hereinafter , an embodiment of the present invention will be described with reference to the drawings . however , the scope of the invention is not limited to the illustrated examples . an image forming apparatus 1 a according to a first embodiment will be described below with reference to the drawings . it is noted that in the drawings , the same components or portions are denoted by the same reference signs , and description thereof will be omitted . as illustrated in fig1 , the image forming apparatus 1 a is an electrophotographic color printer . the image forming apparatus 1 a includes a control unit 4 configured to control each portion and each unit of the image forming apparatus 1 a , an image forming section 10 , two paper feed cassettes 21 , a conveyance unit 40 , a sensing unit 50 and a fuser 60 . the image forming section 10 includes image forming units 11 y , 11 m , 11 c , and 11 k disposed respectively with charging devices , developing devices , or the like around photoreceptor drums 12 configured to form images of yellow ( y ), magenta ( m ), cyan ( c ), and black ( k ) colors , an intermediate transfer belt 15 configured to primarily transfer toner images formed at the image forming units 11 y , 11 m , 11 c , and 11 k and combine the transferred toner images , a secondary transfer roller 18 configured to secondarily transfer the combined toner image from the intermediate transfer belt to a paper sheet p ( recording medium ), and an exposure unit 16 using a laser beam . it is noted that the configuration and function of an image forming section 10 of this kind is conventionally well known , and detailed description thereof will be omitted . the paper feed cassette 21 is a box - shaped case in which paper sheets p are stacked , and is provided to be drawn from a front side ( front side of fig1 ) of the image forming apparatus 1 . the conveyance unit 40 is configured to convey the paper sheets p in the image forming apparatus , and includes a pickup roller 41 , a paper feed roller 42 , a separation roller 43 , a conveyance roller pair 44 , a timing roller pair 45 , and an exit roller pair 46 . in the conveyance unit 40 , one paper sheet p on the uppermost layer of the paper sheets p stacked in the paper feed cassette 21 is picked up by the pickup roller 41 , and fed by the paper feed roller 42 and the separation roller 43 . further , the fed paper sheet p is conveyed downstream of a conveyance direction by the conveyance roller pair 44 , and fed from the timing roller pair 45 to the secondary transfer roller 18 . the secondary transfer roller 18 applies an electric field , and the toner image on the intermediate transfer belt 15 is transferred to the paper sheet p . then , the paper sheet p is fed to the fuser 60 for thermal fusing of toner , and output by the exit roller pair 46 into a paper output tray 2 provided on an upper surface of the image forming apparatus 1 . the sensing unit 50 includes a paper feed sensor 52 positioned between the paper feed roller 42 and the conveyance roller pair 44 , and a registration sensor 54 positioned immediately in front of the timing roller pair 45 . both of the paper feed sensor 52 and the registration sensor 54 detect passage or arrival of the paper sheet p being conveyed . ( detailed description of fuser , see fig2 to 4 ) as illustrated in fig2 , the fuser 60 is configured such that the paper sheet p having the toner image transferred thereon is held between a fusing belt 62 and a pressure roller 74 . then the toner image is fixed on the paper sheet p by heat from the fusing belt 62 . further , the fuser 60 includes a belt guide 64 , a pad 66 , a holding plate 68 , a halogen heater 70 , a reflection plate 72 , a detection unit 76 , and a power cut - off unit 78 , in addition to the fusing belt 62 and the pressure roller 74 . the fusing belt 62 includes a release layer having a base layer including a heat resistant resin such as polyimide , an elastic layer including for example silicone rubber on the base layer , and a surface layer including for example pfa . additionally , the fusing belt 62 is held in substantially a circular shape by the belt guides 64 disposed either end of the fusing belt 62 in a width direction as illustrated in fig3 , and the pad 66 provided on the inner peripheral side of the fusing belt 62 illustrated in fig2 , and extending in the width direction . the holding plate 68 is provided on the inner peripheral side of the fusing belt 62 along the pad 66 for compensation of the rigidity of the pad 66 , holds the pad 66 , and has both ends fixed to the belt guides 64 connected to a frame of an apparatus body . the halogen heater ( heat source ) 70 and the reflection plate 72 are provided on the inner peripheral side of the fusing belt 62 to irradiate the fusing belt 62 with infrared light . radiant heat of the infrared light heats the fusing belt 62 . it is noted that , in the present embodiment , the halogen heater 70 is positioned on one side of an inner periphery of the fusing belt 62 , and the pad 66 and the holding plate 68 are provided on the other side of the inner periphery across the reflection plate 72 . as illustrated in fig2 , the pressure roller 74 forms , together with the fusing belt 62 , a nip portion n