PURPOSE: To obtain the diaphragm of high sound quality having a light weight, a high rigidity and a proper internal loss by mixing metallic boron and/or boron carbide with a thermosetting resin, thermally molding this mixture to a diaphragm form and heating it in an inert atmosphere.

CONSTITUTION: A mold 1 is one for producing a dome shape diaphragm and constituted of a protruding male mold 1A and a recessed female mold 1B. This mold 1 is heated to a prescribed temperature, and the mixture 2 of the metallic boron and the boron carbide is evenly sprayed or applied on the male mold 1A by the use of a sieve or the like. According to this thermocompression processing, the mixture 2 is molten, set and integrated to obtain a dome shape molded member 3. Then, this molded member 3 is taken out from the mold 1, put into a furnace, heated under a non-oxidizing atmosphere, carbonized or graphitized. According to this heating, a resin component in the molded member 3 is carbonized by a thermal decomposition reaction, further a part thereof is graphitized, reacts with a part of the metallic boron to be the boron carbide.