Affixed in intimate contact to the inside surface of a front board 10a of a conductive capsule 10 provided with a receiving sound aperture 10aa is a conductive back electrode board provided with air vent apertures 11a, 11b which are positioned not in alignment with the receiving sound aperture, these apertures 10a and 11a, 11b being connected by a slit 10ad. A polarized FEP film 12 which is an electret film is disposed on the surface of the back electrode board 11 opposite from the front board 10a, and a conductive diaphragm 14 is disposed on the surface of the FEP 12 opposite from the back electrode board 11 with a spacer 13 interposed therebetween which extends around the outer periphery of the opposite surface of the FEP 12. These component parts are accommodate in the capsule 10 to constitute an electret condenser microphone which is capable of suppressing sensitivity degradation due to ingress of grit from the outside to the electret film.