Water-proofed natural pulp or organic synthetic fibers are with polyester-type fibers having a low melting point, and subjected to a paper fabrication process. The fabricated product is dried with hot air at a temperature higher than the melting point of the polyester-type fibers, thereby melt-bonding only intersections of the fibers without completely fusing the polyester-type fibers. The pressure of the hot air contributes to the formation of a predetermined shape. Thus, a water-proof diaphragm for a loudspeaker, having a large thickness, a low density, a high internal loss and a high stiffness, is obtained. By incorporating the thus formed diaphragm, a high-performance loudspeaker having a low distortion and a broad reproducing range is obtained.