a base member connected with either one of a seat cushion and a seat back arranged rotatably to the seat cushion, the base member being formed with a shaft hole;

a rotating member connected with another of the seat cushion and the seat back and arranged circumferentially rotatably to the base member, the rotating member including an internal gear in an inner peripheral surface of the rotating member;

at least three lock members each including an external gear adapted to engage with the internal gear, each of the at least three lock members being adapted to swing in a direction in which the external gear engages with or disengages from the internal gear;

a shaft portion formed integrally with the base member, the shaft portion including a convex guiding surface for swinging the at least three lock members along a circular orbit;

a cam adapted to lock and unlock the rotating member by pressing the at least three lock members and thereby engaging or disengaging the external gear with/from the internal gear, the cam being provided with

an axially projecting portion projecting in an axial direction of the cam and passing through the shaft hole, the axially projecting portion including a centering outer peripheral surface which is slidably in contact with an inner peripheral surface of the shaft hole;

a center shaft press-fitted into the cam to pass through the cam, and adapted to rotate the cam, the center shaft passing through the shaft hole;

a biasing member disposed so as to surround the center shaft and biasing the cam in a direction for locking the rotating member, the biasing member including an inner end portion connected with a tip portion of the axially projecting portion and an outer end portion connected with the base member;

an operating lever attached to the center shaft; and

at least three guide projections formed integrally with the base member, the at least three guide projections and the at least three lock members being disposed alternately in a circumferential direction relative to a rotation axis of the cam, each of the at least three guide projections including

a centering inner peripheral surface provided at an inner peripheral side of the each of the at least three guide projections relative to the rotation axis, and adapted to slide on an outer peripheral surface of the cam to perform a centering of the cam and

a pair of concave guiding surfaces respectively guiding the adjacent two of the at least three lock members so as to swing the adjacent two of the at least three lock members along the circular orbits.