The image forming apparatus includes a fixing device that has a heater, and a required power calculation unit that determines a power Pfsr that is assumed to be required by the heater. Based on a difference between a required power that is determined by the required power calculation unit and a suppliable power, the image forming apparatus selects whether to start printing in a normal mode or to start printing in a power saving mode in which the power that is supplied to the heater is reduced in comparison to the normal mode. The power saving mode includes a plurality of modes, and when a CPU selects the power saving mode, the CPU selects one or a combination of modes from among the plurality of power saving modes based on the aforementioned difference.