[en] The gas detection offered by the present invention is characterized by the detection of CO as distinct from H₂. The optimum structure of the device therefor has an electrolytic cell wherein the electrode surfaces of a working electrode, a counter electrode and a reference electrode are kept in contact with electrolyte and wherein the open sides at both ends of the vessel containing the electrolyte are sealed by the working electrode, the counter electrode and the reference electrode; means for connecting between the working electrode and the counter electrode and means for applying a predetermined a voltage between the working electrode and the reference electrode, wherein sensitivity of the working electrode to carbon monoxide and hydrogen gas is smaller than that of the reference electrode