Scanning optical device and image forming apparatus

A scanning optical device comprising:

• a light source configured to emit a light beam;
• a rotational polygon mirror configured to deflect and scan the light beam emitted from the light source;
• a scanning lens configured to image the light beam defected and scanned by the rotational polygon mirror on a scanned surface;
• an optical box including an opening and configured to accommodate the light source, the rotational polygon mirror, and the scanning lens;
• and
• a cover configured to cover the opening,
• wherein on an opposite surface of the cover opposed to the rotational polygon mirror in a state in which the cover covers the opening, a plurality of ribs projected from the opposite surface toward the rotational polygon mirror are formed,
• wherein as viewed in a rotational axis direction of the rotational polygon mirror, the plurality of ribs are
• disposed so as to have rotational symmetry about a rotational axis of the rotational polygon mirror,
• extended from the rotational axis toward a circumscribing circle of the rotational polygon mirror, and
• disposed at positions apart from a predetermined distance in an upstream side or a downstream side with respect to a rotational direction of the rotational polygon mirror to a plurality of imaginary lines, of the same number as a number of the plurality of ribs, having rotational symmetry about the rotational axis, and
• wherein as viewed in the rotational axis direction, each of ribs has a lengthy shape parallel to the imaginary line corresponding to each of ribs, one end portion thereof in a longitudinal direction of the rib is disposed outside the circumscribing circle and the other end portion thereof in the longitudinal direction of the rib is disposed inside the circumscribing circle.