A fuel cell system includes: a reformer for generating hydrogen from hydrogen-containing fuel; at least one electricity generator for generating electric energy through an electrochemical reaction of hydrogen and oxygen; a fuel supply unit for supplying the fuel to the reformer; and an oxygen supply unit for supplying the oxygen to the reformer and the at least one electricity generator. The reformer includes: a plurality of reaction sections, wherein at least one of the reaction sections has a channel; at least one cover plate; and a bonding joint between two of the reaction sections and between the at least one of the reaction sections and the at least one cover plate to couple the at least one of the reaction sections and the at least one cover plate to each other.