Electro-acoustic converters each include a diaphragm, and a fixed electrode apart from the diaphragm for a certain distance and facing the diaphragm. The electro-acoustic converters are anteroposteriorly disposed on the same axis in a single casing, and are electrically connected in series. The front and rear converters each include impedance converters, and are serially connected with each other together with the impedance converters.