An apparatus can correctly discriminate between types of a recording medium, and obtain good fixed image by performing fixing in the optimum fixing conditions for a variety of different types of recording media. The apparatus includes an LED 101 serving as a first irradiation section, an LED 104 serving as a second irradiation section, a phototransistor 103 serving as a first reading section, and a phototransistor 102 serving as a second reading section. The light output from the LED 101 operating as the light source and passing through a slit 111 illuminates a surface of recording paper P on a recording paper conveyance guide 105. Besides, the recording paper conveyance guide 105 has a window opened for illuminating the recording paper from its underside in the present embodiment. The reflected light from the recording paper P is gathered via slits 112 and 113 and is received by the phototransistors 102 and 103.