To reduce response to various regulations and influence on external equipment, especially flicker, by changing the control period according to the power supply of a heater, in power control of the heater obtained by combining phase control and wave number control.

The image forming apparatus includes an image-forming means 111 for forming a toner image on recording paper, a fixing device 115 for heating the toner image to fix it to the recording paper, and a power supply device for supplying power from an alternating-current power source to the fixing device. The image forming apparatus also includes: a power controlling means that sets a plurality of continuous half waves of the alternating-current power source to be one control period, performs phase control of partial half waves of the plurality of continuous half waves, performs wave number control of remaining half waves, and controls the power supplied to the fixing device by changing the phase angle of the phase control of each half wave and wave number for wave number control in the control period; and a control period-changing means that has two or more types of control periods and changes the control period during image forming.

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