PURPOSE: To offset influence of a surrounding humidity by measuring electrical conductance of a solid electrolyte film and then adjusting voltage of a reference electrode.

CONSTITUTION: An operation electrode 2 which allows oxidation/reduction of a detection gas to be effected, an opposite electrode 3 where oxidation/ reduction reaction is generated, and a reference electrode 4 which functions as a reference potential of oxidation/reduction reaction are provided on an insulation substrate 1 and further a pair of electrodes 6, 6 are formed on a solid electrolyte film 5 covering the insulation substrate 1 between these electrodes by a high-vacuum metal deposition method or the like. A high voltage is applied to the electrodes 6, 6, thus measuring electrical conductance which indicates whether the amount of water contained within the solid electrolyte film 5 is large or small. By adjusting the potential of the reference electrode 4 which functions as a reference potential of the operation electrode 2, gas sensitivity depending on humidity is compensated, thus enabling gas to be detected accurately.