PURPOSE: To provide a diaphragm which is higher in waterproof property than a conventional diaphragm, excellent in flexibility, improved in consistency in mass production, long in durability, excellent in versatility and tensile strength, inexpensive in assembly and manufacture, and simple in design.

CONSTITUTION: A cloth sheet of warp-knitted synthetic fibers having interstices between the fibers is manufactured, the strong tension is applied to the sheet, an elastic polymer coated layer is applied only to one side of the sheet, the coated sheet is placed in a die, the coated sheet is heated and pressurized in the die to press the elastic polymer into the interstices between fibers, an elastomer 88 is cured, and finally, a diaphragm 40 is individually taken out of the die from a formed diaphragm sheet 108'.