In the regions around a magnetic shielding space, curved portions and having a rounded shape, which protrude toward the interior space of a magnetic shielding space, are formed at ends of a first side and a second side close to a first end and a second end of an outer-periphery side. In the vicinity of angular portions and of the magnetic shielding space that are positioned at both ends of the outer-periphery side, the magnetic flux density for a given cross-sectional area of a channel through which a short-circuit magnetic flux flows is reduced, and thus, the level of the short-circuit magnetic flux is reduced.