NOVELTY - A fuel cell system comprises a fuel cell that generates power by reaction of reactive gases, a reactive-gas supply unit, and a control unit to control the supply unit. The control unit includes an average stoichiometric flow-ratio calculation part, and a reactive-gas reduction part that reduces the amount of reactive gases supplied to the fuel cell to lower the stoichiometric flow ratio, when the average stoichiometric flow ratio is equal to or greater than a first predetermined value.    USE - For automotive vehicles.    ADVANTAGE - When the average stoichiometric flow ratio is equal to or greater than the first predetermined value, it is possible to allow enhancement in fuel consumption of the fuel cell by reducing the amount of reactive gases supplied. Because selection of stoichiometric flow ratio is carried out with an average stoichiometric flow ratio as an index, it is possible to prevent frequent changes of stoichiometric flow ratio, thus allowing stabilization of the power generation capacity of the fuel cell. Since reoperation of the reactive-gas reduction part is prohibited before a predetermined time has elapsed after stopping operation, it is possible to prevent the average stoichiometric flow ratio from becoming the second predetermined value consecutively, eliminating unstable power generation.    DETAILED DESCRIPTION - A fuel cell system includes a fuel cell that generates power by reaction of reactive gases. A reactive-gas supply unit supplies reactive gases to the fuel cell. A control unit controls the supply unit. The control unit includes an average stoichiometric flow-ratio calculation part that calculates average stoichiometric flow ratio by averaging stoichiometric flow ratios over a predetermined time, and a reactive-gas reduction part that reduces the amount of reactive gases supplied to the fuel cell to lower the stoichiometric flow ratio, when the average stoichiometric flow ratio is equal to or greater than a first predetermined value. The stoichiometric flow ratio is a ratio of an amount of reactive gases supplied to the fuel cell with respect to a required amount of reactive gases of the fuel cell in accordance with a required amount of power generation. An INDEPENDENT CLAIM is included for a method of controlling a fuel cell for generating power.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram of a fuel cell system.