PURPOSE: To increase strength by formation of fine graphite grains of a cast iron and to prevent the generation of cementite by providing a blowing part for an inoculant in a part of a sprue, blowing the inoculant powder simultaneously with charging of a molten metal and melting said powder into the molten metal flow.

CONSTITUTION: An air permeable casting mold 1 having a sprue 3 on the bottom side is immersed into the melt 4 of a gray cast iron or spheroidal graphite cast iron in a furnace and at the same time, the inside of the mold 1 is evacuated by making use of the air permeability of the mold 1 to suck the molten metal up into the mold. Inoculant powder of Fe-Si, Ca-Si, etc. is blown from an inoculant blowing part 2 provided in a part of the sprue 3 to melt the inoculant into molten metal flow 5 in this stage. The fine graphite grains of the cast iron are thus formed, by which the increase in the strength and the prevention of cementite generation are made possible.