The emission intensity of a laser beam source, at a time when a laser beam emitted from the laser beam source and scanned by an optical scanning device becomes incident to a laser beam detector provided in a predetermined position on the scanning path of the laser beam in order to hold a position of an electrostatic latent image formed on a photoreceptor constant in the main scanning direction, while at the same time, changing the emission intensity of the laser beam, is set so as to become the consistently same emission intensity regardless of a change in emission intensity at the time of electrostatic latent image formation onto a photoreceptor. This allows the detection timing of a laser beam by a laser beam detector to be kept constant.