An elastic body (51) is provided to absorb the energy of collision generated by the collision of a rotor positioning section (31) against a shaft positioning section (29) when the initial position of a shaft section (24) is to be determined by shifting the shaft section (24) toward a female thread section (26), so that the noise of collision generated when the rotor positioning section (31) hits the shaft positioning section (29) can be reduced. Furthermore, the force of collision the rotor positioning section (31) exerts on the shaft positioning section (29) is reduced to prevent the rotor positioning section (31) and the shaft positioning section (29) from being deformed.
