PURPOSE: To maintain surely the nonoxidative atmosphere in a casting mold by continuing introduction of an inert gas to the inside bottom part of a sand mold through a gas replacing hole placed seperately from a runner just prior to pouring so that the atmosphere in the mold is replaced without any hindrance even in the stage of producing a casting having an intricate shape.

CONSTITUTION: A gas replacing hole 5 for replacing the atmosphere in a casting mold 1 consisting of a sand mold for casting, for example, a water wheel runner of a hydraulic generator with an inert gaseous atmosphere by introducing the inert gas into the mold 1 from a supply source 3 is provided separately from a runner 4 in the bottom part of said mold 1. The open end of a riser 8 is capped 9 if necessary and the inert gas is fed to the inside of the mold from the bottom thereof through the hole 5 to replace the atmosphere in the mold 1 with the inert gas. Such feeding of the inert gas is continued just prior to the start of pouring, then the molten metal is poured through the runner 4 into the mold to perform casting. The oxidation or the like of the molten metal is thus effectively prevented and the casting product having high quality is obtd.