A fuel cell system that includes at least one electricity generator that generates electric energy through electrochemical reaction between hydrogen and oxygen, a reformer that generates hydrogen gas by reforming fuel containing hydrogen and supplies the hydrogen gas to the electricity generator, a fuel supply unit which supplies the fuel to the reformer, and an oxygen supply unit which supplies oxygen to the electricity generator and the reformer. The reformer includes a double pipe lines that are arranged concentrically and have independent flow paths through which fuel passes, and catalytic layers that are formed in the flow paths, generate thermal energy through chemical catalytic reaction, and generate hydrogen gas from the fuel.