Abstract: A mixed proton-electron conducting ceramic is a metallic oxide having a perovskite type structure, includes at least one member selected from the group consisting of chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni) and ruthenium (Ru) in a range of from 0.01 or more to 0.8 or less when a molar-ratio sum of metals constituting the metallic oxide is taken as 2, and has proton conductivity and electron conductivity.

Abstract: A direct current voltage is applied between an anodic portion containing an oxidizing catalyst provided on one surface of a proton conductive solid electrolyte which separates the flow path of the gas to be treated into two parts, and a cathodic portion containing a reducing catalyst provided on the other surface of the solid electrolyte, the reducing catalyst comprising at least one selected from oxides of elements of Group 5A, Group 6A, and Group 7A of the periodic table, palladium, palladium alloys, transition elements carried on alumina, cerium oxide, and La.sub.1-x Sr.sub.x Co.sub.1-y Fe.sub.y O.sub.3 (where, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), while bringing a gas containing water vapor into contact with the anodic portion and bringing a gas containing nitrogen oxides into contact with the cathodic portion.