An optical scanning device for scanning plural surfaces including a plurality of laser light units each configured to emit a laser beam, a deflecting device configured to deflect the laser beams from the plurality laser light units to sweep a plurality of predetermined planes, respectively, and a scanning image forming optical unit configured to project the laser beams deflected by the deflecting device on the plural surfaces, respectively, to scan the plural surfaces with the laser beams. The scanning image forming optical unit includes a plurality of optical detecting devices configured to detect the laser beams and a first plurality of reflecting devices positioned to reflect at least two laser beams of the laser beams toward one of the plurality of optical detecting devices.