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. As a result, the water vapor is electrolytically oxidized into protons and oxygen on the anodic portion, so that the protons having been transported to the cathodic portion can be utilized to reduce nitrogen oxides by the reducing power of hydrogen. The method efficiently reduces nitrogen oxides in a gas even under an oxidizing atmosphere.