An oxygen sensor capable of accelerating electrochemical reactions and practically working at a low temperature comprises a solid electrolyte 5, and an outer electrode 31 and an inner electrode 32, provided on the surfaces of the solid electrolyte 5, wherein mixed conductors 11 and 12 capable of adsorbing oxygen molecules and conducting an ionization reaction are provided between the solid electrolyte 5 and the outer electrode 31 and between the solid electrolyte 5 and the inner electrode 32, respectively. The mixed conductors 11 and 12 are porous and have a higher oxygen ion conductivity than that of the solid electrolyte 5 and an electron conductivity substantially equivalent to the oxygen ion conductivity and are made of a fluorite-type oxide or a perovskite-type oxide.