PURPOSE: To obtain a formed object of a belt without void, while the air remaining between tension resisting ropes in a mold is effectively discharged by a method in which an annular air-venting groove is provided, and the groove is drawn to vacuum through vacuum system-introducing part.

CONSTITUTION: An O-ring 8 is set on the mating face between both inner and outer molds M1, M2 forming the cavity for a product i.e., on the parting line of a core. On the side of the outer mold of the mating face, the air-venting recessed part 9 connected to the cavity 5 for a product, is provided. From the recessed part, the cavity 5 for a product is connected to the vacuum system- introducing part 10 provided at the position deviating from the cavity 5. Further, the cavity 5 is connected to an outer vacuum system by communicating to vacuum sucking holes 11 penetrating the outer mold M2. Such mold is used, and after the pressure in the cavity for a product of an injection molding machine has been reduced, injection molding is achieved, whereby the remaining air between tension resisting objects may be discharged outside.