PURPOSE: To prevent the base cracking of a short square steel ingot by inclining a casting mold and molding board in such a manner that only one of the ridge or vertex of the base is lowest, starting pouring, returning the molding board to a horizontal position after a molten steel covers the base of the casting mold and complete the pouring.

CONSTITUTION: While the casting mold 2 is held fixed, the molding board 1 is inclined and the molten steel 5 is poured to the base on the lower side at the time of casting the short square steel ingot having ≤1 (H/A1/2≤1) ratio of the square root of a height H and base area A. The rising speed at the beach 6 is lowered by lowering the pouring speed during the time when the molten steel surface rises, by which the shrinkage of a solidified shell is absorbed to the lower side and the cracking is prevented. While the molding board is returned to the horizontal position in the second half of the pouring, the pouring is continued and finally a heat insulating material is charged into the mold to complete the casting. The base cracking of the ingot is thus prevented and the extremely thick single product is produced at a high yield.