PURPOSE: To prevent a blocking of a gas passage with molten metal from basing occured and the shrinkage cavity from being generated at the time of exhausting gas.

CONSTITUTION: A spiral-state molten metal pool part 15 is arranged at a parting surface part of a gas venting passage 14 from a cavity 11 to the outer part and a hole 16 having smaller diameter than the inner diameter of the molten metal pool part is arranged at the central part of the molten metal pool part orthogonally to the parting surface part and pass through the outer part at the rear part. A pin 17 having a gas venting groove 18 extending in the axial direction on the outer peripheral surface at the rear part of a tip part is slidably arranged in the hole, and a passage 22 which can communicate/shut off the molten metal pool part and the gas venting groove, respectively, at the positions of advancing/retreating limits of the pin, are arranged between the inner peripheral surface of tip part of the hole and the outer peripheral surface of tip part of the pin 17. At the time of exhausting the gas by advancing the pin during filling up the molten metal to communicate the molten metal pool part 15 and the gas venting groove 18, it is prevented that the molten metal flows into the gas venting groove by action of centrifugal force in the spiral-state molten metal pool part. The pin 17 is retreated just before completing the fill-up of the molten metal to shut off the molten metal pool part and the gas venting groove, and the pin 17 is again advanced at the time of solidifying and shrinking the molten metal so as to act a molten metal pressing force.