A carbon dioxide gas-phase reduction apparatus includes: an oxidation tank including an oxidation electrode; a reduction tank which is adjacent to the oxidation tank and into which carbon dioxide is supplied when the inside of the reduction tank is empty; and a porous reduction electrode-supporting electrolyte membrane disposed between the oxidation tank and the reduction tank. The porous reduction electrode-supporting electrolyte membrane is a joint body obtained by joining a porous reduction electrode formed by dispersing a first electrolyte membrane inside voids and a second electrolyte membrane. The second electrolyte membrane is disposed on the oxidation tank side. The porous reduction electrode is disposed on the reduction tank side, connected to the oxidation electrode by a conducting wire, and performs a reduction reaction with the carbon dioxide in the reduction tank by electrons flowing through the conducting wire.