a plurality of light source means; and

a polygon mirror which deflects and scans a plurality of light beams respectively emitted from the plurality of light source means, for imaging the plurality of light beams deflected and scanned by different deflecting facets of the polygon mirror on a plurality of different scanned surfaces,

wherein the number of the light source means is 4 n, the number of the light beams is 4 nm (n and m are positive integers, respectively),

incidence of optical paths of the light beams incident on the polygon mirror are symmetrically disposed with respect to a sub-scanning cross section and a main scanning cross section,

the sub-scanning cross section and the main scanning cross section are perpendicular to each other, the sub-scanning cross section is parallel to a rotational axis of the polygon mirror, and the main scanning direction is perpendicular to the rotational axis of the polygon mirror,

a plurality of optical elements are respectively provided for the plurality of light source means, and are constructed in a united form to form a compound imaging element, and the compound imaging element is disposed between the plurality of light source means and the polygon mirror,

the plurality of optical elements respectively form linear images of the light beams, along a main scanning direction, on the corresponding deflecting facets, and

the compound imaging element includes a first compound optical element formed by the optical elements which make the plurality of light beams of which number is 2 nm among the plurality of light beams, incident onto a first deflecting facet among the different facets of the polygon mirror, and a second compound optical element formed by the optical elements which make the plurality of light beams of which number is 2 nm among the plurality of light beams, incident onto a second deflecting facet among the different facets of the polygon mirror.