When changing the turn-on/off patterns, a controller of a fixing device sets a control interval of the turn-on/off pattern so as to be longer than a first period of time that does not exert any influence to a flicker value and to be shorter than a second period of time that does not exert any influence to a response to temperature control. The controller of the fixing device selects a third turn-on/off pattern in which power difference between power of a first turn-on/off pattern before the change in the turn-on/off pattern and power of a second turn-on/off pattern after the change in the turn-on/off pattern is smaller than a set reference power difference.