PURPOSE: To reduce the magnetic loss very much, by winding the coil around the pole piece consisting of the round bar material directly to reduce the magnetic impedance between the pole piece and the coil.

CONSTITUTION: The magnetic circuit including pole pieces 7 and 8 and common yoke 13 and the magnetic circuit including pole pieces 9 and 10 and common yoke 13 are provided. Coils 17 and 18 are arranged in two magnetic circuit respectively, and one coil 17 is wound around pole piece 8, and the other coil 18 is wound around pole piece 10. In this case, since pole pieces 8 and 10 consist of round bar materials, coils are wound around pole pieces directly without interposition of the coil bobbine or the like. As a result, the magnetic loss is reduced very much.