The optical scanning apparatus has at least two scanning units S1 and S2 which are arranged so as to oppose each other with a deflecting unit interposed therebetween. The scanning units include incident optical systems La and Lb that guide light beams emitted from light source units 1a and 1b to the deflecting unit 4, and imaging optical systems Ma and Mb that cause the light beams for scanning deflected on a deflecting surface of the deflecting unit to form images on surfaces 8a and 8b to be scanned. The two scanning units are configured such that the main scanning planes thereof including an optical reference axis C0 have different heights from a bottom surface 4c of the deflecting unit in the direction of the rotational axis of the deflecting unit.