A diaphragm assembly has a center disk in which use of a metal member is not required. The center disk comprises first and second center disk members, made of resin, embracing a resin diaphragm between them. The first center disk member has an inwardly directed inner flange on an inner peripheral wall defining a resin charging space, and the second center disk member comprises a resin-molded body, formed by charging a resin, and has an outwardly directed outer flange. A portion of the diaphragm is clamped between the inner flange and the outer flange.