This 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) that is able to engage with the clutch housing (50); a gear wheel (57) that holds the pawl (41) and rotates according to 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) due to frictional force. The friction ring (42) is provided with: a protrusion part (42a) that engages with an engagement groove (41b) of the pawl (41); and a plurality of contactpieces (42b) that come into contact with the outer peripheral surface of a small diameter ring part (26c) of the outer gear cover (26) and press against the small diameter ring part (26c) toward theinside in the radial direction. Consequently, assembly of the friction ring onto the outer gear cover can be facilitated.