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.