A heat exchanger 32 comprises a first heat exchanger 40 for allowing heat to be exchanged between water and a reformed gas from a reforming reactor, a second heat exchanger 50 for spraying and vaporizing water in the reformed gas from the first heat exchanger 40, and a third heat exchanger 60 for vaporizing unvaporized or precipitated water. The reformed gas from the reforming reactor is cooled by the heating and vaporizing of water, and is mixed with the vaporized water and fed as mixed gas to a water gas shift reactor. The processes of cooling the reformed gas, heating and vaporizing of the water, and mixing the reformed gas and steam are completed in the heat exchanger 32, making it possible to design a smaller, simpler, and more efficient apparatus.