To provide an optical scanner such that an optical deflector is reducible in thickness in a vertical scanning direction while a mutual interval between pieces of luminous flux is held large enough for separation.

The optical scanner is equipped with a polygon mirror 4 which deflects pieces L1 to L4 of luminous flux emitted by a light source and passed through a front optical system PRE so that their traveling directions change along a first direction corresponding to a horizontal scanning direction (x) and a rear optical system PST which separate the pieces L1 to L4 of luminous flux from the polygon mirror 4 mutually to a second direction corresponding to the vertical scanning direction (y) crossing the horizontal scanning direction at right angles and guides them to scanned surfaces 71 to 74, and scans the scanned surfaces 71 to 74 in the horizontal scanning direction (x) by using the pieces L1 to L4 of luminous flux. The pieces L2 and L3 of luminous flux from the polygon mirror 4 travel in parallel to each other in a plane containing the second direction to impinge on the rear optical system PST and the pieces L1 and L4 of luminous flux impinge on the rear optical system PST at such angles of incidence that they gradually leave each other in a plane containing the second direction.