A fuel cell system includes: an external load connected to a fuel cell; an electric power adjusting unit configured to adjust a generated electric power of the fuel cell in accordance with electric power consumption of the external load; a humidity control unit configured to control humidity of an electrolyte membrane in the fuel cell on the basis of the generated electric power of the fuel cell; an output voltage detecting unit configured to detect an output voltage of the fuel cell; and a cross leakage determining unit configured to cause the humidity control unit to increase the humidity of the electrolyte membrane when the fuel cell generates the electric power, the cross leakage determining unit being configured to determine whether a cross leakage amount increases or not on the basis of a change in the output voltage at that time.