To provide a refining method of molten low nitrogen steel under reduced pressure and a refining apparatus thereof under reduced pressure, with which the pick-up of the nitrogen into the molten steel caused by the outer air invaded into a reduced pressure vessel conveyed in gaps of refractories is prevented, when the decarburizing and degassing are executed by reducing the pressure.

When a refining of the molten steel is executed under reduced pressure in the reduced pressure vessel 10 by dipping an immersion tube 13 in the reduced pressure vessel 10 lined with refractories 19, 20 and 22 into the molten steel in a ladle, in the case that the thickness in at least a part of the lining refractories 22 remaining in the reduced pressure vessel 10 becomes 60-120 mm, after repairing with the refractories 26 and 28, the following refining under reduced pressure, is executed.