The moisture state of a fuel cell is determined without causing any variation in the supply state of the reactant gas to be supplied to the fuel cell. An output current control section temporarily performs a current sweep while maintaining the amount of oxidant gas to be supplied to the fuel cell. A resistance component calculation section calculates the resistance component in the fuel cell by using an output current value and an output voltage value of the fuel cell being that of when the current sweep is temporarily performed. A moisture content calculation section calculates the moisture content in the fuel cell by using the resistance component. A moisture content determination section determines whether or not the moisture content is equal to or lower than a dry state threshold value. A moisture content increasing processing section performs a moisture content increasing process when the moisture content is equal to or lower than the dry state threshold value.
