PURPOSE: To attain small size and high sensitivity, by forming a side plate of a magnetic circuit constituted around a canti-lever type armature 8 vertical to the armature and driving the vicinity of a diaphragm via a drive pin.

CONSTITUTION: A moving end of an armature 8 is inserted between magnets 17 and 17'. Side plates 7, 7' at the side of the armature 8 are provided vertical to the armature 8 not horizontal. An oscillating pin 23 for driving a diaphragm 21 is stood near the center of the armature 8. The center of the diaphragm 21 is driven with the pin 23. An Ac magnetic circuit including the armature 8 is formed independently and the connecting part with a DC magnetic circuit including the magnets 17, 17' is bonded at the neutral position entirely. Thus, small sized device can be attained without losing the magnetic characteristics.