 
     Therefore, this embodiment forms induction heating coils  1130 ,  1140 , and  1150  conforming to the aforementioned condition. Namely, induction heating coils  1130 ,  1140 , and  1150  for the 200 V power source are formed by winding electric wires  1130   b ,  1140   b , and  1150   b  round magnetic material cores  1130   a ,  1140   a , and  1150   a  by 18 turns. Further, within the conventional frequency range from 20 to 40 kHz, the coil impedance is small, so that the number of turns of the coil is increased to 22 turns. On the other hand, in this embodiment, the number of turns of the coil can be reduced to 18 turns, so that the induction heating coils can be miniaturized. 
     Further, electric wires  1130   b ,  1140   b , and  1150   b  of induction heating coils  1130 ,  1140 , and  1150  use heat resistant polyamide-imide copper wires. Electric wires  1130   b ,  1140   b , and  1150   b  are composed of a litz wire of 24 bundled copper wires with a wire diameter of 0.3 mm. Electric wires  130   b ,  140   b , and  150   b  are formed as a litz wire, so that the copper loss can be suppressed. 
     Compared with induction heating coils  1030 ,  1040 , and  1050  for the 100 V power source, the current flowing through induction heating coils  1130 ,  1140 , and  1150  is little, so that the number of twists of copper wires of the litz wire is reduced. 
     Further, rectifier circuit  1070  is formed for 200 V, rectifies a current from commercial AC power source  1071  to a direct current at 200 V, and supplies it to inverter circuit  1060 . 
     When induction heating coils  1130 ,  1140 , and  1150  of this embodiment are driven at a frequency of 60 kHz or 40 kHz, inductance L and load resistance R show the results shown in  FIG. 28 . When the coils are driven at a frequency of 60 kHz (Experiment 3), inductance L is 80 (μH), and load resistance R is 4.1Ω, and L/R=20×10 −6  (H/Ω) is held, and when the coils are driven at a frequency of 40 kHz (Experiment 4), inductance L is 85 (μH), and load resistance R is 3.2Ω, and L/R=27×10 −6  (H/Ω) is held, and both cases conform to L/R&lt;35×10 −6  (H/Ω). 
     As a result, the time required from supply of the drive current to induction heating coils  1130 ,  1140 , and  1150  by inverter circuit  60  to arrival of the surface temperature of heat roller  1027  at 180 degree. C. is 28 seconds in Experiment 3, while it is 32 seconds in Experiment 4, thus a high heating efficiency is obtained. 
     According to this embodiment, the ratio of L/R (H/Ω) of inductance L of induction heating coils  1130 ,  1140 , and  1150  for the power source of 200 V to load resistance R of heat roller  1027  is L/R&lt;35×10 −6  (H/Ω), and coil impedance ZΩ is set to Z&lt;20Ω, and a drive current at a high frequency of 40 to 70 kHz is supplied. Therefore, in the same way as with the first embodiment, the eddy current generated by induction heating coils  1130 ,  1140 , and  1150  is concentrated upon metallic conductive layer  27   c  formed thinly such as 40 μm, and the leakage of the magnetic flux is reduced, and the heat generation efficiency of heat roller  1027  can be improved. By doing this, in fixing device  1026 , rapid fixing, energy conservation, and precise temperature control during fixing can be realized easily. 
     Furthermore, compared with a case of using a drive current at a low frequency, the number of turns of electric wires  1130   b ,  1140   b , and  1150   b  for obtaining the same output can be reduced. As a result, miniaturization and lightweight of induction heating coils  1130 ,  1140 , and  1150  are realized and the degree of freedom of design of fixing device  1026  can be improved. Next, the third embodiment of the present invention will be explained. The third embodiment is different from the first embodiment in that even if the supply voltage used by the induction heating coils is either of 100 V and 200 V, induction heating coils having the same electric characteristics are used and the other is the same as that of the first embodiment. 
     Therefore, in the third embodiment, to the same components as those of the first embodiment, the same numerals are assigned and the detailed explanation will be omitted. 
     As shown in  FIG. 29  of the third embodiment, induction heating coils  1230 ,  1240 , and  1250  may conform to that within the drive frequency range from 20 to 40 kHz of inverter circuit  60  driven by 100 V power source  1270 , coil impedance ZΩ is Z&lt;10Ω and the ratio of L/R (H/Ω) of inductance L of the induction heating coils to load resistance R of the heat roller is L/R&lt;35×10 −6 . 
