PURPOSE: To obtain the diaphragm of a speaker which is lightweight and has a high stiffness and has an adequately large internal loss and can obtain a flat reproducing frequency characteristic by joining easily a diaphragm base board and a high-stiffness sheet with secure strength.

CONSTITUTION: First, a netted thermoplastic resin sheet 1 is laid in a making to sheets machine 2. Next, an aqueous pulp dispersion 3 where a pulp fiber, a binder, etc. are dispersed in the water is charged into the machine 2, and a diaphragm base material 4 like a cone paper is accumulated and formed on the sheet 1 by manufacturing to sheets. This diaphragm base material 4 is taken out together with the sheet 1 in the wet state, and a high-stiffness sheet 5 is put on the lower face of the sheet 1, and they are pressed with a press machine 6 while heating them. Then, the diaphragm base material 4 is dried. Next, the sheet 1 is melted and acts as an adhesive to join the diaphragm base material 4 and the high-stiffness sheet 5. An obtained diaphragm 10 is lightweight and has a high stiffness and has an adequate large internal loss and can reduce resonance sharpness to make the reproducing frequency characteristic flat.