A planetary gear mechanism includes a plurality of planetary gear units each including a sun gear member to be rotationally driven by a driving source, an internal gear member, planetary gear members, and a carrier member, and a case body that internally houses the planetary gear units and includes an inner circumferential surface to be brought into surface contact with outer circumferential surfaces of the internal gear members. Engagement portions having concave shapes or convex shapes that can be fitted to convex shapes or concave shapes formed in the outer circumferential surfaces of the internal gear members are formed in the inner circumferential surface of the case body. The engagement portions are engaged with the convex shapes provided in the outer circumferential surfaces of the internal gear members, from an opposite direction to a rotating direction of the carrier members, to thereby inhibit the internal gear members from rotating.