To provide an electret condenser microphone that is operated by a fantom power supply and the sensitivity of which can be adjusted without relying on an attenuation capacitor and an amplifier.

The electret condenser microphone is used by being connected to the fantom power supply 30 and includes: a microphone unit 10 with a diaphragm and a fixed pole arranged opposite to each other, and with an electret dielectric film provided in either of them and subjected to polarization processing; and an impedance converter comprising a FET 20 whose gate G is connected to the fixed pole 12 and operated as a source follower, wherein the gate G of the FET 20 is connected to the fixed pole 12 via a first capacitive element C1 and a prescribed voltage (e.g. 40V) of a power supplied from the fantom power supply is applied to the fixed pole 12 or the diaphragm 11 for the adjustment of the sensitivity.