A plate-type heat exchanger for use in a fuel cell system that has a fuel cell stack and a reformer is provided. The heat exchanger includes a substrate and a pair of cover plates. The substrate has a first face and a second face opposite to the first face. The substrate is disposed between the cover plates, and combined with the cover plates to form a first passageway and a second passageway. The first passageway is formed in the first face and circulates steam discharged from the fuel cell stack. The steam or water condensed from the steam is supplied to a water supply source. The second passageway is formed in the second face, and circulates water supplied from the water supply source. The water is supplied to the reformer after the circulation. The heat exchanger of the present invention improves performance and efficiency of a fuel cell system.