An optical scan apparatus includes an optical deflection unit (4), an image formation optical system (3), and curved surface mirrors (7a to 7d). Light fluxes from the image formation optical system (3) come onto the deflection surface (4a) of die optical deflection unit (4) in an oblique direction. Light fluxes from the optical deflection unit (4) come onto the curved surface mirrors (7a to 7d) in an oblique direction. When a boundary is a plane (6) including a normal at the center of the deflection surface (4a) and parallel to the main scan direction, the curved surface mirrors (7a to 7d) are arranged in the same space divided by the boundary and the scan speeds of fluxes scanning the surfaces to be scanned (8a to 8d) are almost identical. Thus, it is possible to obtain identical drive frequencies of the light sources (1a to 1d), thereby simplifying the circuit.