Abstract: [PROBLEMS] To provide a method for producing heavy hydrogen gas, which enables heavy hydrogen gas to be efficiently produced from a deuterated solvent as a reaction substrate. [MEANS FOR SOLVING PROBLEMS] (1) A method for producing heavy hydrogen gas, comprising bringing a deuterated solvent into contact with hydrogen gas under pressure in the coexistence with a catalyst selected from a palladium catalyst, a platinum catalyst, a nickel catalyst, a cobalt catalyst, an iridium catalyst, and a rhodium catalyst and a ruthenium catalyst which are not coordinated with a ligand, and (2) a catalytic deuteration method of a compound having a reducible functional group, comprising bringing the heavy hydrogen gas obtained by the above (1) into contact with the compound having a reducible functional group in the coexistence with a catalytic reduction catalyst.