NOVELTY - The system has a supply side temperature sensor (77) that detects the supply cooling-medium temperature which is the temperature of the cooling medium supplied to a cooling-medium supply port (14). The circulating load of medium is selectively defined based on magnitude correlation between fuel cell interior temperature and discharge cooling-medium temperature. A low-temperature starting time control unit provides larger circulating load of medium when supply cooling-medium temperature becomes equal to discharge cooling-medium temperature after defining the circulating load.    USE - Fuel cell system of fixed-type and portable type such as polymer electrolyte fuel cell system mounted in vehicle such as two-wheeled vehicle, boat, ship, airplane and robot. Can also be used in phosphoric acid fuel cell system, molten carbonate fuel cell system and solid oxide fuel cell system.    ADVANTAGE - The emitted-heat amount immediately after sub-zero temperature start-up can be increased. The temperature increase rate of the fuel cell stack can be improved. The shortening of setup time can be achieved.    DETAILED DESCRIPTION - The discharge side temperature sensor (78) detects the discharge cooling-medium temperature which is the temperature of the cooling medium discharged from the cooling-medium discharge port (16). The low-temperature starting time control unit controls the circulating load of the cooling medium with the pump at the time of low-temperature starting of the fuel cell stack (10). The fuel cell interior temperature which is the temperature inside the fuel cell stack is acquired. An INDEPENDENT CLAIM is included for control method of fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view illustrating the schematic structure of the fuel cell system. (Drawing includes non-English language text)    Fuel cell stack (10)    Cooling-medium supply port (14)    Cooling-medium discharge port (16)    Supply side temperature sensor (77)    Discharge side temperature sensor (78)