A carbonaceous acoustic diaphragm whose density is reduced while retaining the required stiffness is provided. Carbon nanofibers and spherical particles of PMMA are mixed into a carbon-containing resin such as a polyvinyl chloride resin, and the mixture is carbonized to vaporize the spherical particles of PMMA, thereby forming a porous structure having pores with the carbon nanofibers in a powdered form uniformly dispersed through amorphous carbon. By forming a multilayer structure by combining the porous layer with a layer that does not use PMMA, the density can be further reduced while retaining the stiffness.