PURPOSE: To increase the inflow speed of molten metal into cavities and to improve productivity by evacuating the inside of the space in the upper part of a molten metal tank and the cavities, then restoring the atmospheric pressure in the space in the upper part of the molten metal tank or pressurizing said space.

CONSTITUTION: While a valve 30 is held open by operating a cylinder 38 upon die clamping, a die 11 is set in the prescribed position in its parting line. Cavities 15 and a space 22 in the upper part of a molten metal tank are connected with the 1st, 2nd selection valves 46, 47 to the 3rd selection valve 48, and to a vacuum tank 43 with the 3rd selection valve. The inside of the cavities 15 and the space 22 are evacuated simultaneously, and after the degrees of vacuum are checked with a pressure switch 53, the space 22 is changed over to a pneumatic tank with the valve 46. Then, the molten metal 25 in a molten metal tank 20 flows quickly into the cavities 15. When it fills the cavity 15 and flows to the valve disc 31 of a valve 30 through vent grooves 17 and vent ports 18, the valve 30 is closed by the effect of the large inertia force of the molten metal.