PURPOSE: To surely pour hollow-forming fluid in synthetic resin and prevent the fluid from leaking.

CONSTITUTION: A groove 31 is provided around a fluid pouring port 10, through which hollow forming fluid 930 is poured. Further, a recessed part 71 is provided between the groove 31 and a runner 21. At the pouring of the fluid, the fluid tends to escape along the boundary between a mold 62 and the synthetic resin in the runner 21. However, due to the solidified layer of synthetic resin formed in the groove 31, resistance develops in the fluid. As a result, by the groove and the recessed part 71, the flow path of the resin is connected to the synthetic resin in the runner 21.