A molten resin is injected into a cavity together with long fibers. Opposite surfaces of the resin are solidified as a fiber-resin mixture temperature drops. These surface layers become non-foam layers. A foam layer is created between the non-foam layers. A pressure reduction process is applied to cause a spring back effect when the foam layer is created. A diaphragm having these three layers can be manufactured in an inexpensive process, but physical characteristics and appearances of the diaphragm are not sacrificed.