An image forming apparatus maintaining the fixing temperature at a fixed level, by detecting the surface temperature of a heating roller provided internally with a heater in a fixing unit, and by controlling an on-time which starts upon the surface temperature reaching a predetermined temperature and ends when the heater is switched on and an off-time which starts upon the surface temperature reaching the predetermined temperature and ends when the heater is switched off. The on-time and off-time data are stored in a memory corresponding to use conditions such as ambient temperature and the type of paper to be used. Initially, an image fixing operation is executed in accordance with the data prestored in the memory, and transition of the surface temperature of the heating roller is detected which results from the image fixing operation. Data concerning new on-time and off-time are calculated on the basis of the detection result, and the calculation result is used as feedback for a next image fixing operation.