NOVELTY - A gas sensor comprises a proton conduction layer between opposing first and second electrodes, a reference electrode in contact with the proton conduction layer, and a diffusion controlling portion between the first electrode and an atmosphere containing a gas to be measured. The diffusion controlling portion is located away from an end portion of the second electrode in a direction towards the reference electrode and on a side opposite to the reference electrode.    USE - Used as a gas sensor, e.g. a hydrogen sensor, for measuring the concentration of hydrogen gas in a fuel gas for use in a fuel cell, e.g. a polymer electrolyte fuel cell.    ADVANTAGE - The diffusion controlling portion is located away from an end portion of the second electrode located on a side towards the reference electrode, in a direction away from the reference electrode, such that an object gas component, e.g. hydrogen introduced from a hydrogen-rich atmosphere, can be pumped before hydrogen reaches a portion of the first electrode which encounters difficulty in pumping hydrogen, preventing the presence of a portion of the first electrode where the concentration of hydrogen is relatively high. In the course of controlling the potential difference between the first electrode and the reference electrode at a constant level, no portion of the first electrode becomes relatively low in concentration of hydrogen, preventing e.g. the dissociation of methanol, allowing the concentration of an object gas component, e.g. hydrogen, to be accurately measured without the influence of e.g. methanol.    DETAILED DESCRIPTION - A gas sensor comprises a proton conduction layer (1), a first electrode (3), a second electrode (5), a reference electrode (7) in contact with the proton conduction layer, and a diffusion controlling portion (19) between the first electrode and an atmosphere containing a gas to be measured. An object gas component contained in the gas to be measured, which is introduced from the atmosphere via the diffusion controlling portion, is dissociatable, decomposable or reactable through the application of a voltage between the first and second electrodes such that the potential difference between the first electrode and the reference electrode becomes constant to generate protons, and 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 first and second electrodes are opposite each other while the proton conduction layer is held between them, and the diffusion controlling portion is located away from an end portion (T) of the second electrode in a direction towards the reference electrode and on a side opposite to the reference electrode.    An INDEPENDENT CLAIM is included for a further gas sensor, similar to that above, in which a first area S1 is an area of a portion of the first electrode on which a projected image of the second electrode is not superimposed when the second electrode is projected onto the first electrode, and a second area S2 is an area of a portion of the second electrode which is superimposed on the first electrode when the second electrode is projected onto the first electrode. The ratio S1/S2 is less than 0.35.    DESCRIPTION OF DRAWING(S) - The figure shows a cutaway view of a hydrogen gas sensor.    Proton conduction layer (1)    First electrode (3)    Second electrode (5)    Reference electrode (7)    Diffusion controlling portions (19)