A laser scanner with excellent productivity is provided in which most parts can be commonly used so as to recycle individually costly and expensive components once used in one product from the points of view of ecology, resource savings and recycling. The laser scanner includes a light source with a laser diode, a polygon motor with a polygon mirror mounted on a rotating shaft thereof, scanning lenses, a BD detection device, and a scanner casing with the BD detection device being held thereby and received therein. The single scanner casing is provided therein with a mounting bearing surface, which serves to permit a plurality of mirror configurations to be housed and arranged therein in accordance with the presence or absence of a reflection mirror, whereby it is possible to selectively form optical paths for irradiating a plurality of laser beams onto a photosensitive drum.