PURPOSE: To detect averaged original density over the entire surface of an original by photodetecting reflected light from an original by a photodetection surface arranged in parallel to a convergent light transmitter, and guiding it to a photoelectric element through a light diffusing member.

CONSTITUTION: A photodetection part 41 consists of a light diffusing member 42 of acrylic resin and a photoelectric element 43 composed of a photodiode; the photodetection surface 44 of the member 42 is flat, nearly as long as an original S, and arranged in parallel along a convergent light transmitter, and a diffusing and reflecting surface 45 on the opposite side to the photodetection surface 44 is beveled like a knife-edge. Reflected light from the original S is diffused in the acrylic resin in every direction, diffused and reflected by the reflecting surface 45, and guided to the photoelectric element 43, which generates an electric signal which corresponds to the quantity of photodetected light, thus performing exposure control. As a result, the reflected light is detected over the overall width of the original surface, so invariably adequate averaged original density is detected.