PURPOSE: To obtain a composite diaphragm with light weight and large rigidity, by bonding a surface member at the external surface of the end of a projection and that of a recessed part of a plate core member where the projections and recessed parts of frustum of triangular pyramid are regularly formed with a prescribed relation.

CONSTITUTION: In a core member 11, a lot of the same size of projections 12 of frustum of regular triangular pyramid are arranged regularly with a prescribed arrangement, and a lot of recessed parts 13 of symmetrical shape as the projections 12, like the inversion of the projections 12 are arranged. The member 11 is a metal mold of shape corresponding to the arrangement of the projections 12 and the recessed parts 13, and the mold can be formed by pressing a light plate such as a sheet of paper and aluminum thin plate. An adhesives is applied to the external side of a regular triangular end 13s of the recessed parts 13, a surface member 14 is bonded, and the adhesives is applied to the external side of the end 13s of regular triangular shape of the recessed parts 13, a surface member 15 is bonded, the entire parts are made flat, allowing to obtain a diaphragm most suitable for the acoustic diaphragm of a speaker.