An image forming apparatus of the present invention includes a fixing device for fixing a toner image on a sheet with heat. A main and an auxiliary power supply feed power to the fixing device. A switching circuit selectively establishes a usual mode in which the main power supply is connected to the fixing device or an auxiliary mode in which the main and auxiliary power supplies both are connected to the fixing device. A control circuit causes the switching circuit to establish the auxiliary mode in the event of continuous fixation, controls, in the usual mode, the amount of power fed from the main power supply to the fixing device in accordance with the output of a temperature sensor responsive to the temperature of the fixing device, and controls, in the auxiliary mode, the amount of power fed from the main and auxiliary power supplies to the fixing device without regard to the output of the temperature sensor.