To provide a simple power transmission mechanism for restricting the movement of a rotating shaft in the extending direction.

The power transmission mechanism comprises a driving side annular member having the rotating shaft whose movement is restricted in the extending direction and a plurality of triangular recessed portions formed in the inner periphery, a driven side annular member externally fitted and fixed to the rotating shaft and having a plurality of recessed portions formed in the outer periphery, balls engaged with the recessed portions of the driving side annular member and the recessed portions of the driven side annular member and movable from the recessed portions of the driving side annular member to the recessed portions of the driven side annular member, a thrust ring externally fitted to the driven side annular member and tendered outward in the radial direction and in one extending direction of the rotating shaft for abutting on the balls, a spring externally fitted to a boss portion of the driven side annular member for energizing the thrust ring in one extending direction of the rotating shaft, and a nut threaded to the boss portion of the driven side annular member for holding the spring, the ball thrust ring and the balls in cooperation with the outer periphery of the driven side annular member. Both end faces of the driving side annular member face the outer periphery of the driven side annular member and the outer periphery of the thrust ring, respectively, in slightly spaced relation.