A fixing apparatus, comprising: a belt including a ferromagnetic layer; a roller facing the belt, an image on a sheet being fixed as the sheet passes through a nip formed between the belt and the roller; a ferromagnetic plate disposed inside the belt; an induction heater configured to cause heat generation in the ferromagnetic layer and the ferromagnetic plate; a driving circuit configured to output a high frequency current to the induction heater by switching on and off switching elements thereof; and a controller configured to determine whether or not a temperature of the ferromagnetic plate is higher than the Curie point thereof, and control the driving circuit to reduce a level of the high frequency current when the temperature of the ferromagnetic plate is determined to be higher than the Curie point thereof, characterized in that the fixing apparatus further comprises: a detector including a coil and a detection circuit that is configured to detect an electrical resistance of the coil, which changes depending on whether or not a magnetic flux is formed, wherein the detector is configured to detect whether or not a magnetic flux having a magnetic path that passes through a surface of the ferromagnetic plate is formed; and the controller determines whether or not the temperature of the ferromagnetic plate is higher than the Curie point thereof, based on a detection result of the detector. a belt including a ferromagnetic layer; a roller facing the belt, an image on a sheet being fixed as the sheet passes through a nip formed between the belt and the roller; a ferromagnetic plate disposed inside the belt; an induction heater configured to cause heat generation in the ferromagnetic layer and the ferromagnetic plate; a driving circuit configured to output a high frequency current to the induction heater by switching on and off switching elements thereof; and a controller configured to determine whether or not a temperature of the ferromagnetic plate is higher than the Curie point thereof, and control the driving circuit to reduce a level of the high frequency current when the temperature of the ferromagnetic plate is determined to be higher than the Curie point thereof, characterized in that the fixing apparatus further comprises: a detector including a coil and a detection circuit that is configured to detect an electrical resistance of the coil, which changes depending on whether or not a magnetic flux is formed, wherein the detector is configured to detect whether or not a magnetic flux having a magnetic path that passes through a surface of the ferromagnetic plate is formed; and the controller determines whether or not the temperature of the ferromagnetic plate is higher than the Curie point thereof, based on a detection result of the detector. a detector including a coil and a detection circuit that is configured to detect an electrical resistance of the coil, which changes depending on whether or not a magnetic flux is formed, wherein the detector is configured to detect whether or not a magnetic flux having a magnetic path that passes through a surface of the ferromagnetic plate is formed; and the controller determines whether or not the temperature of the ferromagnetic plate is higher than the Curie point thereof, based on a detection result of the detector.