PURPOSE: To make an exposure quantity uniform in the moving direction of a photosensitive body by providing an arm whose one end is engaged to a light shielding member which varies the width of a slit and whose other end is fitted with a cam follower, and a cam member having a curved surface engaged to the cam follower.

CONSTITUTION: A cam 11 to which a cam follower 10 is engaged is provided so that its base line becomes parallel to the scanning direction extending over almost all the scanning range of a scanner, and the projecting quantity from the base line of the cam face is larger in both the end parts of the scanning range than in the center part. Light shielding plates 8 of both sides of a slit 18 extend-provide a spring 13 on an anchor 12 provided vertically on each end face, and by energizing it in each approaching direction, the cam follower 10 is brought into press-contact with the cam 11, an arm 9 rotates round a shaft 16 in accordance with the projecting quantity of the cam, and the width of the slit 18 is prescribed. Accordingly, when the cam shape of the cam 11 is set variable in slit opening width so as to correct a drop of the intensity of illumination, which is caused by an incident angle of a main beam to a lens by a variation of the scanning position, the exposure quantity of a photosensitive body in the scanning process is always held uniformly.