     Further, simultaneously, as shown in  FIG. 30 , induction heating coils  1230 ,  1240 , and  1250  make it a condition that the frequency when driving inverter circuit  1060  by 200 V power source  1280  is within the range from 50 to 80 kHz, and coil impedance ZΩ is Z&lt;20Ω, and the ratio of L/R (H/Ω) of inductance L of the induction heating coils to load resistance R of the heat roller is L/R&lt;35×10 −6  (H/Ω). The electric characteristics of induction heating coils  1230 ,  1240 , and  1250  are shared by 100 V power source  1270 , so that the coil impedance has a tendency to be reduced. 
     Therefore, when driving induction heating coils  1230 ,  1240 , and  1250  by 200 V power source  1280 , the frequency is increased to 50 to 80 kHz to obtain a predetermined output. 
     When the induction heating coils do not conform to the aforementioned condition, to generate a minimal quantity of heat fixable in fixing device  1026 , the drive frequency of inverter circuit  1060  driven by 100 V power source  1270  is reduced to 20 kHz or lower. Namely, inverter circuit  1060  must be driven at a frequency in the audible zone and noise is caused at the time of driving. 
     Further, the coil impedance is preferably not too low. When the coil impedance is low, if the coils are driven by 200 V power source  1280 , the frequency must be made higher. However, when the frequency is increased, highly efficient switching elements  1063   a  and  1064   a  must be used and the cut-down of cost is disturbed. On the other hand, when low-priced general-purpose switching elements  1063   a  and  1064   a  are used, as the frequency is increased, the characteristics of switching elements  1063   a  and  1064   a  are deteriorated and the switching efficiency is reduced. Therefore, coil impedance for enabling the frequency when driving inverter circuit  1060  to retain a range of not deteriorating switching elements  1063   a  and  1064   a  is desirable. 
     Therefore, this embodiment forms induction heating coils  1230 ,  1240 , and  1250  conforming to the aforementioned condition. Namely, induction heating coils  1230 ,  1240 , and  1250  shared by 100 V power source  1270  and 200 V power source  1280  are formed by winding electric wires  1230   b ,  1240   b , and  1250   b  round magnetic material cores  1230   a ,  1240   a , and  1250   a  by 16 turns. 
     Electric wires  1230   b ,  1240   b , and  1250   b  of induction heating coils  1230 ,  1240 , and  1250  are composed of a litz wire of 50 bundled heat resistant polyamide-imide copper wires with a wire diameter of 0.3 mm. Electric wires  1230   b ,  1240   b , and  1250   b  share the same induction heating coils  1230 ,  1240 , and  1250  for 100 V power source  1270  and 200 V power source  1280 , so that the electric wires have a litz wire structure of 100 V power source  1270  in which a large current flows. Further, when 200 V power source  1280  is used, in the conventional fixing device in which the frequency is within the range from 20 to 40 kHz, the coil impedance is low, so that the number of turns of the coil is increased to 22 turns. On the other hand, induction heating coils  1230 ,  1240 , and  1250 , since the number of turns can be reduced to 16 turns, are miniaturized. 
     When induction heating coils  1230 ,  1240 , and  1250  of this embodiment are driven at a frequency of 40 kHz or 20 kHz by 100 V power source  1270 , inductance L and load resistance R show the results shown in  FIG. 31 . When the coils are driven at a frequency of 40 kHz (Experiment 5), inductance L is 28 (μH), and load resistance R is 1.7Ω, and 
     L/R=16×10 −6  (H/Ω) is held, and when the coils are driven at a frequency of 20 kHz (Experiment 6), inductance L is 30 (μH), and load resistance R is 1.1Ω, and 
     L/R=27×10 −6  (H/Ω) is held, and both cases conform to L/R&lt;35×10 −6 ×10 −6  (H/Ω). 
     Further, when induction heating coils  1230 ,  1240 , and  1250  are driven at a frequency of 80 kHz or 50 kHz by 200 V power source  1280 , inductance L and load resistance R show the results shown in  FIG. 32 . When the coils are driven at a frequency of 80 kHz (Experiment 7), inductance L is 26 (μH), and load resistance R is 2.6Ω, and 
     L/R=10×10 −6  (H/Ω) is held, and when the coils are driven at a frequency of 50 kHz (Experiment 8), inductance L is 27 (μH), and load resistance R is 1.9Ω, and 
     L/R=14×10 −6  (H/Ω) is held, and both cases conform to L/R&lt;35×10 −6  (H/Ω). 
