In an image forming apparatus according to the present invention, when an image forming operation is started from a standby state, electric power is supplied to a heater by switching to a first fixing control to turn on and off the heater to heat a heating roller with a fixed duty ratio based on the detected temperature of the heating roller, and thereafter, when detecting a state that the falling detected temperature reaches a lower limit value at which the detected temperature turns to rising, electric power is supplied to the heater by switching to a second fixing control which changes a duty ratio in accordance with a temperature difference between the detected temperature and a target control temperature.