NOVELTY - The gas sensor for detecting electromotive force, comprises a gas diffusion barrier (114) supporting a catalyst to promote a reaction between oxygen and combustible components, a plate-shaped solid electrolyte having oxide ion conductivity, and paired electrodes (116, 120) formed on opposite surfaces of the solid electrolyte (118). The first electrode is formed on a surface of the solid electrolyte located in a region in which ambient gas diffuses at a rate limited by the gas diffusion barrier, and catalyzes a reaction between oxygen and combustible gas.    USE - Useful for detecting electromotive force and a current flowing between electrodes.    ADVANTAGE - The gas sensor detects the air-fuel ratio precisely and controls the amount of fuel supplied to the engine properly from the time immediately after the start of the engine.    DETAILED DESCRIPTION - The gas sensor for detecting electromotive force, comprises a gas diffusion barrier (114) supporting a catalyst to promote a reaction between oxygen and combustible components, a plate-shaped solid electrolyte having oxide ion conductivity, and paired electrodes (116, 120) formed on opposite surfaces of the solid electrolyte (118). The first electrode is formed on a surface of the solid electrolyte located in a region in which ambient gas diffuses at a rate limited by the gas diffusion barrier, and catalyzes a reaction between oxygen and combustible gas. The second electrode is located outside the region to diffuse the ambient gas. The gas diffusion barrier supports a catalyst to promote a reaction between oxygen and combustible components. The combustible components is organic compounds. The density of the catalyst is supported on the side of the gas diffusion barrier to face ambient gas higher than the gas diffusion barrier facing an electrode housing space. The electrode housing space is provided between the gas diffusion barrier and the solid electrolyte. A catalyst-supporting layer is provided outside the gas diffusion barrier. The catalyst is supported in the gas diffusion barrier. The catalyst-supporting layer has density of 6.25-125 mg/cm3. INDEPENDENT CLAIMS are included for:    (1) a fuel supply system; and    (2) a method for using a gas sensor.    DESCRIPTION OF DRAWING(S) - The diagram shows a gas sensor:    Dense protective layer; (112)    Gas diffusion barrier; (114)    Electrodes; (116, 120)    Solid electrolyte; (118)    Catalyst layer. (128)