PURPOSE: To eliminate all driving means and also to prevent contamination of a photodetecting sensor, by using a specified multiplayer film interference filter as a mirror for forming an exposure use optical system, and providing the photodetecting sensor on its back face.

CONSTITUTION: A filter 11 is formed by a multilayer film interference filter which transmits light of a spectrum area in which a photoreceptor is not induced, and reflects light of other area. Also, a photodetecting sensor 12 is provided on the back face of this multilayer film interference filter 11. Accordingly, even if the sensor 12 is fixed and placed in an optical path, no bad influence is exerted on a copying work, therefore, all driving members for moving the photodetecting sensor become unnecessary. Also, the sensor 12 is not contaminated since the sensor 12 is placed on the back face of the filter 11. In this way, all driving means come into disuse and contamination of the photodetecting sensor can be prevented.