This torque limiter (64) is configured from a ring body (65) and a spring-form part (66) provided in the ring body (65). The ring body (65) comprises a band-form metal plate and is bent approximately into a C-shape along the circumferential direction of a motor shaft (34) and a lock holder (52). The spring-form part (66) is formed so as to be capable of radial elastic deformation. The spring-form part (66) is configured from main protrusions (71) arranged in a compressed state between the motor shaft (34) and the lock holder (52), and from sub-protrusions (72) arranged in a non-compressed state. Further, grease is stored in a part or all of the concavities formed by the main protrusions (71) and the sub-protrusions (72).