Electrode having high activity to oxygen gas and low activity to flammable gas is provided. An oxygen pump includes oxide-ion conductive solid electrolyte 2, electrode 8 which is an inactive electrode, and active electrode 10. Electrode 8 is an electrode that includes Ce.sub.0.8Sm.sub.0.2O.sub.2-.alpha.. Electrode 8 is disposed on the gas detection chamber 12 side of solid electrolyte 2. Active electrode 10 is disposed on the open space side of solid electrolyte 2. Gas detection chamber 12 is an enclosed space defined by solid electrolyte 2, insulation layers 6, and diffusion control layer 4.