To adjust the quantity of curvature of a laser scanning line on the photoreceptor drum of the image formation device using a multiplex transfer system.

A glass G of a plate shape which is long along a scanning direction is arranged halfway in the optical path of a laser beam with which a photoreceptor drum 1 is scanned through a polygon mirror 2. This glass G is slanted at an angle α in a cross section. The quantity of curvature of the laser scanning line on the photoreceptor drum 1 can be adjusted by varying the tilt angle α of the glass G. The laser beam can, therefore, be superposed with high precision and tint variation and a color slurring quantity are reduced.