PURPOSE: To prevent clogging of a sprue in low pressure casting by providing a void part between a stalk and a heat insulating cover provided on the outside circumference thereof and insulating the heat of the molten metal.

CONSTITUTION: Molten Al 2 is charged from a heat insulating furnace 1 into a sprue 11 of a bottom part 10 through a stalk 5 and a sub-riser 9 under low pressure, and is filled into product parts. When the stalk 5 and the sub-riser 9 cool quickly, the molten metal 2 returning to the furnace 1 through the sprue 11 then through the sub-riser 9 and the stalk 5 upon ending of the filling into product parts solidifies and clogs the sprue. However, air is forced through a pressurizing pipe 4 into the furnace 1 to maintain the metal 2 always at a constant height in the stalk 5. Since a hole 7 is provided in a void part 8 for heat insulation, the molten metal 2 does not rise. Since the temp. drop of the molten metal in the stalk 5 is prevented by the heat insulating effect of the air in the part 8, the clogging of the sprue is obviated.