To provide an optical device capable of decreasing the dimension of the optical unit in the direction of an optical axis when not in use, while suppressing an increase in the dimension perpendicular to the optical axis.

The optical device is disposed on the optical axis J1 of an optical unit 18 when in use, and includes lens units 13, 14, and 16 retracted to an area outside the optical axis, i.e., outside a light passage area including the optical axis J1, when not in use. In the optical device, a storage space 185 for storing at least one of the lens units 13, 14, and 16 is provided in the area outside the optical axis, which is outside the light passage area of an optical unit 18.