A fixing device for use in an electrophotographic copying machine comprising a fixing plate having a width substantially equal to the maximum width of recording sheets in use and a heater disposed transversely of a fixing plate. The heater is divided into heater sections in the direction of widths of the recording sheets corresponding to the widths thereof. A temperature sensing element is associated with each of the divided heater sections and supplied power to the heater sections is controlled in response to the output of the temperature sensing element.