PURPOSE: To copy an original to a copy paper, which has a size corresponding to the original size, with a proper density by constituting an original size detecting part and an original density detecting part of a light emitting parts and a light receiving part and extinguishing the light emitting part of the other detecting part while one detecting part is operated.

CONSTITUTION: The device is so constituted that a light emitting part 16a of an original density detector 16 does not emit light because of detecting an original size when an optical system 4 consisting of an exposure light source 3 and etc. is moved in the direction of an arrow A on the going path and the exposure light source 3 is not lit because of detection of an original density when the optical system 4 is moved reverse on the returning path. That is, the original size and the original density are detected in periods different from each other, and lights other than a light required for detection is not made incident to each detecting part. Consequently, the original is always copied accurately to a copy paper, which has a size corresponding to the original size, with a proper density.