PURPOSE: To make uniform the illuminating distribution in the slit longitudinal direction on the surface of a photosensitive body drum irrespective of variable power by displacing the pupil of an image forming lens in the direction orthogonal to an optical path and also in the direction orthogonal to the optical path of a light shielding member.

CONSTITUTION: Picture information 2 on an original platen 1 slit-illuminated by an illuminating means 3 is light-condensed by the image forming lens 6 through mirrors 5 to 7, and an image is formed in the shape of a slit on the surface of the photosensitive body drum 12 (image carrier) through a mirror 11 at a prescribed magnification. According to the magnification, a diaphragm plate 9 (pupil) arranged in a lens 8 is moved in the direction orthogonal to the optical axis to change the pupil position of the lens 8. The relative position relation between the pupil and the light shielding member 10 is changed, and the pupil area on the optical axis is made constant, observed from the side of an image plane. Thus, the illumination distribution in a range of overall displacement is made uniform, and absolute illuminance becomes roughly constant.