Abstract: Disclosed is a fuel cell system (100) comprising a solid polymer fuel cell, wherein a cathode off-gas containing moisture which is discharged from a cathode of a fuel cell stack (10) is circulated and supplied back to the cathode. In addition, when flooding occurs in this fuel cell stack (10), an air pump (42), a circulation throttle (44), a supply throttle (46) and a back pressure control valve (38) are controlled for increasing the circulation amount of the cathode off-gas, thereby discharging the excess moisture. Consequently, flooding problems can be resolved in this fuel cell system while surely keeping the electrolyte membrane wet.