PURPOSE: To improve inclusion separation and shell forming by only a little quantity of heat insulating material, and to improve ingot quality, by introducing the inert gas from a ventable refractory at shifting wall bottom part in a horizontal mold and charging an adsorptive heat insulating material between the wall and the skimmer placed in front of the wall.

CONSTITUTION: The inert gas, such as Ar gas, etc., is introduced into molten steel (S) in the horizontal hold 1 from the ventable refractory 6 at wall 3 bottom part, which shifts relatively in the mold 1, solidifying the molten steel at one side from the bottom face. Further, the skimmer 12 is arranged in front of the wall 3 and a submerged nozzle 2 is arranged at the narrow range between the skimmer 12 and wall 3, and also the heat insulating material 4 having good adsorptivity of inclusion is charged. The inclusions in the molten steel (S) poured from the submerged nozzle 2 are floated up compulsorily by bubbling flow 9 of Ar gas and adsorbed into the heat insulating material 4 under condition of good separation, and also consumption of the heat insulating material 4 is reduced. Further, the development of the primary solidified shell on the bottom face of the horizontal mold 1 is uniformly stabilized and occurence of the constrained cracking is prevented. Therefore, the ingot quality is improved.