To reduce leakage magnetic flux and to surely perform shielding by setting distance between the edge part of a cancellation magnet and the peripheral wall part of a shield cover larger than the thickness in the forward/ backward direction of the cancellation magnet.

An annular-type cancellation magnet 4 is adhered on the rear surface part of a yoke 1. The shield cover 5 is adhered on the rear surface part of the cancellation magnet 4. The distance t2 between the edge part of the cancellation magnet 4 and the peripheral wall part of the shield cover 5 is set larger than the thickness t1 in the forward/backward direction of the cancellation magnet 4. The leakage magnetic flux from a driving magnet 3 is reduced and flux density in a magnetic gap part 7 is increased by the cancellation magnet 4. The leakage magnetic flux is reduced and the shielding is surely performed, and the magnetic energy of the driving magnet 3 and that of the cancellation magnet 4 are efficiently used.