A rotor has a projection part projected from an inner wall of a central hole part toward a central axis and a rib part defined between opening parts adjacent with each other in a circumferential direction. A first imaginary straight line is defined to extend from the central axis outward in a radial direction through a center of the projection part. The projection part is formed so that the first imaginary straight line passes the opening part. A second imaginary straight line is defined to extend from the central axis outward in the radial direction through a center of the rib part. The rib part is formed so that the second imaginary straight line passes a center of a pole part or an intermediate position between pole parts adjacent with each other in the circumferential direction.