An image forming apparatus including a main power supply unit, a chargeable auxiliary power supply unit, a fixing unit including a heater configured to receive power from at least one of the main power supply unit and the auxiliary power supply unit, and a voltage or temperature detecting part configured to detect a voltage input to the main power supply unit from a commercial power supply or to detect a voltage of the auxiliary power supply unit or to detect an environment temperature. Energy accumulated in the auxiliary power supply unit can be controlled to not be supplied to the fixing unit at a time of starting the fixing unit if the voltage or temperatures detected are less than predetermined values.