PURPOSE: To improve the efficiency of a casting operation in a reducing or inert gaseous atmosphere by evacuating the inside of a pouring chamber which can be hermetically closed, introducing an inert or reducing gas therein and performing pouring under the condition when the relation between the degree of vacuum in the chamber and the inflow rate of the gas attains a prescribed equil. state.

CONSTITUTION: A molten Mg alloy is filled in a ladle 36 provided to the inside wall of a pouring chamber 50 and when the molten metal attains a specified temp., a shell mold 32 dewaxed in a calcining furnace is fed by a conveyor into the chamber 50. The opening of the chamber 50 is then closed hermetically with a cover plate 14 and the cover 42 of the ladle 36 is closed. After the inside of the chamber 50 is evacuated by a vacuum pump 56, at least one of an inert gas and reducing gas is fed under the reduced pressure from a cylinder 58 into the chamber 50. When the prescribed degree of vacuum is attained, the molten metal is cast through a valve 44 into the mold 32. A valve 62 is closed and the atmospheric air is conducted through a valve 64 into the chamber upon completion of casting, then the mold 32 is conveyed to a cooler. The casting operation is thus accomplished efficiently in the atmosphere of the reducing gas or inert gas.