A carbon dioxide sensor includes: a solid electrolyte layer that is anion conductive; a reference electrode disposed on one surface of the solid electrolyte layer; and a detection electrode disposed on the other surface of the solid electrolyte layer. The detection electrode is made of a mixture containing: (a) one or more metals selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Os, and Ir; (b) a cation conductive carbonate; and (c) an oxide containing Li and at least one of Ce and Sm. The solid electrolyte layer is preferably oxide ion conductive, and the cation conductive carbonate is preferably lithium ion conductive.