A multi-beams optical scanning device is constituted such that a width in a main scanning direction of each of a plurality of light beams emitted from a light source unit is wider than a moving area in the main scanning direction in which a deflection facet of a deflection unit can move and exist when the plurality of light beams emitted from the light source unit are deflected toward an entire effective scanning area on the surface to be scanned and are deflected toward the synchronous detection means provided outside the effective scanning area.