A fuel cell system of the present invention can suppress an excessively wet or dry state of a fuel cell stack so as to thereby ensure the durability of the fuel cell stack. The fuel cell system supplies an oxidant gas with a reduced flow rate per unit time and for a long time period if the rate of voltage decrease of the stack becomes faster than a threshold rate, and supplies the oxidant gas with an increased flow rate per unit time and for a short time period if the rate of voltage decrease becomes slower than a threshold rate.