In a scanning optical apparatus, light emitted from each of a plurality of light sources is converted by a first optical element into a beam of light, which in turn is converted by a second optical element into a linear image extending in a main scanning direction incident on a deflecting mirror at which the beams of light are deflected in the main scanning direction. A third optical element configured to convert the beams from the deflecting mirror into spot-like images is a single lens, and each of opposite lens surfaces thereof has a curvature in a sub-scanning direction varying continuously from a position corresponding to an optical axis thereof outward in the main scanning direction in such a manner that MTF values in a sub-scanning direction of an image formed on the scanned surface vary less with image height.