In a ring body of a stator, permanent magnets are arranged in alternate inter-pole magnetic path portions located between an even number of stator poles so that magnetic fields pointing in the circumferential direction of the ring body are generated. Coils are wound around alternate inter-pole magnetic path portions where permanent magnets are not provided so that magnetic fields that oppose the magnetic fields generated by the permanent magnets are generated. When electric power is not supplied to the coils, a ring-shaped magnetic circuit is formed by the permanent magnets, and therefore, magnetic flux does not leak to a rotor. When electric power is supplied to the coils, the magnetic flux of the magnetic fields of the permanent magnets and that of the coils combine with each other and flow from the stator poles to rotor poles of the rotor, whereby strong attraction force is obtained.