PURPOSE: To completely remove the free hydrogen from the gas mixture at relatively low temperature by providing a catalyst body to keep oxidation of hydrogen and a hydrogen storage device to be thermally combined therewith, generating the hydride by the hydrogen storage device at the temperature below the effective reaction temperature of the catalyst body to combine the hydrogen, and to generate the heat. CONSTITUTION: A free hydrogen removing device comprises a catalyst constituent body (plate) 1, and two hydrogen storage devices 2 provided along its edge, and the plate 1 and two hydrogen storage devices 2 are tightened by bolts through a flange 4 so as to be thermally combined. When an accident occurs in a reactor, and the mixed gas containing hydrogen, oxygen, steam, etc., is leaked, the maximum concentration of hydrogen appears in the vicinity of a leaked part after several hours, and hydrogen is filled in a reactor container at slower pace. A hydrogen occluded substance 6 stored in a cap 3 occludes hydrogen to generate the hydride and to generate the heat. The generated heat is transferred to the plate 1, the catalyst covered on each side of the plate 1 is activated, oxidation of hydrogen is promoted, and the free hydrogen in the mixed gas is removed.