The fixing device of the image forming apparatus of the present invention has a cleaning pad arranged on the upper frame which supports a heat roller. An infrared temperature sensor is supported by a supporting mechanism that is capable of supporting the infrared temperature sensor at the temperature detecting position and the evacuating position. When the right and left frames are attached/removed to/from the main body of the apparatus at the time of maintenance, the infrared temperature sensor is moved to the evacuating position and pressed against the cleaning pad and then, the right and left frames are moved by sliding to the main body of the apparatus. The cleaning pad is also slid and moved jointly with the slide moving of the right and left frames, and the infrared temperature sensor contacts the cleaning pad and dirt on the infrared temperature sensor is removed. As a result of the improvement of temperature detecting accuracy of the infrared temperature sensor, the temperature control of the heat roller is improved and fixed image of high quality is obtained.