PURPOSE: To save the absorbing member behind the reflecting mirror, ease manufacture and perform the wavelength selection of reflected light effectively by using the absorbing member of light to the reflecting mirror within the lens holder provided to the exposure optical path of a copying machine or the like.

CONSTITUTION: A reflecting flat plate 3 in a lens holder 1 is composed of a member which absorbs light, e.g., ceramic aluminum close to black color. The surface of the incident side of light is a mirror surface and a reflecting layer 4 composed of the materials which reflect light of the required wavelength region, e.g., alternate layers of CeO2 and MgF2, are evaporated on said mirror surface. When the light l emitted from the original transmits through a converging lens 2 and reaches the reflecting flat plate 3, only the light of the required wavelength region corresponding to the spectral sensitivity characteristic of the photosensitive object is selectively reflected by its reflecting film layer 4, whereby the photosensitive object is exposed. On the other hand, the light of the unnecessary wavelength region is completely absorbed in the flat plate 3.