PURPOSE: To make the magnetic flux uniform by the pole teeth of the end and the center by providing both chamfered ends of a core, and providing a ladder-shaped auxiliary bias permanent magnet at the chamfered part.

CONSTITUTION: A pair of C-shaped cores 14 provided with both chamfered ends 13 are provided at the lower surface of a bias permanent magnet 12. A ladder-shaped auxiliary bias permanent magnet 16 magnetized in the same direction as that of the magnet 12 is provided in close contact with the end of the magnet 12 between the chamfered ends 13e of both the outsides of the pair of cores 14 and a back yoke 11. An armature 1 is formed of the yoke 11, the magnet 12, the cores 14, a coil 17 and the magnet 16. The magnetic flux ad of the magnet 16 is added to the magnetic flux b formed by the magnet 12 and the magnetic flux c formed by the coil. Accordingly, the magnetic flux is strengthened by the flux ad at the pole teeth 15e of the end. As a result, the edge effect is compensated.