An optical scanner includes a plurality of scanning optical systems each of which scans a different surface to be scanned and includes a light source configured to emit a light beam, an optical deflector having a plurality of reflection surfaces, and a synchronization detector configured to receive the light beam deflected by the optical deflector and detect a timing to scan an effective area of the surface to be scanned with the deflected light beam before a scanning is started or after the scanning is completed. The synchronization detector of one of a pair of scanning optical systems is disposed on a scanning end side to determine the scanning timing, and the synchronization detector of the other one of the pair of scanning optical systems is disposed on a scanning start side to determine the scanning timing.