To reduce flicker even when power consumption at a printing time is small in an image forming apparatus provided with a fixing device to fix a toner image formed on a recording material by heating it.

In the image forming apparatus provided with an image forming part 108 to form the toner image on the recording material S, the fixing device 109 to fix the toner image on the recording material by heating it and an electric power controller to control electric power supplied to the fixing device, the electric power controller has two kinds of electric power controlling means of phase control and the number of waves control and also has an electric power control changeover means to switch two kinds of electric power controlling means, and the number of waves control is set as the electric power controlling means at a usual time in the electric power controller, so that the changeover of the electric power controlling means from the number of waves control to the phase control is performed in accordance with a printing condition.