To avoid the generation of a defective product due to porosity caused by insufficient deaeration from an unvulcanized belt molded product during a vulcanizing process and an adhesion failure between a core and a rubber.

This unvulcanized belt molded product 5 of a double ribbed type has a core material 5b sandwiched between an inside rubber 5a and an outside rubber 5c. In addition, this molded product is used for a vulcanizing device K of such a structure that a heating steam and a pressurizing steam are freely supplied to the inside of a mold 2 to be internally fitted to the molded product and to the outer peripheral side of a jacket 6 to be externally fitted to the molded product respectively. Further, a nonwoven fabric 20 which is of such a length that it is vertically short and wound around an upper and a lower end part between the core material 5b and the outside rubber 5c and makes a single wrapping round around the unvulcanized belt molded product 5, is interposed in a ring-like form in such a state that the nonwoven fabric 20 slightly protrudes in the axial direction ahead of the unvulcanized belt molded product 5.