A method of detecting degradation of a membrane electrode assembly in a fuel cell system is provided. In the method, it is possible to detect degradation (cross leakage) of a fuel cell. In the state where variation of a load is suppressed, variation of the gas pressure where a predetermined amount of the fuel gas is supplied from a fuel tank to a fuel cell is measured by a pressure sensor provided in a fuel gas circulation channel extending from the fuel gas outlet to the fuel gas inlet.