End rings are coupled with upper and lower ends of a cylindrical body tube. A plurality of first heat exchanger tubes are fitted and held in holes provided in a pair of first tube plates. A plurality of second heat exchanger tubes are fitted and held in holes provided in a pair of second tube plates. The inside of each first heat exchanger tubes forms a first-fluid passage for carrying a reformed gas. The space defined between an outer wall of each first heat exchanger tube and an inner wall of the corresponding second heat exchanger tube forms a second-fluid passage for carrying a coolant. The space defined among an inner wall of the body tube and outer walls of the second heat exchanger tubes form a third-fluid passage for carrying compressed air.