An optical scanning device includes a control unit, a light deflector, light detection units, and a light source. A mirror unit of the light deflector has a flat reflection part for generating scanning light and a groove-shaped reflection part for generating twice reflected light, and performs reciprocating rotation about a rotation axis. The light detection units are disposed at positions on the scanning trajectory of the scanning light where the twice reflected light is received, and are each divided into light detectors in the scanning direction of the scanning light by a division line. The control unit detects the deflection angle θ of the mirror unit based on both the output of the light detector and the output of the light detector.