A fixing apparatus of an image forming apparatus of the present invention strikes a support plate having an infrared temperature sensor against a roller shaft of a heat roller, thereby, regardless of the part accuracy, keeps a gap between the heat roller and the infrared temperature sensor constant. By improvement of the temperature detection accuracy of the infrared temperature sensor, the temperature control of the heat roller is improved and toner images can be fixed always at a fixed temperature. The fixing property is improved and fixed images of high image quality free of ripple marks are obtained.