A fuel cell system comprising a generation chamber, a plurality of fuel cells arranged in the generation chamber, fuel gas feeding means for feeding a fuel gas to the fuel cells, oxygen-containing gas feeding means for feeding an oxygen-containing gas to the fuel cells, power converter means for converting the DC output of the fuel cells into an alternating current, power control means for controlling the electric power output to the power converter means from the fuel cells, and generation control means for controlling the flow rate of the fuel gas fed to the fuel cells and the flow rate of the oxygen-containing gas fed to the fuel cells. A fuel gas buffer amount specific to the fuel cells is not smaller than a fuel gas amount that is needed within a maximum period of increase-needing time that is necessary until the flow rate of the fuel gas increases to a required amount when the amount of increase in the flow rate of the fuel gas is set to be a maximum. The generation control means sets the flow rate of the fuel gas and the flow rate of the oxygen-containing gas based on an electric current of the fuel cells.