A diaphragm assembly used for a condenser microphone has a diaphragm made of a resin film including a metallized film on one surface of a supporter ring. The diaphragm is made by a first step of bonding a ring jig of a larger diameter than the supporter ring to the resin film having the metallized film composed of a ductile metallic material on the one surface via an adhesive without exerting tension on the resin film; a second step of heating and contracting the resin film bonded to the ring jig without applying the tension at a temperature over a glass transition point of a film material; and a third step of bonding the supporter ring to the resin film via an adhesive in a state of exerting predetermined tension on the resin film. The diaphragm assembly is cut out of the resin film after the adhesive becomes hardened.