configured to hold the paper sheet p therein . additionally , the pressure roller 74 is rotationally driven in a direction indicated by an arrow a , and this rotational driven force drives the fusing belt 62 to be rotated in a direction indicated by an arrow b , while maintaining the substantially circular shape . it is noted that the paper sheet p is conveyed to the nip portion n , while holding a surface having the toner transferred thereon , toward the fusing belt 62 . the detection unit 76 is configured to detect contraction of the fusing belt 62 ( peripheral inward deformation of the belt ) immediately before ignition of the belt . additionally , the detection unit 76 is a conductive rod - shaped member , and provided at a place ( on one side ) between the inner peripheral surface of the fusing belt 62 and the halogen heater 70 . as illustrated in fig3 , the detection unit 76 extends in the width direction of the fusing belt 62 . additionally , both ends of the detection unit 76 are connected to terminals t of the power cut - off unit 78 described below . specifically , the terminals t of the power cut - off unit 78 are disposed in rod insertion holes provided in the belt guides 64 . both ends of the detection unit 76 as the rod - shaped member are inserted into the rod insertion holes of the belt guides 64 , respectively , and the ends of the detection unit 76 are put on the terminals t of the power cut - off unit 78 , and connected with the terminal t by their own weight . the power cut - off unit 78 is a so - called relay ( drive circuit ) for controlling power supply to the halogen heater 70 . normally , the detection unit 76 is in contact with the terminals t of the power cut - off unit 78 by its own weight . in this condition , the detection unit 76 and the power cut - off unit 78 forma closed circuit , and the power supply to the halogen heater 70 is maintained . however , when the fusing belt 62 is abnormally overheated and contracted , the inner peripheral surface of the fusing belt 62 makes contact with the detection unit 76 , and the detection unit 76 is pressed toward the inner periphery of the fusing belt 62 . therefore , the detection unit 76 having a rod shape is bent , and finally separated from and comes off the terminals t of the power cut - off unit 78 . as a result , as illustrated in fig4 , the circuit formed by the detection unit 76 and the power cut - off unit 78 is changed from the closed circuit to an open circuit , and power supply to the halogen heater 70 is stopped by the power cut - off unit 78 . that is , the detection unit 76 as the rod - shaped member is disposed to be operated as described above at a position where the detection unit 76 makes contact with part of the fusing belt 62 deformed and moved peripherally inward . in the fuser 60 and the image forming apparatus 1 a including the fuser 60 , when the fusing belt 62 is deformed peripherally inward , the detection unit 76 having the rod shape is pressed peripherally inward , bent , and finally comes off . as a result , the circuit including the detection unit 76 and the power cut - off unit 78 is changed to the open circuit , and the power supply to the halogen heater 70 is stopped . that is , in the fuser 60 and the image forming apparatus 1 a including the fuser 60 , the peripheral inward deformation of the fusing belt 62 is directly detected , and the power supply to the halogen heater 70 is stopped based on the detection of the deformation . accordingly , the fuser 60 and the image forming apparatus 1 a including the fuser 60 overcomes the problem with the conventional fuser having the mechanism for cutting off power supplied to the heat source based on monitoring of temperature of the fusing belt , for example , difficulty in measurement of the temperature by the detection unit due to separation in distance between the fusing belt and the detection unit provided on the outer peripheral side of the fusing belt . that is , the fuser 60 and the image forming apparatus 1 a including the fuser 60 can further accurately detect the abnormal overheating of the fusing belt 62 , and prevent the ignition of the fusing belt 62 . further , the detection unit 76 is provided on one side of the inner periphery of the fusing belt 62 where the halogen heater 70 is positioned . the one side where the halogen heater 70 is positioned has a temperature higher than that of the other side where the pad 66 and the holding plate 68 are positioned . accordingly , in the fuser 60 and the image forming apparatus 1 a including the fuser 60 , the detection unit 76 is provided at a part of the fusing belt 62 where the temperature tends to rise , so that the detection unit 76 can detect the contraction caused by the abnormal overheating of the fusing belt 62 earlier , and can prevent the ignition of the fusing belt 62 , compared with the detection unit provided at the other part of the fusing belt 62 . an image forming apparatus 1 b according to a second embodiment and the image forming apparatus 1 a according to the first embodiment are different in configuration of the detection unit 76 of the fuser 60 . the detection unit 76 according to the second embodiment includes a lever 80 , and an optical sensor 82 having a set of a light