A temperature sensor for detecting the temperature of a heating roller contacts the heating roller with the bottom surface of a contact section provided on the front end of the temperature sensor. A temperature detecting element is disposed on the upper surface of the front end at a position corresponding to the contact section. The front edge of the sensor is bent away from the heating roller. During a printing process, a portion of the toner that is liquefied in the heat fixing process remains on the heating roller and is subsequently deposited on the bottom surface of the contact plate as the heating roller rotates. However, by forming the surface from the contact section to the front edge as a smooth curved surface to eliminate edges or corners that can lead to the accumulation of toner, a clump of toner does not form on the bottom surface of the contact plate.