To prevent a positional deviation of recording dots as a result of a scanning speed change by setting a rotary polygon mirror for rotating and scanning a light from a light-emitting element, an optical system for scanning the scanned light at a uniform velocity on a body to be scanned, and a photodetection means for detecting that the light passes through a light scanning area.

A light emitted from a semiconductor laser 2 irradiates a rotary polygon mirror 5 rotating at high speed of a uniform rotation angular velocity, and scanned. A scanned light 6 is turned to a uniform velocity scanning light by an fθ lens 8 and exposed to a photosensitive body or the like object to be irradiated with the light. In this case, a moving mirror 10 is arranged behind the fθ lens 8, so that the scanning light 6 can be switched to a vertical direction as an exposure direction of the photosensitive body and a horizontal direction as an exposure position detecting direction. A plurality of photodetection means 11 are set in a light scanning are in the horizontal direction, so that a pass timing of the scanned light can be detected. Scanned image information is changed on the basis of a detect signal 13, whereby the exposure is executed to a correct position on the photosensitive body.