PURPOSE: To approximately make an image formed dot interval and image forming spot diameter of respective light beam constant when plural light beams, which are arrayed in accordance with the circumference direction of a photosensitive drum, illuminate the drum by making a light beam, which is located at the center of a multibeam column consisting plural light beams, incident on the drum perpendicularly to form the images.

CONSTITUTION: When a photosensitive drum 6 are irradiated with plural light beams arrayed in the circumference direction of the photosensitive drum 6, the light beam located at the center of the multibeam column consisting plural light beams is made incident on the drum 6 perpendicularly and an image is formed and the radius of the drum 6 and the arrangement of a light source are selected in accordance with the distance between imaged dots and the number of multibeams and allowable errors in the distance between imaged dots and the diameter of imaged dots. Thus, the dot interval and spot diameter are approximately made constant and imaged dots without unevennes are formed.