A first light guide guides first light in a first light guiding path. The first light is of light irradiated from the light source to a sheet. A first detection unit receives reflected light from the sheet and outputs an image signal indicating an image of a surface of the sheet. A second light guide guides second light in a second light guiding path different from the first light guiding path. The second light is of the light irradiated from the light source and is different from the first light. A second detection unit receives the second light and output a detection signal corresponding to a light amount of the second light. A control unit controls a light emission amount of the light source based on the detection signal.