PURPOSE: To form the image of an original on a photosensitive body with excellent precision by changing the deflecting/scanning speed 4 of the luminous flux for illuminating the original as the intensity of the luminous flux is changed by means of the angle of view.

CONSTITUTION: The luminous flux from a semiconductor 6 is turned into almost parallel light by a collimator lens 7, reflected by a rotary polygon mirror 8, and forms a spot on the original 1 through an original platen glass 2 by a focusing lens 9, to scan the original 1. However, the illumination of an image surface by a lens 3 is decreased by COS4ω when the half angle of view is defined as (ω), according to a cosine 4-th power formula. At this time, the scanning speed is changed according to the position of the image surface, so that the illumination of the image surface is kept constant. Concretely, the scanning speed is slower as the angle of view is widened, when the distortion characteristic of the lens 9 is defined as a negative function. On the other hand, a fθ lens is used as the lens 9, and the light intensity of a laser 6 is controlled according to the scanning position of the spot. In other words, the light intensity is strengthened according to the widening of the angle of view.