NOVELTY - A controller (48) is connected to water removing device, water recirculating device and a shut-off valve (7) for blocking passage of water from water tank (5) into water recirculating device. The controller opens shut-off valve and controls water-removing device to supply purge gas to water passage (11) when shut-down operation of fuel cell system begins, after which valve is closed, to halt supply of purge gas.    USE - Fuel cell system for generating electric power, using fuel gas and oxygen-bearing gas.    ADVANTAGE - The water is prevented from freezing in the vicinity of the constitutional components of water supply system and water recirculation system by controlling the shut-off valve provided between water tank and water recirculating device using controller.    DETAILED DESCRIPTION - A sensor is used for detecting temperature of water inside the fuel cell system. The detected temperature is compared with predetermined temperature based on which water removing device is controlled, to supply purge gas to water passage.    A water passage pipe is provided with humidifying device. A bifurcated portion (49) is provided between water passage pipe and drainage pipe (49a). A drain valve (42) is provided on the drainage pipe, for draining water inside the water passage. A purge gas releasing device releases purge gas from water passage and comprises release pipe through which purge gas flows. A gas release valve is positioned on the release pipe and is connected to apex portion of the water passage. A temperature sensor (22) is provided in the vicinity of the purge gas release valve (14b) for detecting the temperature of purge gas release pipe (14a).    A water passage suction outlet is formed in the interior of the water tank within which a coolant passage (15) is provided to adjust the temperature of water inside the tank. A water pressure adjustment valve (20) is closed to hermetically seal the purge gas supplied into the water passage.    An INDEPENDENT CLAIM is also included for fuel cell system control method.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit diagram of the fuel cell system.    Water tank (5)    Shut-off valve (7)    Water passage (11)    Purge gas release pipe (14a)    Purge gas release valve (14b)    Coolant passage (15)    Water pressure adjustment valve (20)    Temperature sensor (22)    Drain valve (42)    Controller (48)    Bifurcated portion (49)    Drainage pipe (49a)