Abstract: A fuel cell system comprising the stack having a gas distribution means of guiding reaction gas into electrodes of respective single cells, the gas supplying sections for supplying the reaction gas to the stack, the gas humidifying section for humidifying the reaction gas which are disposed between the gas supplying sections and the electrode inlet of the gas distribution means, and the controlling section for controlling the water vapor content of the reaction gas supplied at least to one of the electrodes so as to exceed the saturated water vapor content at the temperature of at least one of the electrode inlets of the gas distribution means.