PURPOSE: To allow smelting of an extremely low oxygen steel and highly clean steel by blowing an inert gas from blowing devices provided to the outside of riser and downcomer immersion pipes to stir the slag on the molten steel.

CONSTITUTION: The molten steel 20 in a ladle 18 is risen from the riser immer sion pipe 14A into a vacuum vessel and is subjected to a degassing treatment. The molten steel subjected to the degassing is returned by the downcomer immersion pipe 14B into the ladle 18. The inert gas is blown to the molten steel from the inert gas blowing devices 28A, 28B provided to the respective outer sides of the riser and downcomer immersion pipes 14A, 14B in such reflux type degassing treatment method for the molten steel. The slag 26 suspended on the molten steel is stirred by this blowing. The extremely low oxygen steel and the highly clean steel are smelted in this way.