To detect a shift of an irradiating position of an optical beam on a photosensitive body drum with high accuracy by a method wherein a position of at least one of a light source, an optical means and an image forming body is controlled based on a position error signal corresponding to a shift quantity shifted from a reference position by detecting the irradiating position of an optical beam on an image forming body.

A focus lens position control section 11A supplies a driving voltage which is based on the input of a position error signal in a direction X from a reference position to a horizontal movement stage 12B, then a focus lens 3 is moved in a main scanning direction and an irradiating position in the main scanning direction of a laser beam for detection emitted on a photosensitive drum 5 is corrected. Also, a focus lens position control section 11B supplies a driving voltage which is based on the input of a position error signal in a direction Y to a vertical movement stage 12A, then the irradiating position of the laser beam for detection in the sub-scanning direction is corrected. As a result, it is possible to detect a shift of the irradiating position of the optical beam emitted from the light source with high accuracy.