An image forming apparatus includes an image forming unit for forming and transferring an image of a document to a copy sheet, fixing unit including a fixing roller for fixing the document image to a copy sheet while transporting it, a heater for heating the fixing roller, and heater control unit for turning on and off the heater at specified timings so as to increase and maintain temperature of the fixing roller to and in a predetermined steady-state temperature range whose upper limit is a fixing temperature, the copy sheet being heated by the fixing roller having temperature of about the predetermined fixing temperature while transported, whereby the document image transferred to the copy sheet is fixed thereto, and control unit for controlling the heater control unit to control the heater within a copying period during which a copying operation is completed. The heater control unit on-off controls the heater only during the copying operation. Accordingly, power consumed by the heater is reduced and a temperature increase within the apparatus can be suppressed.