PURPOSE: To quickly operate a temperature oven-rise preventing element when the temperature of a fixing roller is overrisen with the abnormality of a temperature control circuit by using a bimetal plate as the supporting member of the temperature overrise preventing element.

CONSTITUTION: This temperature overrise preventing device 11 is laid on the fixing roller 1, and the temperature overrise preventing element 12 of the device 11 is supported by a supporting member 14 so as to come into contact with the fixing roller 1 when its temperature is an ordinary one. The supporting member 14 is made of a material so as to be deformed by a change in the temperature of a bimetal, a shape storage memory alloy, etc. The supporting form of the temperature overrise preventing element 12 by the supporting member 14 is set as follows. In other words, time until tone temperature over-rise preventing element 12 is separated from the fixing roller 1 after warming up is started, is set about the same as that until a heating source 3 consecutively supplies power after the ordinary temperature is obtained, in a state where the element 12 comes into contact with the fixing roller 1, and the element 12 is operated. Therefore, the element 12 comes into contact with the fixing roller 1 at the time of overrising the temperature, so that quick correspondence can be attained.