An image forming apparatus according to the invention includes an image forming apparatus main body and at least one option apparatus that is mounted on the main body when functions are extended. The main body includes a main power supply unit that converts a commercial power supply into a predetermined voltage, a first load that is driven by the main power supply unit, a first auxiliary power supply that is charged by the commercial power supply, and a fixing device including a main heater heated by the power of the commercial power supply and an auxiliary heater heated by the power of the first auxiliary power supply. The at least one option apparatus includes a second load driven by the power of the main power supply unit and a second auxiliary power supply that is charged by the commercial power supply when the at least one option apparatus is mounted, while heating the auxiliary heater in conjunction with the first auxiliary power supply.