In an image forming apparatus, a temperature of a heating member heated by a heat source to fix a developer image on a recording sheet is detected by a temperature sensor disposed apart from the heating member. A controller controls the heat source based upon a temperature determined mathematically by application of a specific function to the temperature detected by the temperature sensor. In the controller, the specific function is switched among a plurality of functions according to a control mode which is switchable among a plurality of modes and in which operation of the heat source is regulated.