NOVELTY - A fuel cell system comprises fuel cell, hydrogen gas supply line, gas exhaust line for passing exhaust hydrogen gas containing nitrogen, gas recirculation line for recirculating the exhaust gas to the fuel cell, purge valve adapted to discharge the exhaust gas to an outside, and controller operative to close the purge valve when flow rate of the exhaust gas passing through the purge valve is at least a predetermined value.    USE - For fuel cell powered automobile and electric power generators for industrial use or domestic use.    ADVANTAGE - Nitrogen diffused in a hydrogen line is discharged while minimizing the amount of hydrogen to be concurrently discharged with nitrogen for providing a high operating efficiency.    DETAILED DESCRIPTION - A fuel cell system (S) comprises:    (a) fuel cell supplied with hydrogen gas and oxidizing gas;    (b) gas supply line supplying the hydrogen gas to the fuel cell;    (c) gas exhaust line passing exhaust hydrogen gas expelled from the fuel cell, with the exhaust hydrogen gas being able to contain nitrogen;    (d) gas recirculation line recirculating the exhaust hydrogen gas to the fuel cell at its upstream;    (e) purge valve (8) adapted to discharge the exhaust hydrogen gas to an outside; and    (f) 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.    An INDEPENDENT CLAIM is also included for a method of controlling a fuel cell system, comprising closing 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 at least a predetermined value.    DESCRIPTION OF DRAWING(S) - The figure is a view of a structure of a fuel cell system.    Fuel electrode (1a)    Oxidizing electrode (1b)    Electrolyte membrane (1c)    Regulator valve (3)    Pressure sensor (4, 20)    Ejector (6)    Purge valve (8)    Temperature sensor (21, 22)    Controller (100)    Fuel cell system (S)