A high-performance variable-magnetic-field dipole-ring magnetic circuit is provided. Specifically, provided is a permanent-magnet magnetic field generator having inner and outer dipole rings which share a central axis, and a yoke which surrounds the dipole rings, wherein a unidirectional variable magnetic field is generated in a space within the inner dipole ring. The generator has a fastener for fixing the inner dipole ring, and a rotator for allowing the outer dipole ring to rotate around the central axis. In the generator, each of the inner and outer dipole rings has a plurality of permanent magnets arranged in such a ring shape that a magnetization direction of the permanent magnets can make a full rotation per half circle of the ring.