A fuel cell system having a reforming reactor is disclosed, in which condensed water generated in the reforming reactor is drained during the warming-up of the system, so as to quickly complete the warming-up. The system comprises a fuel cell for generating power by using a supplied fuel gas and oxidizing gas; a reforming reactor for reforming original fuel gas so as to produce a reformed fuel gas which is supplied to the fuel cell; and a drain for condensed water stored in the reforming reactor. A water-collecting portion is formed by a plurality of sloped portions and is attached to the bottom of the reforming reactor, and the drain is connected to the water-collecting portion.