A power transmission mechanism (19) for transmitting power from a motor (34) to a spindle (12) is provided with a clutch housing (50), a pawl (41) which is capable of engaging with the clutch housing (50), a gear wheel (57) which holds the pawl (41) and rotates in accordance with the rotation of the motor (34), an outer gear cover (26), and a friction ring (42) which engages with the pawl (41) and is held on the outer gear cover (26) due to frictional force. The friction ring (42) is provided with a protruding portion (42a) which engages with an engagement groove (41b) of the pawl (41), and a plurality of contact pieces (42b) which come into contact with an outer peripheral surface of a small diameter ring portion (26c) of the outer gear cover (26) and press against the outer peripheral surface towards an inner side in a radial direction. As a result, the friction ring can be easily coupled to the outer gear cover.