An addendum of a gear portion and a tip end of a first cam follower are located on similar-diameter cylindrical planes centering at a center axis of a cam ring. A gear housing recess is provided in an inner peripheral surface of a fixed ring. A bottom surface of a first-cam-ring-movement cam groove engaging with the first cam follower, and a bottom surface of the recess are located on similar-diameter cylindrical planes centering at a center axis of the fixed ring. A release portion is provided at the inner peripheral surface of the fixed ring to provide communication between a protruding position cam groove portion of the first-cam-ring-movement cam groove and the recess. At the protruding position of the cam ring, a portion of the gear portion near the first cam follower passes through the release portion and is located inside the protruding position cam groove portion.