PURPOSE: To uniform the surface charge density of an electret excellently and to improve the stability greatly by preventing charge leakage due to the supporting structure for a fixed electrode and setting the distance from a diaphragm freely.

CONSTITUTION: When charges of an electret element 23 are minus, an electret structure 25 induces plus charges on the reverse side of a dielectric 24 mounted on the element 23 and minus charges having the same polarity with the element 22 on the top side, namely, performs dielectric (polarization) operation. Firstly, a switch SW connected to the dielectric 24 is closed to ground the dielectric 24, and then the switch SW is opened to obtain insulation from the earth. For example, an insulating pair of tweezers are used to mount the dielectric 24 on the surface of the electret element 23 while holding the zero charge state, thus obtaining the dielectric operation.