PURPOSE:To perfectly prevent the absorption of nitrogen into molten steel, to restrain the development of spalling phenomenon, crack, shortage of joint, etc., of a refractory forming the inner and the outer cylinders of an immersion tube, to prolong the service life of the immersion tube and to prevent the stickiness of a large quantity of slag to the outer periphery of the non-immersion part of the immersion tube and a flange part by restraining spatter of the slag. CONSTITUTION:At the time of executing vacuum degassing by dipping the immersion tube of an RH vacuum degassing apparatus into molten steel, and inert gas filling member 7 is embedded and arranged at the upper part from the molten steel surface M in the outer periphery of the seal member of the immersion tube, and while holding the pressure at an area of the inert gas filling member higher than the atmosphere, the degassing treatment is executed to the molten steel to be treated to prevent the absorption of the nitrogen in the RH vacuum degassing treatment. Further, in some case, the inert gas filling member is divided and embedded and arranged to the outer periphery of the seal member, and further, the pressure at the area of the inert gas filling member is made to be in the range of 0.1-1MPa.