PURPOSE: To obtain a large steel ingot having high quality without using a large apparatus by arranging a base-plate in an assembling mold consisting of long side molds with upward widened taper and short molds and descending the ingot as expanding intermittently the clearance of both long side molds and an ingot holding apparatus.

CONSTITUTION: The base-plate 16 is arranged as holding the long side molds 2 with upward widened taper by the short side molds, and at first stage, molten steel 22 is poured from a pouring tube 20 as covering the molten steel surface by powders 24. As pouring the molten steel 22, the molten steel 22 at side faces and bottom face is solidified to form shell 26. After finishing the first stage, inner faces of the long side molds 2 and the ingot holding skids 10 are made as the same plane, and as keeping the upward widened taper, they are expanded intermittently for the clearance between the opposite faces by operating jacks 12 and at the same time, the base-plate 16 is removed. As the solidified shell, that is, the ingot is descended, the molten steel 22 is poured by volume corresponding to the descended length to keep the molten steel 22 to a fixed surface. The ingot is descended intermittently at low speed and is sprayed by water from spraying nozzles 14. At reaching to necessary length for the ingot, the pouring stage is finished.