PURPOSE: To improve the quality of a steel ingot by moving horizontally a casting mold with respect to a gas nozzle for spraying a molten steel and blowing a prescribed flow rate of an inert gas to the molten metal surface of the casting mold disposed at a specific distance or below.

CONSTITUTION: Mold side walls 1 each consisting of a heat insulating material and having a flat rectangular shape are fixed on a flat ingot mold stool 2 to form the casting mold 3. A molten steel discharging pipe 4 is provided above the mold 3 and the gas nozzle 6 for spraying is disposed near the aperture end thereof. The moten steel flow 5 flowing downward from a discharge pipe is atomized by an inert gas such as Ar gas ejecting from a nozzle 6. The mold 3 is moved at ≥2mm/sec relative horizontal speed with respect to the nozzle 6 and the inert gas is ejected at 20W700Nm3 for each one ton of the steel plate from the nozzle 6. The distance between the molten metal surface 8 and the nozzle 6 is maintained at ≤3m. Blowholes and the segregation in the head part of the ingot are prevented and the quality of the ingot is improved by the above-mentioned method.