PROBLEM TO BE SOLVED: To provide an effluence prevention method of nonmetallic inclusion of molten metal which facilitates aggregation thickening of the nonmetallic inclusion in the molten steel in tundish, makes floating separation easier and also can catch the nonmetallic inclusion incapable of perfectly floating.SOLUTION: A method of preventing effluence of nonmetallic inclusion 13 included in molten steel 2 in tundish 4 is provided. A cylindrical weir 11 is provided so as to surround a molten steel injection part to a casting mold 8 in the tundish 4, a whirling flow is formed in the molten steel 2 in the cylindrical weir 11, while retaining a catching device 12 which attaches the nonmetallic inclusion 13 to a center part of the whirling flow and catches the nonmetallic inclusion 13 such that the tip 12a gets to such a height position as to secure a channel to the molten steel injection part to inject the molten steel 2 in the tundish 4 to the casting mold 8. The nonmetallic inclusion in the molten steel c
