PURPOSE: To enable a hydrogen concentration measuring time within a molten metal to be reduced and an action for a post process to be performed readily for improving a working efficiency by enhancing an initial hydrogen concentration within an inactive gas to be circulated within a closed circuit previously.

CONSTITUTION: While an inactive gas such as N2 is supplied into a supply side gas passage 10 from a cylinder 14 by preheating a probe 2 and opening a valve 16. the probe 2 is inserted into a molten metal 4. The valve 16 is closed when the closed circuit is filled with the inactive gas, a heat conductivity of the inactive gas within a collection side gas passage 12 is detected by a heat conductivity detector 22 and the output is adjusted to zero point. Then, a valve 20 is opened and H2 gas or H2/N2 mixture gas is introduced, thus enabling the inactive gas within the closed circuit to be substituted. In this case, a concentration of H2 gas within the mixed gas is brought closer to a balanced water concentration within the molten metal 4. Then, the valve 20 is closed, measurement of heat conductivity of the mixed gas is initiated by a detector 22 while operating a circulation pump 8 and the concentration of hydrogen within the molten metal is obtained from the heat conduction value when the aging disappears.