A second ball bearing is installed from a first axial side toward a second axial side into an interior of a bearing holder of a rotor through an opening of the bearing holder, so that an outer race of the second ball bearing is press fitted to an inner peripheral portion of the interior of the bearing holder. A first ball bearing is inserted from the first axial side toward the second axial side into the interior of the bearing holder of the rotor through the opening of the bearing holder after the installing of the second ball bearing, so that an outer race of the first ball bearing is press fitted to the inner peripheral portion of the interior of the bearing holder and is axially spaced from the outer race of the second ball bearing.