An effective scanning range of laser light in a scanning direction is configured so that a center position of the effective scanning range is displaced by a distance .beta. along the scanning direction from an optical axis f an f.theta. lens. The incidence angle .alpha. between a center of luminous flux of the laser light on a deflection surface of polygon mirror and the optical axis of the f.theta. lens can beset to satisfy a relational expression (4.pi./N)-(W/D).ltoreq.|.alpha.|<(4.pi./N)-{(W-.beta.)/D}.