An optical scanning device includes a plurality of scanning optical systems that focus light beams deflected by an optical deflector onto corresponding scanning surfaces. The systems include a first scanning optical system and a second scanning optical system. The first scanning optical system and the second scanning optical system are disposed respectively at each side of a plane including a rotation axis of a polygonal-mirror optical deflector. Each of the scanning optical systems includes a synchronous detection optical system that determines a timing to start scanning the scanning surfaces with the light beams. When a time from the end of an effective scanning area in the second scanning optical system to synchronous detection in the first scanning optical system is Ta, and a time from the end of an effective scanning area in the first scanning optical system to synchronous detection in the second scanning optical system is Tb, Ta>Tb.