PURPOSE: To minimize the working point permeance coefficient by a mechanism wherein a permanent magnet consisting of a magnetic circuit such as a speaker is made into a bent solid body of a certain thickness, thus resulting in an increased magnetic pole area and smaller thickness without increasing the outer diameter.

CONSTITUTION: The isotropic permanent magnet formed by sintering a rare earth elenent such as cobalt is buried in a pole piece 12 installed to a yoke plate 14, thus, a magnetic void 16 is installed between the plate 14 and the piece 12. In this structure, the magnet 10 is made into a shell type structure of a certain thickness and of a bent shape. Or, the magnet can be buried in each yoke plates at both sides holding the pole piece between them, to generate the magnetic void between them. In this way, the magnetic potential difference between the magnet 10 and the piece 12 or the plate 14 both of which are connected therewith becomes negligible and the magnetic pole area is substantially increased.