An optical scanning device includes a light source device having light emitting members of a number N (N is an integer not less than 2) disposed with a tilt with respect to a sub-scan direction and a main-scan direction, a deflecting device for scanningly deflecting light beams of the number N from the light emitting members, an input optical system for directing the light beams from the light source means to the deflecting device, an imaging optical system for directing the deflected light beams to a surface to be scanned, a synchronism signal detecting device for detecting a portion of a light beam scanningly deflected by the deflecting surface and producing writing start timing signals for the light beams upon the surface to be scanned, and a synchronism detecting optical system for directing the deflected light beams to the synchronism signal detecting device, wherein the synchronism signal detecting device detects a light beam emitted from a light emitting member among the light emitting members of the number N, except a light emitting member which emits a light beam that provides a least light quantity when the same is incident on the synchronism signal detecting device, to determine the writing start timing signal.