emitting element and a light receiving element . detailed description will be made below . as illustrated in fig5 , the lever 80 of the detection unit 76 according to a second embodiment has one side positioned on the inner peripheral side of the fusing belt 62 . additionally , the lever 80 has the other side mounted with a plate - shaped member 80 a configured to block light from the light emitting element of the optical sensor 82 . normally , the light from the light emitting element is blocked by the plate - shaped member 80 a , and power is supplied to the halogen heater 70 . however , as illustrated in fig6 , when the fusing belt 62 is abnormally overheated and contracted , the inner peripheral surface of the fusing belt 62 makes contact with the one side of the lever 80 of the detection unit 76 . in this condition , the one side of the lever 80 is pressed toward the inner periphery of the fusing belt 62 . when the one side of the lever 80 is pressed , the other side of the lever 80 is moved around the fulcrum , accompanied by movement of the plate - shaped member 80 a blocking light from the light emitting element of the optical sensor 82 . accordingly , blocking of the light from the light emitting element by the plate - shaped member 80 a is released , and the light from the light emitting element is received at the light receiving element . when the light is received by the light receiving element , a signal is generated . the power cut - off unit 78 receives the signal to be operated , and power supply to the halogen heater 70 is stopped . even in the image forming apparatus 1 b configured as described above , the abnormal overheating of the fusing belt 62 can be detected further accurately , and the ignition of the fusing belt 62 can be prevented . the other configurations of the present second embodiment are similar to those of the first embodiment . accordingly , description of the present second embodiment is identical to that of the first embodiment except for the detection unit 76 . an image forming apparatus 1 c according to a third embodiment and the image forming apparatus 1 a according to the first embodiment are different in configuration of the detection unit 76 of the fuser 60 . the detection unit 76 according to the third embodiment includes an optical sensor having a set of a light emitting element 84 and a light receiving element 86 . detailed description will be made below . as illustrated in fig7 , in the detection unit 76 according to the third embodiment , the light emitting element 84 is provided at the belt guide 64 on one end side of the fusing belt 62 in a width direction , and the light receiving element 86 is provided at the belt guide 64 on the other end side . normally , light b 1 from the light emitting element 84 is received at the light receiving element 86 . in this condition , the power supply to the halogen heater 70 is maintained . however , as illustrated in fig8 , when the fusing belt 62 is abnormally overheated and contracted , the inner peripheral surface of the fusing belt 62 intrudes an optical path of the light b 1 . therefore , the light from the light emitting element 84 is blocked . accordingly , a signal is generated from the light receiving element 86 , the power cut - off unit 78 receiving the signal is operated , and the power supply to the halogen heater 70 is stopped . even in the image forming apparatus 1 c configured as described above , the abnormal overheating of the fusing belt 62 can be detected further accurately , and the ignition of the fusing belt 62 can be prevented . the other configurations of the present third embodiment are similar to those of the first embodiment . accordingly , description of the present third embodiment is identical to that of the first embodiment except for the detection unit 76 . an image forming apparatus 1 d according to a fourth embodiment and the image forming apparatus 1 a according to the first embodiment are different in configurations of the detection unit 76 and the power cut - off unit 78 . detailed description will be made below . as illustrated in fig9 , a detection unit 76 according to the fourth embodiment includes an optical sensor having a set of a light emitting element 88 and a light receiving element 90 provided outside the fusing belt 62 . the power cut - off unit 78 according to the fourth embodiment represents a triac 92 configured to control the power supply to the halogen heater 70 . here , the triac 92 is operated according to an ignition prevention control program of a sub cpu 100 provided at a control unit 4 . it is noted that , in the image forming apparatus 1 d , temperature control of the halogen heater 70 is performed by a triac 94 connected to a main cpu 102 provided at the control unit 4 . in the detection unit 76 and the power cut - off unit 78 according to the fourth embodiment , normally , an optical axis b 2 of the light emitting element 88 crosses the outer peripheral surface of the fusing belt 62 . that is , the fusing belt 62 blocks light from the light emitting element 88 . therefore , the light from the light emitting element 88 cannot be received by the light receiving element 90 . in this condition , the triac 92 is on , and power is supplied to the