a fuel cell system enables time required for purging to be reduced without a major increase in discharge gas concentration at a time of purging . it comprises a fuel cell ; a fuel gas supply path for supplying the fuel gas to an anode ; an oxidizing gas supply path for supplying an oxidizing gas to a cathode ; a fuel gas circulating path for returning an unreacted fuel gas to an anode inlet side ; a dilution box for diluting the fuel gas by the oxidizing gas and for discharging it to outside ; and a fuel gas discharge path connecting the fuel gas circulating path and a dilution box discharge gas inlet . a drain valve , a purge valve and an air discharge valve are provided , opening areas of which are different from one another . the drain valve with a smallest opening area is initially opened .