A fuel cell system (10A to 10D) comprising: a partial oxidation reformer that produces a fuel gas by subjecting a raw fuel to a partial oxidation reforming; a shift reactor that causes a shift reaction of carbon monoxide in the fuel gas produced in the partial oxidation reformer and water vapor; an exhaust gas mixing unit that is provided on a flow field upstream of the shift reactor; an oxygen-containing gas supply path for supplying an oxygen-containing gas; a fuel cell that is provided with: a fuel gas flow field in which a fuel gas guided from the shift reactor flows; and an oxygen-containing gas flow field in which an oxygen-containing gas guided from the oxygen-containing gas supply path flows, and that generates electric power by an electrochemical reaction between the fuel gas in the fuel gas flow field and the oxygen-containing gas in the oxygen-containing gas flow field; a fuel gas discharge path that communicates with the fuel gas flow field and in which a fuel exhaust gas flows, the fuel exhaust gas being a fuel gas that has at least partially been used by the fuel cell; an oxygen-containing gas discharge path that communicates with the oxygen-containing gas flow field and in which an oxygen-containing exhaust gas flows, the oxygen-containing exhaust gas being an oxygen-containing gas that has at least partially been used by the fuel cell; an exhaust gas combustor that combusts the fuel exhaust gas guided from the fuel gas discharge path and the oxygen-containing exhaust gas guided from the oxygen-containing gas discharge path to produce a combustion gas; a startup combustor that is disposed on the oxygen-containing gas supply path and that combusts a fuel to produce a combustion gas that contains water vapor; and a combustion gas introducing path that connects the exhaust gas combustor and the exhaust gas mixing unit, wherein the combustion gas produced by the startup combustor is guided to the shift reactor through the oxygen-containing gas flow field of the fuel cell, the oxygen-containing gas discharge path, the exhaust gas combustor, the combustion gas introducing path, and the exhaust gas mixing unit. a partial oxidation reformer that produces a fuel gas by subjecting a raw fuel to a partial oxidation reforming; a shift reactor that causes a shift reaction of carbon monoxide in the fuel gas produced in the partial oxidation reformer and water vapor; an exhaust gas mixing unit that is provided on a flow field upstream of the shift reactor; an oxygen-containing gas supply path for supplying an oxygen-containing gas; a fuel cell that is provided with: a fuel gas flow field in which a fuel gas guided from the shift reactor flows; and an oxygen-containing gas flow field in which an oxygen-containing gas guided from the oxygen-containing gas supply path flows, and that generates electric power by an electrochemical reaction between the fuel gas in the fuel gas flow field and the oxygen-containing gas in the oxygen-containing gas flow field; a fuel gas discharge path that communicates with the fuel gas flow field and in which a fuel exhaust gas flows, the fuel exhaust gas being a fuel gas that has at least partially been used by the fuel cell; an oxygen-containing gas discharge path that communicates with the oxygen-containing gas flow field and in which an oxygen-containing exhaust gas flows, the oxygen-containing exhaust gas being an oxygen-containing gas that has at least partially been used by the fuel cell; an exhaust gas combustor that combusts the fuel exhaust gas guided from the fuel gas discharge path and the oxygen-containing exhaust gas guided from the oxygen-containing gas discharge path to produce a combustion gas; a startup combustor that is disposed on the oxygen-containing gas supply path and that combusts a fuel to produce a combustion gas that contains water vapor; and a combustion gas introducing path that connects the exhaust gas combustor and the exhaust gas mixing unit, wherein the combustion gas produced by the startup combustor is guided to the shift reactor through the oxygen-containing gas flow field of the fuel cell, the oxygen-containing gas discharge path, the exhaust gas combustor, the combustion gas introducing path, and the exhaust gas mixing unit.