Sheet and coated steel have been demanded to meet quality requirements of ever increasing severity in recent years. Reduction in nonmetallic inclusions is an important challenge in this respect. Protection on the molten steel in the tundish against contamination is of special importance in not only improving steel product quality, but also avoiding immersion nozzle clogging and increasing the number of sequence-cast heats. To supply molten steel with high cleaniliness in a stable manner, the cleaning behavior of the molten steel in the tundish was investigated and mechanism whereby the molten steel is kept clean in the tundish was quantified. Based on the results achieved, technology was studied for keeping the molten steel clean in the tundish.