NOVELTY - A carbon monoxide (CO) gas sensor comprises a detecting unit which has a solid electrolyte membrane (3) between a detection electrode (1) and counter electrode (2). The electrode (1) comprises a reaction layer on a electrically conductive porous material containing an electrochemically active catalyst A. The counter electrode comprises electrochemically active catalyst B on electrically conductive porous material.    USE - For a fuel cell (claimed) and for detecting the concentration of small amounts of CO gas in a hydrogen-rich atmosphere.    ADVANTAGE - The current flowing between the detection electrode and counter electrode is minimized, therefore the drying of the electrolyte membrane due to a temperature raise in the sensor is prevented and the shelf life of the sensor is improved. The sensitivity of the sensor to CO gas is improved. The temperature range and humidity range in which the CO gas sensor is effectively operated are widened. CO gas can be measured stably even when a reference electrode is omitted.    DETAILED DESCRIPTION - The CO gas sensor comprises a detecting unit and a voltage applying unit. The detecting unit has a solid electrolyte membrane between detection electrode and counter-electrode. The voltage applying unit applies a voltage between the two electrodes. The detection electrode comprises an electrochemically active catalyst A on an electrically conductive porous material. A reaction layer having density of 1 ng/cm2 to 100 mug/cm2 and thickness of 0.3 nm to 15 mum, is formed on the porous material. The counter electrode comprises electrochemically active catalyst B on electrically conductive porous material.    INDEPENDENT CLAIMS are also included for the following:    (i) a fuel cell apparatus with a CO gas sensor;    (ii) a detection electrode which has a reaction layer containing electrochemically active material A on porous material which is electrochemically inactive and electrically conductive; and    (iii) a counter-electrode which comprises electrochemically inactive and electrically conductive porous material containing platinum-ruthenium catalyst.    DESCRIPTION OF DRAWING(S) - The figure shows the carbon monoxide gas sensor.    Detection electrode (1)    Counter electrode (2)    Solid electrolyte membrane (3)