A fuel cell system including: a reformer for generating hydrogen from a fuel containing hydrogen; at least one electricity generator for generating electric energy through an electrochemical reaction between hydrogen and oxygen; a fuel supply unit for supplying the fuel to the reformer; and an oxygen supply unit for supplying oxygen to the reformer and the electricity generator. Here, the reformer includes a plurality of plates stacked to form at least one passage for allowing a material selected from the group consisting of the fuel and a gas to flow therethrough, and at least one catalyst layer formed on entire surfaces of the at least one passage.