     According to this embodiment, the ratio of L/R (HΩ) of inductance L of induction heating coils  1230 ,  1240 , and  1250  to load resistance R of heat roller  1027  is set to L/R&lt;35×10 −6  (H/Ω), and when 100 V power source  1270  is used, coil impedance ZΩ is set to Z&lt;10Ω, and when 200 V power source  1280  is used, coil impedance ZΩ is set to Z&lt;20Ω. By doing this, induction heating coils  1230 ,  1240 , and  1250  common to both 100 V power source  1270  and 200 V power source  1280  can be used. Therefore, by common use, induction heating coils  1230 ,  1240 , and  1250  can be mass-produced and the cost can be reduced. 
     Further, when driving induction heating coils  1230 ,  1240 , and  1250  by 200 V power source  1280 , the frequency is increased to 50 to 80 kHz. Therefore, even if metallic conductive layer  1027   c  is formed thinly such as 40 μm, the eddy current generated by induction heating coils  1230 ,  1240 , and  1250  is concentrated upon metallic conductive layer  1027   c  by the skin effect, and the leakage of the magnetic flux is reduced, and the heat generation efficiency of heat roller  1027  can be improved. 
     Furthermore, compared with the conventional induction heating coils for the 200 V power source driven at a low frequency, induction heating coils  1230 ,  1240 , and  1250  of this embodiment are driven at a high frequency of 50 to 80 kHz, so that even if the number of turns of electric wires  1230   b ,  1240   b , and  1250   b  is reduced, the same output can be obtained. Therefore, compared with the conventional induction heating coils, in induction heating coils  1230 ,  1240 , and  1250  of this embodiment, the number of turns of electric wires  1230   b ,  1240   b , and  1250   b  can be reduced, and miniaturization and lightweight of induction heating coils  1230 ,  1240 , and  1250  are realized, and the degree of freedom of design of fixing device  1026  can be improved. 
     Further, the present invention is not limited to the aforementioned embodiments and within the scope of the present invention, can be modified variously, and for example, the endless member may be in a belt shape, and the material of the metallic conductive layer may be unrestrictedly stainless steel, aluminum, or a composite material of stainless steel and aluminum. Further, the thickness of the metallic conductive layer is not restricted and optional. However, to reduce the thermal capacity, shorten the warming-up time, realize energy conservation, and exactly control the temperature, the metallic conductive layer is desirably thinned to 10 to 100 μm or so. Further, the conveying direction of a medium to be fixed by the fixing device is also optional and an apparatus for conveying vertically a medium to be fixed is acceptable. Further, if the induction heating coils conform to L/R&lt;35×10 −6  (H/Ω) and the coil impedance setting condition, the shape thereof, the wire thickness and kind, and the number of turns of wires are not limited. Furthermore, the kind and characteristics of electronic parts such as the switching elements used in the inverter circuit are not limited and they may supply a desired current to the induction heating coils. Further, this embodiment is based on the fixing device in which the coils are arranged outside the rollers. However, the embodiment can be applied also to a fixing device in which the coils are arranged inside the heat rollers. 
     As described above, according to the present invention, by the induction heating coils, the heat generation efficiency of the metallic conductive layer can be improved and rapid fixing, energy conservation, and precise temperature control can be realized easily. 
     Further, miniaturization and lightweight of the induction heating coils are realized and the degree of freedom of design of the fixing device can be improved. 
     Furthermore, in the present embodiments, any conductive material that satisfies the above-described conditions is applicable to the conductive layer, and there is used, for example, a stainless steel alloy, copper, a composite material of stainless steel and aluminum or the like. 
     In addition, there has been described an example of a half bridge circuit as the induction heating control circuit shown in  FIG. 3 , but the present invention is not limited to this example, and there is not any restriction on the circuit as long as the circuit can change its frequency. There may be used, for example, a semi-E-class inverter circuit (one switching element) for general use. 
     Moreover, the end areas A 1  have been referred to also as the end portions because they are disposed in the opposite ends of the central area A 2  in the above embodiments, but the present invention is not limited to this constitution, and the end area A 1  may be disposed on only one side of the central area A 2 . 
     Furthermore, in the above embodiments, a generated heat distribution is divided by two types of metals, but the distribution may include three or more different types of metals in a constitution whose frequency can be changed among three or more types of frequencies.