A first member (40a) has a magnet assembly (20) that includes a plurality of permanent magnets (10) held with their homopoles contacting one another. A second member (50a) includes magnet coils (30), and is designed to be changeable in position relative to the first member. The magnet assembly (20) generates the strongest magnetic field in a magnetic field direction lying in the homopolar contact plane at which the homopoles contact one another, the magnetic field direction being oriented outward from the magnet assembly (20) along the magnetic field direction.