PURPOSE: To absorb and remove nucleation site in molten steel by repeating cooling and heating operations, between the temp. at lower than the solidus of a steel and the temp. exceeding the liquidus and injecting glassy silicon dioxide together with inert gas into the molten steel.

CONSTITUTION: The molten steel 8 is charged into a vessel 1 for molten metal coating the inner surface with glassy silicon dioxide 4 and the glassy silicon dioxide powder is injected together with the stream of inert gas from a nozzle 6. After this molten steel 8 is cooled at the lower temp. than the solidus, the molten steel is heated to the temp. exceeding the liquidus. After there coolings and heatings are repeated to become the aimed degree of undercooling, a stopper 2 is lifted to cast the molten steel 8 into a mold. The injected silicon dioxide powder is dispersed into the molten steel 8 and the nucleation site is absorbed and removed to obtain a casting having little micro segregation.