PURPOSE: To separate and float quickly and efficiently nonmetallic inclusions in a cylindrical auxiliary tundish provided between a tundish and a mold, by forming swirling flow in molten steel and flocculating the inclusions at the center.

CONSTITUTION: The molten metal charged from a ladle 1 into a tundish 2 is so supplied as to generate swirling flow in a cylindrical tundish 2' through a flow passage 8. The swirling flow is further intensified by a magnetic induction turning device 10 for molten steel to flocculate the inclusions having the specific gravity smaller than the specific gravity of the molten steel to the center of the swirling flow thereby making the molten steel on the outside of the swirling flow clean. The clean molten steel is preferentially discharged from an outflow port 7' provided on the outside circumferential side of the circular bottom surface of the tundish 2' into a mold 3. It is possible to float effectively the flocculated inclusions by blowing an inert gas from the porous refractory material 6 provided in the central part on the bottom surface of the tundish 2'.