halogen heater 70 . however , as illustrated in fig1 , when the fusing belt 62 is abnormally overheated and contracted , the optical axis b 2 of the light emitting element 88 and the outer peripheral surface of the fusing belt 62 are uncrossed , the light from the light emitting element 88 is received by the light receiving element 90 , and the light receiving element 90 transmits a signal to the sub cpu 100 . in the sub cpu 100 receiving the signal , the ignition prevention control program is activated , and the triac 92 is turned off . accordingly , power supply to the halogen heater 70 is stopped . even in the image forming apparatus 1 d configured as described above , the abnormal overheating of the fusing belt 62 can be detected further accurately , and the ignition of the fusing belt 62 can be prevented . further , in the image forming apparatus 1 d , the triac 92 configured to control the power supply to the halogen heater 70 is connected to the sub cpu 100 , and the triac 94 configured to control the temperature is connected to the main cpu 102 . accordingly , in the image forming apparatus 1 d , even if the main cpu 102 is out of control , an ignition prevention control program of the sub cpu 100 independent of the main cpu 102 is activated , and thus , the image forming apparatus 1 d has higher safety than an image forming apparatus having one cpu to control the triacs 92 and 94 . the other configurations of the present fourth embodiment are similar to those of the first embodiment . accordingly , description of the present fourth embodiment is identical to that of the first embodiment except for the detection unit 76 and the power cut - off unit 78 . an image forming apparatus 1 e according to a fifth embodiment is different from the image forming apparatus 1 d according to the fourth embodiment in that the detection unit 76 of the fuser 60 is a distance sensor . detailed description will be made below . as illustrated in fig1 , in the detection unit 76 according to the fifth embodiment , the detection unit 76 provided outside the fusing belt 62 is a laser displacement meter , and a distance between the detection unit 76 and the outer peripheral surface of the fusing belt 62 is measured . normally , the distance d between the detection unit 76 and the outer peripheral surface of the fusing belt 62 is maintained constant . in this condition , the triac 92 is on , and power is supplied to the halogen heater 70 . however , when the fusing belt 62 is abnormally overheated and contracted , the distance d between the detection unit 76 and the outer peripheral surface of the fusing belt 62 is increased . then , when the distance d between the detection unit 76 and the outer peripheral surface of the fusing belt 62 exceeds a threshold , the detection unit 76 transmits a signal to the sub cpu 100 . in the sub cpu 100 receiving the signal , the ignition prevention control program is activated , and the triac 92 is turned off . accordingly , power supply to the halogen heater 70 is stopped . even in the image forming apparatus 1 e configured as described above , the abnormal overheating of the fusing belt 62 can be detected further accurately , and the ignition of the fusing belt 62 can be prevented . further , in the image forming apparatus 1 e , even if the main cpu 102 is out of control , the ignition prevention control program of the sub cpu 100 independent of the main cpu 102 is activated , and thus , the image forming apparatus 1 e has higher safety than an image forming apparatus having one cpu to control the triacs 92 and 94 . another method of measuring the distance d by the detection unit 76 includes a method of measuring a distance from an exciting coil to the outer peripheral surface of the fusing belt 62 , as the change of a capacitor component of an induction circuit , when the heat source of the fuser 60 uses ih . the other configurations of the present fifth embodiment are similar to those of the fourth embodiment . accordingly , description of the present fifth embodiment is identical to that of the fourth embodiment except for the detection unit 76 . it should be understood that the fuser and the image forming apparatus according to the present invention are not limited to the above - mentioned embodiments , and various alterations or modification may be made without departing from the scope of the invention . for example , as the power cut - off unit , a transistor may be used in place of the relay . further , the heat source is not limited to the halogen heater , and the fusing belt may include an arbitrary material . still further , the detection unit or the power cut - off unit according to each embodiment may be combined . as described above , the present invention is useful for a fuser and an image forming apparatus , and particularly , the present invention is superior in further accurately detecting abnormal overheating of a fusing belt , and preventing ignition of the fusing belt . although the present invention has been described and illustrated in detail , it is clearly understood that the same is by way of illustrated and example only and is not to be taken by way of limitation , the scope of the present invention being interpreted by terms of the appended claims .