PURPOSE: To prevent the breakout of a molten steel and to unidirectionally solidify the molten steel using a water-cooled mold stool and plural water-cooled side walls which are not fixed by the same to constitute a casting mold which is freely movable in a horizontal direction and controlling the spaces between gates and the side walls to specified spaces.

CONSTITUTION: The molten steel is poured into the side walls 25 of the casting mold via a pouring nozzle 28 from a tundish 27 and a heat insulating material is sprayed atop the molten steel from a heat insulating material spraying machine 43. The casting mold is horizontally moved by a casting carriage 23 when the molten steel attains the desired pouring height. The side walls 25 of the casting mold are divided to a plurality and are further cooled with water and therefore, the generation of large thermal strain is obviated. Since the side walls 25 of the casting mold are not fixed by the mold stool 24, the position adjustment is easily executed by top surface rollers 33 for supporting the side walls. The gates 26 which remain stopped in the same position and rub the inside surfaces of the casting mold continue to stably maintain the specified space from the inside surfaces of the casting mold and the gates 26 do not stick to the side walls 25 of the casting mold. The molten steel is thereby unidirectionally solidified from the base without inducing breakout and the steel ingot having high quality is obtd.