Optical scanning apparatus and method for adjusting the same

An optical scanning apparatus comprising a light source, an incident optical system for guiding a light beam from the light source to a deflecting means, and an imaging optical system for guiding said light beam onto a surface to be scanned, wherein the incident optical system comprises an anamorphic condenser lens of plastic material having an optical power in a main scanning direction and an optical power in a sub-scanning direction being different from each other, and means for adjusting a beam diameter in the main scanning direction of the imaging spot on the surface to be scanned, and the anamorphic condenser lens comprises a diffractive portion having an optical power on at least one surface, and both the optical power in the main scanning direction and in the sub scanning direction of the refractive portion of the anamorphic condenser lens have a positive optical power and both the optical power in the main scanning direction and in the sub scanning direction of the diffractive portion of the anamorphic condenser lens have a positive optical power and satisfy a conditional expression.