An image forming apparatus includes a power switch for turning on and off a power source. An electric power saving mode designation acceptor can accept a designation to set a power saving mode to suppress power consumption. The apparatus further has an image former for forming a toner image on a sheet; a fixing device for thermally fixing the toner image; a heater for heating the fixing device; and an electric power saving controller for shifting to the power saving mode if the power saving mode designation is accepted by the power saving mode designation acceptor, and for controlling the heating section to heat the fixing device to a predetermined fixing temperature necessary for fixing the toner image on the sheet if the power saving mode designation is not accepted by the electric power saving mode designation acceptor, in response to shifting of the power switch from off to on.