PURPOSE: To reduce the distortion in the frequency characteristic by mixing a titanium fiber or potassium titanate fiber and flaky graphite or mica with polypropylene to form a diaphragm so as to eliminate orientation.

CONSTITUTION: A reinforcement fiber such as titanium fiber, potassium titanate fiber or the like, and a filler such as flaky graphite or mica are mixed with a polypropylene, or its copolymer or a mixture incorporating the polypropylene as a major component, the mixture is formed into a film and this film is formed into the shape of diaphragm. Thus, the rigidity is larger than that of the diaphragm of single polypropylene, the orientation is lost, and no split damping exists even at a low frequency region and a smooth frequency characteristic is obtained.