A fuel cell system includes a fuel cell stack, an oxygen-containing gas flow rate control device, a reformer, a raw fuel flow rate control device, and an exhaust gas flow rate control device. In the fuel cell system, the oxygen-containing gas flow rate control device includes a start-up oxygen-containing gas flow rate regulator valve for supplying an oxygen-containing gas to the reformer to perform partial oxidation reforming from start-up of fuel cells until the temperature of the fuel cell stack is raised to a predetermined temperature. After the temperature of the fuel cell stack reaches the predetermined temperature, the exhaust gas flow rate control device supplies an exhaust gas to the reformer to perform the partial oxidation reforming and steam reforming.