A power transmission mechanism (19) for transmitting power from a motor (34) to a spindle (12) having a clutch housing (50), a pawl (41) engageable with the clutch housing (50), a gear wheel (57) which holds the pawl (41) and rotates with the rotation of the motor (34), an outer gear cover (26), and a friction ring (42) engaging with the pawl (41) and held on the outer gear cover (26) by 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) in contact with an outer peripheral surface of a small diameter ring portion (26c) of the outer gear cover (26) and press radially against the outer peripheral surface towards an inner side The friction ring can be easily coupled to the outer gear cover.