PURPOSE: To detect hydrogen which has passed through iron and steel by bringing V, Nb or Ta in a plate form into contact with an inner wall of a steel plate which seals a space in iron and steel base metal and by measuring an expansion coefficient due to hydrogen absorption.

CONSTITUTION: An inner space in iron and steel base metal 1 is sealed by an iron and steel plate 2 by means of welding 4, and if metal which can easily absorb hydrogen, that is, V, Nb or Ta is joined with the steel plate 2 which is sensitive to hydrogen weakening in this space, hydrogen entering the steel plate 2 is concentrated in hydrogen detecting metal 3. Concentration of the hydrogen causes the metal 3 to expand, and by measuring the expansion coefficient by a distortion gage 5, hydrogen can be detected.