a small gear having a gear body which is rotated by an electric motor and on which a tooth portion is formed and shaft portions respectively formed so as to protrude from both end portions of said gear body and being smaller in diameter than said gear body;

bearings for respectively bearing both said shaft portions of said small gear so that said small gear can move in an axial direction;

two circular elastic bodies having flat portions formed on an inner circumferential side, and a tapered portion formed on an outer circumferential side connected to the flat portions, the elastic bodies being respectively externally fitted on both said shaft portions of said small gear, for suppressing movement of said small gear in the axial direction to shaft end portions; and

a large gear meshing with the tooth portion of said gear body of said small gear and connected to a steering means; and being configured to assist steering by rotation of said electric motor, comprising:

limiting members, arranged along outer circumferences of said shaft portions respectively at its both end portions of said gear body, for limiting deflection amounts of said elastic bodies,

wherein each elastic body is disposed adjacent to a respective one of the bearings and adjacent to a respective one of the limiting members,

wherein the bearings are roller bearings, and

wherein the circular elastic bodies are belleville springs with the flat portions at its end portions and multiple flexible pieces formed so as to protrude from the tapered portion to the inner circumferential side,

the limiting members being disposed between inner rings of the roller bearings and its both end portions of the gear body of the small gear, respectively, and

the inner rings of the roller bearings are configured so as to make contact with protruding side faces of the flat portions, and the limiting members are configured so as to make contact with rear faces of the protruding side faces.