To easily, safely, intensely and uniformly stir molten metal and to obtain a high efficient refining reaction by holding the molten metal in a horizontally long cylindrical vessel so that the cross sectional area is different in the horizontal direction, electromagnetic-stirring this molten metal and developing the flow in the vertical upward direction.

The molten metal 8 is incorporated in a torpedo car 1 which is the horizontally long cylindrical vessel changing the inner diameter. Into this molten metal 8, refining agent together with stirring gas are supplied through a lance 4 for injection to execute pre-refining. Further, the molten metal 8 is electromagnetic-stirred to execute the stirring of the refining agent. At this time, shifting magnetic field generating means 10 for torpedo car are arranged at the parts gradually changing the inner diameters at both ends of the torpedo car 1. The flows of the electromagnetic stirrings in vertical upward generated with shifting magnetic fields are shifted to the center direction along the inner walls of the inner diameters gradually changed in the vessel to intensely stir the whole molten metal 8 therein. By this method, the refining agent is uniformly dispersed to efficiently execute pre-refining treatment.