A fixing device has a heating roller with a heat source therein. A power
supply operates the heat source for heating a transfer material and
fixing a toner image thereon. A pressure roller is spaced from the
heating roller and a facing member faces the heating roller. An endless
fixing belt is mounted on the facing member and the heating roller and
contacts the pressure roller to form a nip. A temperature detector faces
the outer surface of the fixing belt for detecting the surface
temperature thereof. An excessive temperature preventer faces part of the
outer surface of the heating roller where the fixing belt is not mounted
for detecting the surface temperature of the heating roller and cutting
off the power supply to the heat source when the surface temperature
rises above a preset temperature.