An optical scanning apparatus effectively blocks undesirable-light generated in opposed scanning units, and forms high-quality image with simple configuration. The apparatus includes two scanning units disposed with a polygon mirror therebetween, each scanning unit including: an incident optical system guiding a beam from a light source to the polygon mirror; and an imaging optical system including an imaging optical element to cause the deflected beam form an image on a scanning surface. One of the scanning units includes a member to block an undesirable light reflected on optical surfaces of the imaging optical element of the other scanning unit and traveling toward the scanning surface of said one scanning unit. rp