PURPOSE: To obtain excellent torque transmitting characteristics, and improve durability.

CONSTITUTION: A cylindrical clutch outer 23 is integrally molded in a boss part 21 to engage with an output shaft 1 by a helical spline, and its opening side is formed in an port opening shape. In an inner peripheral surface of this clutch outer 23, a circumferential directional shape is a wedge shape, and notches 23a having a uniform inside diameter in the shaft direction are extendedly arranged in a plurality at an equal angle pitch in the shaft direction, and have a wedge-shaped profile. A clutch inner 24 to be housed in the inner periphery of the clutch outer 23 is formed so that a shaft directional inclination of its outer peripheral surface becomes almost equal to a shaft directional inclination of an inner peripheral surface of the clutch inner 24. A roller 8 is movably housed in the circumferential direction in a wedge-shaped space between the notches 23a and an outer peripheral surface of the clutch inner 24.