PURPOSE: To simplify assembling and obtain a magnetic circuit having dimensional accuracy by performing setting of each parts through press fitting into the frame, and by using a resin magnet shaped to be possible to make the press fitting.

CONSTITUTION: An upper plate 2 is designed to be of the same size as inner diameter of a recessed part 6 of a frame 1 and press fitted in the recessed part 6. Then, making a central part of resin magnet 9 into a shape possible to be press fitted in the center pole part 11 of a bottom plate 4, providing collar 10 in the inner diameter, and further making the size of outer peripheral part possible to be presse fitted in the insertion part 7 of a magnet of the frame 1. Thus, the magnet 9 is press fitted in the frame 1 and the plate 4 is press fitted in the collar 10, which results in operation of magnet 9 itself as a jig to make the size of a gap 8 of the magnetic circuit correct. Thereafter, bonding material 12 is applied on outer peripheral part of the plate 4 to fix a whole part of the magnetic circuit.