A phase shift element having a central area (for transmitting part of an incident laser beam in the vicinity of the beam central axis) and a first area (giving the laser beam incident thereon a phase difference π relative to a beam passing through the central area) is placed on an optical path between a laser light source and a polygon mirror of a scanning optical system. The scanning optical system including such a phase shift element is installed in a printer. By the effect of the phase shift element, the intensity of side lobes (several rings of light accompanying the main beam) is prevented from exceeding a threshold value even when optical surfaces of an imaging optical system of the scanning optical system have certain microscopic undulations, by which black stripes occurring in halftone printing can be eliminated.