A controlled potential electrolysis gas sensor including, as gas electrodes 10 for detecting gas, a working electrode for electrochemically reacting gas to be detected, a counter electrode with respect to the working electrode, and a reference electrode for controlling potential of the working electrode, the gas electrodes 10 being disposed to face an electrolysis solution accommodating portion of an electrolysis tank accommodating electrolytic solution 20. Each electrode 10 includes a noble metal catalyst layer 50. On the noble metal catalyst layer 50 of at least one of the working electrode and the reference electrode, there is formed a polymer layer 60 having ion conductivity and water permeability.