A shift mechanism includes first and second ring bodies which are relatively rotatable; a shift cam surface formed on the first ring body on an end surface thereof which faces the second ring body, the shift cam surface being inclined with respect to a circumferential direction of the first ring body; a follower projection formed on the second ring body for engaging with the shift cam surface; and a rotating mechanism for relatively rotating the first and second ring bodies. The shift cam surface and the follower projection are arranged to move the first and second ring bodies toward and away from each other in the axial direction as the first and second ring bodies are relatively rotated. An annular rib is formed along the circumference of the first ring body outside the shift cam surface over the follower projection of the second ring body.