To solve the problem of a voice coil generator, where the thickness of a magnetic field forming unit is increased and hence a dimensional increase can not be avoided and, further, since the magnetic field forming unit has an opening on one side only, a structure having openings on both sides cannot be obtained.

The polarity of a magnetic field formed between an outer cylinder 32a and an inner cylinder 32b is partially changed in an axial direction and, further, the winding direction of a coil 31c provided in a cylindrical gap formed between both the cylinders 32a and 32b is reversed successively corresponding to the polarity of the magnetic field. With such a constitution, it is not necessary to close one-side openings of the outer cylinder 32a and the inner cylinder 32b, so that the total length can be reduced and, by forming vibrating plates 31b on both sides, the change of pressure can be transmitted easily.