A fuel cell system includes an air supply unit for supplying air containing oxygen to a fuel cell stack. A humidifier humidifies the supplied air. An air supply line is connected between a cathode inlet of the fuel cell stack and the humidifier to supply the humidified air to the fuel cell stack. A cathode-side exhaust line is connected between a cathode outlet of the fuel cell stack and the humidifier to supply a cathode exhaust gas discharged from a cathode of the fuel cell stack to the humidifier for humidification in the humidifier. A unified valve module includes a bypass line connected between the air supply line and the cathode-side exhaust line and an outward discharge port discharging the cathode exhaust gas to outside, and controls a gas flow between the humidifier and the fuel cell stack and between the bypass line and the outward discharge port.