PURPOSE: To eliminate uneven illumination, by providing plural slit plates for forming an exposing slit opening, between a projection lens and a photosensitive body, and forming the opening ends of the slit plates of one side in a nonlinear shape by placing the optical axis between them.

CONSTITUTION: A light quantity control member 13 is provided with a pair of slit plates 30, 31 for forming an exposing slit opening, and these pair of slit plates 30, 31 move in the direction as indicated with an arrow, synchronizing with movement of a projection image. Also, end parts 30a, 31a of the slit plates 30, 31 have a nonlinear shape in the slit lengthwise direction in order to make an illumination distribution uniform in the slit lengthwise direction in an exposure area on a photosensitive drum 1. Also, when the slit plates 30, 31 are placed at a position for controlling a luminous flux of a main light beam 42, their turning is controlled so that the slit opening 52 falls at almost right angles with the main light beam 42. In this way, the illumination distribution on the photosensitive body becomes uniform, and uneven intensity of illumination can be eliminated.