An image fixing device that is controllable with a high degree of reliability. The image fixing device is configured with a heat generator for fixing a toner image onto a recording medium that is opposed to an induction heater. Heat is generated in the heat generator through electromagnetic induction. An inverter circuit drives the induction heater. A power controller controls a power value that is output from the inverter circuit, whose power value is calculated by a power-value calculator. A power-value sensor acquires the power value output from the power controller, and a temperature sensor senses a temperature at least at one point of the heat generator. Thus, either the power value sensed by the power-value sensor or a temperature value sensed by the temperature sensor may be used as a reference value to calculate the power value in the power-value calculator.