PURPOSE: To enable a bias magnetic field to be inverted at high speed with a simple mechanism and a drive circuit and achieve a low power consumption by allowing a drive coil to be in a bent shape in a direction for approaching a magnetic rotary body along an axis which is in parallel to a rotary axis.

CONSTITUTION: A drive coil 1 is mounted on a fixed base 2 and is in a shape which is bent in a direction for approaching a rotary body 4 along an axis which is in parallel to a rotary axis 4a of the magnetic rotary body 4. Also, the yoke 4 as a magnetic rotary body with the axis 4a at both edge portions is mounted on the base 2. Permanent magnets 3a and 3b for generating magnetic field which are magnetized in a same direction in a direction which crosses the axis 4a are applied to both surfaces of the yoke 4 in one piece. Then. when a current is allowed to flow to the coil 1, a magnetic field which is generated by it allows an asymmetrical magnetic field to be generated around a magnetic pole surface and Maxwell stress which is operated upon the magnetic pole surface generates a rotary torque.