TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Catalyst: The catalyst A and B are selected from platinum, gold, copper, nickel, palladium, silver, rhodium and ruthenium or its alloy, preferably catalyst A comprises platinum.Preferred Sensor: A reaction layer comprising catalyst, is formed on the porous material of counter electrode. The total weight of catalyst in the reaction layer of counter electrode is 10-1000-fold of total weight of catalyst in the reaction layer of detection electrode. The electrochemically active area of counter electrode is 10-1000-fold of the electrochemically active area of detection electrode. The void rate of porous material in detection electrode is 10-90%, preferably 35-45% and its average void diameter is 1 nm to 100 mum, preferably 10 nm to 0.3 mum. The CO gas sensor further comprises a diffusion controlling member disposed on detection electrode. The voltage applying unit applies CO oxidation potential and CO adsorption potential between the two electrodes. The intensity of current flowing between the two electrodes is 0.001 mA/cm2 to 1 A/cm2, preferably 0.1 mA/cm2 to 0.01 A/cm2 when the CO oxidation potential is changed to CO adsorption potential. The thickness of reaction layer of detection electrode is preferably 5-10 mum. The porous material which is electrochemically inactive and electrically active contains carbon. The amount of electrically conductive material carried on the porous material of detection electrode is 1 ng/cm2 to 10 mg/cm2, preferably 0.1-10 mug/cm2. The ratio of specific area of porous material to electrochemically active material of detection electrode is 1:0.001-100, preferably 1:0.1-1. The porous material of counter electrode comprises a material which hardly adsorbs CO. The amount of material incorporated in counter electrode is 0.1-100000-fold, preferably 10-1000-fold by the weight ratio of electrochemical material in the detection electrode. Alternately an electrolyte is disposed between detection electrode and counter electrode instead of solid electrolyte membrane.