An optical scanning device includes a light source, a beam detector that takes a main scanning start time of a light beam emitted from the light source and deflection-scanned in a predetermined main scanning direction by a deflection-scanning component, and a housing with a support portion that supports reflecting mirrors in different arrangement positions so arrangement angles are different from each other. The support portion includes a first support portion that supports a first side surface of the reflecting mirror in a plurality of arrangement positions. A second support portion supports the first side surface of the mirror with the first support portion, causing a first arrangement angle in the first arrangement position of the reflecting mirror. A third support portion supports the first side surface of the reflecting mirror with the first support portion, causing a second arrangement angle in a second arrangement position of the reflecting mirror.