NOVELTY - A fuel cell system comprises a controller functioning to compute a first energy amount required when the system is under a first protection mode preventing freezing of water in the system and a second energy amount required when the system is under a second protection mode preventing freezing of water, respectively based on the estimated restart time and outside air temperature shift; and to select a protection mode based on the energy amount.    USE - Used in vehicles.    ADVANTAGE - The system can suppress the energy amount required to protect the system from freezing of water to the absolute minimum.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell generating power by supplying an oxidizing agent and a hydrogen-containing gas to an electrolyte membrane (1 m); a water supply device (9, 12) supplying water to the fuel cell; and a controller functioning to:    (a) estimate a restart time when the fuel cell is restarted;    (b) estimate a shift of an outside air temperature;    (c) compute a first energy amount required when the system is protected by a first protection mode which prevents freezing of water in the system by heating water supplied to the fuel cell when the fuel cell has stopped, and a second energy amount required when the system is protected by a second protection mode which prevents freezing of water in the system by discharging water from the fuel cell when the fuel cell has stopped, respectively based on the estimated restart time and outside air temperature shift;    (d) select the first protection mode when the first energy amount is less than the second energy amount and select the second protection mode when the first energy amount is larger than the second energy amount as a protection mode used when the fuel cell has stopped; and    (e) protect the system with the selected protection mode.    An INDEPENDENT CLAIM is also included for protecting a fuel cell by estimating a restart time when the fuel cell is restarted; estimating a shift of an outside air temperature; computing a first energy amount required when the system is protected by a first protection mode which prevents freezing of water in the system by heating water supplied to the fuel cell when the fuel cell has stopped, and a second energy amount required when the system is protected by a second protection mode which prevents freezing of water in the system by discharging water from the fuel cell when the fuel cell has stopped, respectively based on the estimated restart time and outside air temperature shift; selecting the first protection mode when the first energy amount is less than the second energy amount and select the second protection mode when the first energy amount is larger than the second energy amount as a protection mode used when the fuel cell has stopped; and protecting the system with the selected protection mode.    DESCRIPTION OF DRAWING(S) - The figure shows a fuel cell system.    Coolant tank (6)    Heat exchanger (8)    Water supply device (9, 12)    Gas-liquid separator (15)    Heater (21)    System position sensor (32)    Outside air temperature sensor (33)    Illumination sensor (34)    Oxygen concentration sensor (35)    Remaining energy amount sensor (36)    Restart time input device (37)    Temperature sensor (38)    Water amount sensor (39)    Alarm device (41, 42)    Day and time sensor (51)    Climactic information input device (52)    Switch (53)