PURPOSE: To largely reduce the generation of leaking magnetic flux and to easily make dimensional accuracy.

CONSTITUTION: A ring-like main magnet 3 is provided on a lower part plate 1 provided with a center pole 2, a ring-like upper part plate 4 is provided on the upper part of the main magnet 3 and respective components provided up and down with the upper plate 4 as a center are covered by a shield cover 7. Thus, a magnetic circuit 8 is constituted and further, a gap between the outer circumferential edge part of the upper plate 4 and the inner surface of the shield cover 7 is made as small as possible to be less than 0.5mm. Thus, by making the gap between the outer circumferential edge part of the upper plate 4 and the inner surface of the shield cover 7 as small as possible, since the magnetic potential difference between a frame 9 and the shield cover 7 can be made as small as possible, the generation of the leaking magnetic flux from the frame 9 towards the shield cover 7 can be reduced.