PURPOSE: To perform cutting out of products in a vulcanization molding process by arranging an unvulcanized sheet material between upper and lower molds and producing a diaphram therefrom.

CONSTITUTION: A circular annular rib molding groove 36 is arranged to the upper surface being the mating surface of a lower mold 12 and an upper mold 11 in concentric relation to inner and outer cutting blade moving grooves 14, 18 so as to be downwardly depressed in a vertical direction. This rib molding groove 36 is constituted and arranged so as to be capable of forming the rib 4 protruding from the rear surface of the diaphragm 1 being the product of a diaphragm manufacturing apparatus. Heaters are built in the upper and lower molds 11, 12 so as to heat both molds 11, 12. Since the main body 2 and through-hole 3 of the diaphragm 1 being a processed product can be removed in a vulcanization molding process by this constitution, it is unnecessary to provide a pulling-out process other than the vulcanization molding process and the number of production processes can be reduced to contribute the reduction of production cost.