To solve the problem that the temperature of the end part of a heat roller is continuously raised, so that bearings made of resin that support the shaft of a heat roller are melted when small sized papers such as post cards or the like are continuously printed.

A first thermistor 31 is positioned at the center section of a heat roller 7, while a second thermistor 32 is positioned at the tip end section of the roller 7. When the temperature detected by the thermistor 32 is raised to a heating prohibiting temperature T1, a lamp heater 9 which heats the roller 7 is turned off, image forming operations are stopped and the roller 7 is cooled off. When the temperature detected by the thermistor 32 is reduced to a heating allowable temperature T2, the heater 9 is turned on and heating of the roller 7 is resumed.