Abstract: A hydrogen gas sensor comprising a detection electrode, a reference electrode, and an electrolyte contacting with these electrodes, in which the reference electrode and the detection electrode are composed of a material having a property that hydrogen molecule doesn't voluntarily dissociate into atomic hydrogen on a surface of the electrode in Standard State such as nickel, titanium, copper, tungsten and the like, in which hydrogen gas is detected by an electromotive force generated between the reference electrode and the detection electrode while at least the detection electrode is maintained at a temperature not less than a temperature that hydrogen molecule begins to dissociate into atomic hydrogen voluntarily on the surface of the detection electrode.