A fuel cell system includes a fuel cell, a fuel gas supply unit for supplying a fuel gas to the fuel cell, an oxygen containing gas supply unit, which has a heat exchanger for heating an oxygen-containing gas, for supplying the oxygen-containing gas heated by the heat exchanger to the fuel cell, an exhaust gas discharge unit for supplying an exhaust gas used in a generating reaction and discharged from the fuel cell, as a heating medium for heating the oxygen-containing gas to the heat exchanger, and a combustion gas supply unit, which has a combustor disposed out of a passageway of the exhaust gas discharge unit for generating a combustion gas by combusting a raw fuel with an oxygen-containing gas supplied thereto, for supplying the combustion gas, together with the exhaust gas, to the heat exchanger.