PURPOSE: To remove hydrogen gas in the presence of Na vapor, and make a hydrogen absorbing material exchangeable and regeneratable without stopping the flow of cover gas by using a hydrogen gas removing device consisting of a hydrogen gas selective permeative metallic membrane and the hydrogen absorbing material.

CONSTITUTION: The cover gas in a liquid metal Na tank is introduced into a hydrogen gas removing device 5. When it is in contact with a metallic membrane 15 which selectively allows the permeation of hydrogen gas such as Pb, the hydrogen gas dissociates and dissolves into the membrane 15. Further, hydrogen atoms pass through a thin metallic membrane of Au or the like which is thinly adhered to the membrane 15 and has no hydrogen gas dissociating and adsorbing capacity, thence they are absorbed in a hydrogen absorbing material 17 such as Ti. The cover gas removed of the hydrogen gas is conducted to an Na vapor removing device, and after it is removed of Na by this, it is returned into the tank. To regenerate the hydrogen absorbing material, the material 17 is heated by a heater 20, and the hydrogen gas in the material 17 is expelled by a vacuum exhausting apparatus 20. At this time, the hydrogen pressure in a chamber 21 may rise higher than the hydrogen partial pressure on the cover gas side, but the hydrogen gas does not permeate through the membrane 16 and there are no problems.