PURPOSE: To prevent the intrusion of ladle slag into a molten steel and to improve the cleanliness of the molten steel by blowing an inert gas out of the outside circumference in the outside part of an immersion pipe to the outside.

CONSTITUTION: Plural pieces of gas supply openings 7 are arranged circumferentially to the outside circumference in the lower part of a downcomer 11 and are provided to face upward. The inert gas is blown out of the openings 7 into the molten steel 12 through a gas supply system 5 and a gas passage 6 embedded into the refractories of the downcomer 11. The downward flow generated when the gas is sucked toward the outside circumference of the downcomer 11 is, therefore, blocked by utilizing the buoyancy of the inert gas and the intrusion of the slag 4 in a ladle 1 into the molten steel is prevented even if the flow rate of the molten steel 12 flowing down in the downcomer 11 is high. This method is, therefore, effective for the degassing treatment intended to improve the cleanliness of the molten steel.