An optical scanning apparatus includes a plurality of light sources which emit first and second beams. First and second coupling lens units respectively couple the first and second beams emitted by the plurality of light sources. A rotary deflector which has pairs of mutually-opposite reflection surfaces and deflects the first beam in a first direction by one of the reflection surfaces and deflects the second beam in a second direction opposite to the first direction, by another of the reflection surfaces. First and second imaging units scan an image surface of a photoconductive medium along a first half of a main scanning line by focusing and deflecting the deflected first beam onto the image surface, and scan the image surface along a second half of the main scanning line by focusing and deflecting the deflected second beam onto the image surface, such that the first and second halves form a substantially straight scanning line on the image surface when the rotary deflector is rotated.