PURPOSE: To attain miniaturization of a device and high performance by mounting a ring permanent magnet with a magnetic pole having different inner circumference and outer circumference to the inner and/or outer circumference of a cylindrical air gap so as to induce a magnetic field in the radial direction to the cylindrical air gap.

CONSTITUTION: A ring permanent magnet with a magnetic pole having different inner circumference and outer circumference is mounted to the inner and/or outer circumference of a cylindrical air gap so as to induce a magnetic field in the radial direction to the cylindrical air gap. For example, the permanent magnet 1 of ring shape magnetized in the radial direction shown in the arrow is mounted to the inner circumference of a yoke 22 of a yoke section 2 where a center core 21 and the yoke 212 are incorporated, that is, the outer circumference of the cylindrical air gap 3. Since the ring shape magnet 1 faces directly the cylindrical air gap 3 and the magnetic field in the part is induced directly by the permanent magnet 1, a magnetic flux with high density is concentrated onto the cylindrical air gap 3 and magnetic leakage is less. Thus, the device is made small in size the high performance is attained.