An elastic body 51 is provided to absorb impact energy generated by the collision between rotor positioning portions 31 and shaft portion positioning portions 29 when a shaft portion 24 is moved into a female thread portion 26 to determine the initial position of the shaft portion 24, and so the impact noise generated by the collision between the rotor positioning portions 31 and the shaft portion positioning portions 29 can be reduced. Also, the force of impact imparted to the shaft portion positioning portions 29 by the rotor positioning portions 31 decreases, and so it is possible to prevent deformation of the rotor positioning portions 31 and the shaft portion positioning portions 29.