FUEL CELL SYSTEM

A fuel cell system including a gas supply part supplying cathode gas to a fuel cell stack through a supply flow path, a humidifier connected to the supply flow path and a discharge flow path of the fuel cell stack and humidifying cathode gas with a moisture included in cathode exhaust gas, and an electronic control unit including a microprocessor. The microprocessor is configured to perform acquiring information on a first pressure of the cathode gas flowing through a first flow path inside the humidifier, acquiring information on a second pressure of the cathode exhaust gas flowing through a second flow path inside the humidifier, and calculating a leakage amount of the cathode gas from the first flow path to the second flow path inside the humidifier, based on a difference between the first pressure and the second pressure.