A fuel cell system has an oxidizer gas flow rate adjuster that adjusts a flow rate of oxidizer gas of the fuel cell stack and a controller that determines a humidity state in the fuel cell stack. The controller has an average cell voltage calculator that calculates an average cell voltage, a minimum cell voltage calculator that calculates a minimum cell value, a voltage difference calculator that calculates a voltage difference between the average cell voltage and the minimum cell voltage, and a voltage comparator that compares the absolute voltage difference with a preset value. The flow rate of the oxidizer gas is changed based on a comparison between the absolute voltage difference and the preset value, and then the humidity state in the fuel cell stack is determined based on a change in the absolute voltage difference.