A fixing apparatus capable of reducing deterioration in productivity while preventing a fixing roller from being heated to a high temperature. The fixing roller has a heating unit incorporated therein, and a rotatable pressurization roller abuts on the fixing roller. A thermistor detects a surface temperature of the fixing roller. A fixing operation is controlled by selectively switching between a first mode and a second mode in which the number of sheets subjected to fixing per unit time is smaller than in the first mode. One of the first and second modes is selected based on a first temperature detected by the thermistor at a first time, a second temperature detected by the thermistor at a second time, and a minimum temperature of the fixing roller at which the toner image can be fixed.