NOVELTY - A hydrogen sensor comprises a support for a first electrode, a second electrode and a reference electrode. The electrodes are provided in contact with a proton conduction layer. The hydrogen sensor has an annular sealer for sealing against the support and against the proton conduction layer, and is disposed to surround the first electrode.    USE - Measuring hydrogen gas concentration in a fuel gas for use in a polymer electrolyte fuel cell (claimed) for e.g. automobile or household use.    ADVANTAGE - The sensor has excellent heat resistance and durability.    DETAILED DESCRIPTION - A hydrogen sensor comprises a support (9) for a first electrode (3), a second electrode (5) and a reference electrode (7). The first, second and reference electrodes are provided in contact with a proton conduction layer (1). The support has a diffusion controlling portion (19) for establishing communication between an atmosphere containing a gas to be measured, and a measurement chamber (11a) accommodating the first electrode. An object gas component contained in the gas to be measured, which is introduced from the atmosphere via the diffusion controlling portion, is caused to be dissociated, decomposed or reacted by applying a voltage between the first electrode and the second electrode, such that a constant electric potential difference is produced between the first electrode and the reference electrode, to generate protons. The concentration of the object gas component is obtained on the basis of a limiting current generated, as a result of the generated protons being pumped out via the proton conduction layer from the first electrode to the second electrode. The hydrogen sensor has an annular seal for sealing against the support and against the proton conduction layer. The annular seal is disposed to surround the first electrode.    DESCRIPTION OF DRAWING(S) - The figure is an explanatory cutaway view showing a hydrogen gas sensor.    Proton conduction layer (1)    First electrode (3)    Second electrode (5)    Reference electrode (7)    Support (9)    First support (9a)    Second support (9b)    Measurement chamber (11a)    Diffusion controlling portion (19)