a cylindrical inner yoke made of a magnetic material and having an outer peripheral surface;

a first cylindrical inner permanent magnet joined to the outer peripheral surface of the inner yoke and including an inner peripheral portion which is magnetized so as to have one of north and south poles and an outer peripheral portion which is magnetized so as to have the other pole, the first inner permanent magnet having an outer diameter and an outer peripheral surface;

a second cylindrical inner permanent magnet joined to the outer peripheral surface of the inner yoke so as to be spaced away from the first inner permanent magnet and including inner and outer peripheral portions which are magnetized so as to have polarities opposite to polarities of the inner and outer peripheral portions of the first inner permanent magnet respectively, the second inner permanent magnet having an outer diameter;

an outer yoke made of a magnetic material and formed into such a cylindrical shape that the outer yoke has an inner diameter larger than the outer diameters of the first and second inner permanent magnets, the outer yoke being disposed on the outer peripheral portions of the first and second inner permanent magnets, the outer yoke having an inner peripheral surface;

an interconnecting member interconnecting the outer and inner yokes so that the inner peripheral surface of the outer yoke is radially opposed to the outer peripheral surfaces of the first and second inner permanent magnets;

a first outer permanent magnet joined to the inner peripheral surface of the outer yoke with a first gap being defined between the first inner and outer permanent magnets and formed into such a cylindrical shape that the first outer permanent magnet is radially opposed to the outer peripheral surface of the first inner permanent magnet, the first outer permanent magnet including inner and outer peripheral portions both of which are magnetized so as to have the same polarity as the inner and outer peripheral portions of the first inner permanent magnet respectively;

a second outer permanent magnet joined to the inner peripheral surface of the outer yoke so as to be axially spaced away from the first outer permanent magnet, the second outer permanent magnet being formed into such a cylindrical shape as to be radially opposed to the outer peripheral surface of the second inner permanent magnet with a second gap therebetween;

a first armature coil made by winding a magnet wire into a cylindrical shape and inserted into the first gap so as to be axially movable; and

a second armature coil made by winding a magnet wire into a cylindrical shape and inserted into the second gap so as to be axially movable, the second armature coil being mechanically connected to the first armature coil,

wherein the inner yoke includes an inner thicker portion axially opposed to the first and second inner permanent magnets and having a larger radial thickness than a remaining portion of the inner yoke, and the outer yoke includes an outer thicker portion axially opposed to the first and second outer permanent magnets and having a larger radial thickness than a remaining portion of the outer yoke.