An image forming apparatus according to the invention includes an auxiliary power supply configured to detachably mountable on the image forming apparatus, a fixing device including a main heater heated by a commercial power supply and an auxiliary heater heated by the auxiliary power supply, a detector that detects mounting and detachment of the auxiliary power supply, and a control unit that performs control to carry out, when the mounting of the auxiliary power supply is detected by the detector, continuous printing of sheets at printing speed higher than printing speed at the time when the auxiliary power supply is not mounted. According to the image forming apparatus according to the invention, the image forming apparatus is capable of operating without deteriorating power use efficiency during continuous printing at normal speed and, on the other hand, can secure stable fixing performance even when the speed of the continuous printing is increased.