In a reforming reactor (31), a partial oxidation reaction is performed between a hydrocarbon fuel and air, and in a mixer (32), water is injected into hot gas heated by the partial oxidation reaction to vaporize the water, and the vaporized water is mixed with the hot gas. In a shift reactor (33), the vaporized water is made to undergo a shift reaction with the hot gas. In this way, a device for promoting vaporization of the water or a complex fuel injection device is not required.