Optical scanning apparatus, multi-beam optical scanning apparatus, and image-forming apparatus

An optical scanning apparatus comprising:

• light source means for emitting a light beam;
• a rotary polygon mirror for reflecting and deflecting the light beam emitted from the light source means;
• entrance optical means for guiding the light beam emitted from the light source means to the rotary polygon mirror;
• and
• imaging optical means for forming an image of the light beam reflectively deflected by the rotary polygon mirror on a surface to be scanned, wherein:
• in a main scanning section, the light beam enters onto a deflection surface of the rotary polygon mirror with a predetermined inclination angle which is formed between a direction of the light beam entering onto the deflection surface and an optical axis of the imaging optical means,
• the imaging optical means has a plurality of asymmetric change surfaces each of which has a curvature in a sub-scanning direction that changes asymmetrically in a main scanning direction, assuming the optical axis of the imaging optical means as a center, and
• in the plurality of asymmetric change surfaces, a curvature of a surface located at a side with respect to the optical axis of the imaging optical means, where an optical path of the light beam entering onto the deflection surface of the rotary polygon mirror with the predetermined inclination angle exists, is larger in a whole of an effective scanning region than a curvature of a surface located at a side with respect to the optical axis of the imaging optical means, where an optical path of the light beam entering onto the deflection surface of the rotary polygon mirror with the predetermined inclination angle does not exist.