PURPOSE: To form a unidirectionally solidified ingot which is defectless and has high quality by lining the inside of an ingot case on a molding board with heat insulating refractories, passing cooling water to the passage in the molding board and making the ingot in such a manner that the cooling rates in vertical and horizontal directions attain a specific ratio.

CONSTITUTION: Cooling water of an adequate temp. and flow rate is circulated to the passages 8 of the molding board 7 and the molten steel is supplied from a pouring port 9 into the casting mold. Since the ingot case 5 is lined with the heat insulating refractories 6, the formation of the solidification boundary to a diagonal line shape is suppressed without the growth of the columnar crystals of the fine grains in the outermost layer and toward the center in the cross section by quick cooling. On the other hand, the molten steel is continuously and quickly cooled by the water cooled molding board 7 and therefore the columnar crystals are grown upward from the bottom of the ingot as the molten steel rises and solidifies. The conditions of the ratio v1/v2>10 between the cooling rate v1 in the vertical direction and the cooling rate v2 in the horizontal direction are controlled and satisfied by taking the temp. and flow rate of the cooling water, the material and size of the heat insulating refractories, the material, size, etc. of the molding board into consideration in order to suppress the formation of the solidification boundary to the diagonal line shape and to obtain the ingot grown unidirectionally with columnar crystals.