NOVELTY - A gas sensor has a diffusion ratio (a/b) between the diffusion resistance (a) of a diffusion limiting inlet (19) and the diffusion resistance (b) of a diffusion limiting outlet (21) of not greater than 2.    USE - The gas sensor is used for measuring or detecting a hydrogen component in a gas of interest, in particular, to a hydrogen sensor for measuring concentration of hydrogen contained in a fuel gas for use in a fuel cell, particularly a polymer electrolyte fuel cell (PEFC) (claimed).    ADVANTAGE - By having a ratio (a/b) of not greater than 2, the amount of hydrogen gas which reaches the second electrode through the gas diffusion-limiting outlet is not very large as compared with that of the hydrogen gas which reaches the first electrode through the diffusion limiting inlet, even during transition when the concentration of hydrogen gas in a fuel gas for use in a polymer electrolyte fuel cell increases abruptly. Therefore, the generation of unusual hydrogen sensor output, specifically the generation of unusual electromotive force that induces undershoot current, can be effectively or completely suppressed so that, even during transition, accurate measurement of hydrogen gas concentration can be accomplished.    DETAILED DESCRIPTION - A gas sensor comprises:    (a) a proton conductive layer (1);    (b) a first electrode (3) and a second electrode (5) on the proton conductive layer (1);    (c) a gas diffusion-limiting inlet (19) between the first electrode (3) and a measurement gas atmosphere (6) containing a hydrogen gas; and    (d) a gas diffusion-limiting outlet (21) between the second electrode (5) and the atmosphere (6).    The diffusion ratio (a/b) between diffusion resistance (a) of the diffusion limiting inlet (19) and diffusion resistance (b) of the diffusion limiting outlet (21) is not greater than 2.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory cutaway view showing a hydrogen sensor of the invention.    Proton conductive layer (1)    First electrode (3)    Second electrode (5)    Gas atmosphere for measurement (6)    First support element (7)    Second support element (8)    Gas diffusion limiting inlet (19)    Gas diffusion limiting outlet (21)