A fuel cell system includes: a fuel cell to which a reaction gas is supplied to generate electricity; a circulating system which returns unreacted reaction gas discharged from the fuel cell to an upstream of the fuel cell to thereby circulate the unreacted reaction gas; a discharge device which discharges the unreacted reaction gas from the circulating system; a first pressure detector which detects a pressure at a downstream of the discharge device; and a controller which controls an amount of discharged gas to be discharged from the circulating system by the discharge device, based on the pressure detected by the first pressure detector.