A magnetic shield made of magnetic material in the shape of a thin cylinder is externally fitted and fixed on a cylindrical mold member holding magnetic flux collecting rings so that it has extended sections with appropriate length on both sides in the axial direction, and therefore magnetic noise from outside in the radial direction is shielded directly by the outer surface of the magnetic shield, magnetic noise from both sides in the axial direction is shielded by concentrating the magnetic noise on the end faces of the magnetic shield, and the influence on the magnetic flux collecting rings held in the mold member is eliminated.