A fuel-cell system is provided, which includes a fuel cell, a reactive-gas supply unit and a control unit. The reactive-gas supply unit supplies the reactive gases to the fuel cell. The control unit includes an average stoichiometric flow-ratio calculation part and a reactive-gas reduction part. The average stoichiometric flow-ratio calculation part calculates an average stoichiometric flow ratio by averaging stoichiometric flow ratios. A stoichiometric flow ratio is a ratio of an amount of the reactive gases supplied to the fuel cell with respect to a required amount of the reactive gases of the fuel cell in accordance with a required amount of power generation. When the average stoichiometric flow ratio is equal to or greater than a first predetermined value, the reactive-gas reduction part reduces the amount of the reactive gases supplied to the fuel cell so as to lower the stoichiometric flow ratio.