A 3D-image pickup optical system includes: a pair of optical systems each of which includes a magnification-varying lens unit, an aperture stop, a first lens unit and a first reflective member in a sequential order from an object side, the optical systems having optical axes offset from each other; a second lens unit that receives object light fluxes from the pair of optical systems; a second reflective member that guides the object light fluxes from the pair of optical systems to the second lens unit; and a switching unit that is disposed in an object side of the second reflective member, and alternately guides the pair of the object light fluxes respectively from the pair of optical systems to the second reflective member.