A fuel cell system comprises a humidifier which can humidify a fuel cell even when humidification has become insufficient at start-up of the fuel cell and during normal operation thereof. A water collecting apparatus which collects water in the exhaust gas of the fuel cell, and an auxiliary humidifier which humidifies the gas supply by using collected water from the water collecting apparatus, are provided separate to the water-permeable-type humidifier. The water collecting apparatus comprises a vapor/liquid separator and a collected water storage tank, and the auxiliary humidifier comprises a check valve, a collected water supply pump, an auxiliary humidification pipe, and an injector. The collected water in the collected water storage tank is transferred by a collected water supply pump, atomized by the injector, and supplied to the intake side of the fuel cell.