Abstract: A casing 10 includes second gears 20 having second locking cams 25 disposed in axial symmetry with a rotational axis of the casing 10, and guides 11A each having a slide-contacting face 11c, disposed in axial symmetry with a rotational axis of the casing 10 and on a straight line L4 perpendicular to a line L3 passing through the second locking cams 25. A cam 40 includes a plurality of first locking cams 41 for pressing the second locking cams 25, disposed in axial symmetry with a rotational axis of the cam 40, and guiding faces 45 disposed in axial symmetry with a rotational axis of the cam 40 and on a straight line L2 perpendicular to a line L1 passing through the first locking cams 41. A shaft 30 includes coupling portions 32, 34 for transmitting external torque and is press-fitted into the center hole 42 of cam 40 at a predetermined rotational angle.