An optical scanner comprises: a light source; beam shaping optics for transforming an optical beam from the light source into a convergent beam; a rotating polygonal mirror having at least a first reflecting face for deflecting the convergent beam and a second reflecting face; and transmission optics having a lens, with which the optical beam deflected by the first reflecting face of the rotating polygonal mirror is allowed to be incident on the second reflecting face of the rotating polygonal mirror, with the optical beam incident on the second reflecting face of the rotating polygonal mirror being deflected therefrom to produce a scanning optical beam which scans a predetermined surface to be scanned, wherein the convergent optical beam forms a focused image at a point located between the first reflecting face of the rotating polygonal mirror and the lens in the transmission optics.