Abstract: An air-fuel ratio sensor for detecting the air-fuel ratio in intake gas introduced into an engine by way of the oxygen concentration in engine exhaust comprises an oxygen ion transmissive solid electrolyte body and first and second porous electrodes provided on opposite sides of the solid electrolyte body, the first porous electrode being adapted to be brought into contact with the exhaust. The first porous electrode is formed of a material which exhibits semi-catalytic property. Metal oxide which oxidizes HC to produce CO is provided near three-phase points at which said first electrode, the solid electrolyte body and the exhaust adjoin each other when the first electrode is brought into contact with the exhaust.