A light beam L from a laser source 62 impinges upon a deflection mirror surface 651 from the direction of a pivot axis (sub scanning direction Y) at an acute angle .gamma. with respect to the surface normal NL to the deflection mirror surface 651, and the light beam scans in a main scanning direction X. Since the light beam is incident upon the deflection mirror surface 651 along the direction of the pivot axis, the length of a movable plate 653 in the main scanning direction X may be relatively short. Further, a first optical system 63 shapes the light beam incident upon the deflection mirror surface 651 into an elongated cross sectional shape which is long in the main scanning direction X, and the movable member 653 is finished as an elongated strip which elongates in the main scanning direction X. Hence, the movable plate 653 is light-weight and can pivot stably at a faster speed than in a conventional apparatus.