An optical device includes an optical system mount plate, a casing, and a screw-hole forming part. An optical system is mounted on the optical system mount plate. The optical system mount plate is housed in the casing. The screw-hole forming part is provided on a bottom of the casing such that a screw provided on a tripod head can threadedly fit in the screw-hole forming part. The screw-hole forming part and at least one portion of the bottom are engaged with the optical system mount plate so as to counteract a rotational tightening force that is applied to the screw-hole forming part when the screw is threadedly tighten in the screw-hole forming part.