PURPOSE: To decrease the generation of inverse V segregation and macro segregation, etc. by providing a riser part in the upper part at a convergent taper end of a hermetic casting mold of which the base is the max. wall surface of a flat casting mold having a taper, disposing a heat insulating material to the periphery thereof and protecting the surface of the molten steel to be poured into the mold with the heat insulating material.

CONSTITUTION: The flat casting mold 1 is placed on a mold stool 4 with the max. wall surface of said mold as a bas and the apertures at both ends are held in place between side face molds 2 to form the hermetic casting mold. The riser part 3 is provided atop the end on the open side of the taper in a manner as to notch the mold 1 and the heat insulating material 7 is provided to the periphery thereof. The molten steel 6 is poured from the riser part 3 into the mold constituted in the above-mentioned manner and the surface of the molten steel is protected by a heat retaining material 8. Then the solidification on the riser part side is last as compared with the other sections and the cooling mainly consisting of the heat extraction from the base of the max. wall surface of the mold 1 is made possible. The solidification boundary approximately horizontally rises.