The size Hbs of a beam spot of an incident light beam on a deflection mirror surface 651 is larger than the width Hb of the deflection mirror surface 651 when a pivot angle .theta. is zero, and therefore, Hbs>Hb is satisfied at any pivot angle .theta. and the deflection mirror surface 651 is overfilled with the incident light beam. Hence, even when beam rotation occurs, only a central portion of the incident light beam is reflected by the deflection mirror surface 651 and guided as a scanning light beam to a scanning lens. Owing to this, the central axis CLs of the outgoing light beam (scanning light beam) Ls from a deflector becomes close to the central axis CLs as it is when the pivot angle .theta. is zero.