PURPOSE: To prevent casting defects and to improve yield by using a core made of ceramic fibers and solidifying unidirectionally a molten steel upward from the bottom part.

CONSTITUTION: Refractories 26 such as magnesia are impregnated into the surface of the cylindrical ceramic fibers 25 and further molding sand 27 is packed into the axial center part thereof to form the core 23. The molten steel 8 is packed into the region between a molding board 1, the core 25 and a heat insulating sleeve 22 by top pouring and a heat insulating material 28 for the molten steel surface is introduced therein. Since the inside and outside circumferential surfaces of the molten steel 8 are heat-insulated by the ceramic fiber core 23 and a heat insulating sleeve 22, the molten steel 8 is subjected to heat extraction from the molding board 1 in the bottom and is thereby unidirectionally solidified upward in the form of columnar crystal structure. The shrinkage during the solidification of the steel ingot is absorbed by the deformation of the core and the casting defects can be prevented. The yield of production is improved as the smaller feeding ratio is allowed.