A membrane electrode assembly is constituted of a solid electrolyte membrane 1 made of a polymeric material, and a fuel electrode 2 and an air electrode 3 which are disposed on both sides of the solid electrolyte membrane 1. A porous membrane 8 is attached to the back surface of the fuel electrode 2, and a fuel tank 12 is formed between a casing 16 of the fuel electrode 2 side and the porous membrane 8. The fuel tank 12 is filled with liquid-retaining sheets 14. An air chamber 13 is formed on the back surface of the air electrode 3, and the air chamber 13 is filled with moisture-retaining sheets 15. A casing 17 of the air electrode 3 side is provided with a number of inlet ports 21. Further, fins 22 are formed on an outer surface of the casing 17 of the air electrode side.