An optical scanning device includes a housing, a light source, a rotatable polygon mirror deflecting a light beam from the light source, a scanning optical system scanning a scanning object with the light beam from the polygon mirror, first and second reflective mirrors one of which is provided inside a deflection area, and a photodetector detecting the timing of scanning of the scanning object with the light beam from the one of the first and second reflective mirrors. The scanning object is one of first and second scanning objects. The housing includes first and second supports configured to support the first and second reflective mirrors at first and second positions, respectively, that are outside first and second scan-use areas, respectively. The first position is also inside the second scan-use area. The first and second scan-use areas are used for the scanning of the first and second scanning objects, respectively.