A fuel cell system (S) is provided with a fuel cell (1) supplied with hydrogen gas and oxidizing gas, a gas supply line (5) supplying the hydrogen gas to the fuel cell, a gas exhaust line (31) passing exhaust hydrogen gas expelled from the fuel cell with the exhaust hydrogen gas being able to contain nitrogen, a gas recirculation line (7) recirculating the exhaust hydrogen gas to the fuel cell at an upstream thereof, a purge valve (8) adapted to discharge the exhaust hydrogen gas, containing the nitrogen, to an outside, and a controller (100) operative to close the purge valve when judgment is made that a flow rate of the exhaust hydrogen gas passing through the purge valve during an opened state of the purge valve is equal to or greater than a predetermined value.