A heater control circuit performs ON/OFF control of a switching converter for a FET so that an AC line current supplied from a DI terminal becomes approximate to a predetermined value. A current flowing to a heater is stabilized to a predetermined value, and, unless an AC line voltage fluctuates, electric power supplied to the heater is held to a predetermined value. With this contrivance, in an image forming apparatus including a fixing heater (heating heater), a printing speed of the image forming apparatus can be improved to the greatest possible degree under such a condition that a total amount of utilizable electric power is restricted.