NOVELTY - Operating fuel cell (1) involves: measuring internal temperature, power generation voltage and current flowing in cell; controlling, operation amount of cooling water (10) pump or fan (8), or cooling fluid amount in bypass valve (9); and when displacement amount of cell stack voltage to time is smaller than 0, displacement amount of cell stack temperature to time is larger than 0 and displacement amount of cell stack current to time is greater than or equal to 0, changing operation amount of cooling water pump, operation amount of fan, and/or cooling fluid amount to increase heat discharged from cell.    USE - For operating fuel cell system (claimed).    ADVANTAGE - The fuel cell system provides improved system having excellent start characteristic. The fuel cell has a characteristic that due to property of electrolytic membrane, when moisture in cell is reduced, cell performance is lowered. With improved cell system, even at start time when temperature is changed particularly terribly, cell temperature can be controlled finely, and vapor pressure in cell is controlled, thus cell characteristic can be stabilized without excessively reducing moisture in electrolytic membrane. By use of the control system that increases at least one of operation amount of cooling water pump, operation amount of fan, and ratio of cooling flow rate flowing to radiator to cooling flow rate flowing through bypass valve; temperature is controlled at very early timing when internal drying condition of cell affects cell characteristic, and moisture content in cell can be controlled, so that particularly even when cell temperature is changed greatly at start time, cell characteristic is hardly lowered and cell behavior is stabilized. Further, when at least one of operation amount of cooling water pump, operation amount of fan, and ratio of cooling flow rate flowing to radiator to cooling flow rate flowing through bypass valve is decreased, cell temperature is judged to be lowered as required, and cooling effect is regulated by changes in operation amounts of pump and fan, thus without unnecessarily lowering cell temperature i.e. without generating condensed water causing flooding, minimal cooling necessary for stabilization of cell performance is executed, and the system can be started at high speed. Also, to set value Tc of stack temperature at time of rated power generation, when present time T is less than Tc minus 5, it is proposed to drive repeatedly operation of cooling section. At start time, it is natural that stack and auxiliary device are greatly changed in temperature, though when power is generated under rated condition and then power is generated under partial load condition, heat value is reduced, so that temperature set by Tc cannot be maintained, and stack temperature lowers. Furthermore, then when power is generated again under rated condition, stack temperature rises greatly, so that condition coincides with condition at start time. By this proposal, not only at start time but also in correspondence to sudden temperature change of stack due to change in power generation condition, cell can be operated stably. In system using self moistening method using generated water accompanying power generation of fuel cell, immediately after start, collection amount of generated water is small, and self moistening is not executed sufficiently, so that cell performance may be lowered or may become unstable. With the improved fuel cell, even fuel cell power generation system using moistening method for collecting generated water at time of power generation and adding it to gas to be supplied can be started in short time and can perform operation at stable power generation voltage.    DETAILED DESCRIPTION - Operating fuel cell system comprising fuel cell stack (1), fuel cell temperature sensor (13), voltage sensor (12), current sensor (14), radiator (7), fan (8), cooling water (10) pump and bypass valve (9), involves: measuring internal temperature of fuel cell with fuel cell temperature sensor; measuring power generation voltage of fuel cell with voltage sensor; measuring current flowing from fuel cell with current sensor; and controlling, on basis of voltage information measured by voltage sensor, temperature information measured by temperature sensor, and current information measured by current sensor, at least one of operation amount of cooling water pump, operation amount of fan, and cooling fluid amount flowing through bypass valve; and when displacement amount dV/dt of a fuel cell stack voltage V to time t is smaller than 0, displacement amount dT/dt of fuel cell stack temperature T to time t is larger than 0, and displacement amount dI/dt of fuel cell stack current I to time t is greater than or equal to 0, changing at least one of operation amount of cooling water pump, operation amount of fan, and cooling fluid amount flowing through bypass valve to increase quantity of heat discharged externally from fuel cell. The operating of fuel cell system involves, when displacement amount dV/dt is smaller than 0, displacement amount dT/dt is larger than 0, and displacement amount dI/dt is greater than or equal to 0, increasing at least one of operation amount of cooling water pump, operation amount of fan, and ratio of cooling flow rate to radiator to cooling fluid amount flowing through bypass valve; and when displacement amount dV/dt is larger than 0, and displacement amount dT/dt is smaller than 0, decreasing at least one of operation amount of cooling water pump, operation amount of fan, and ratio of cooling flow rate to radiator to cooling fluid amount flowing through bypass valve. To set value Tc of stack temperature at time of rated power generation, when present time T is within range of T is less than Tc minus 5, operation of cooling section is driven repeatedly.    DESCRIPTION OF DRAWING(S) - The figure shows part of the power generation system configuration of fuel cell.    Fuel cell stack (1)    Radiator (7)    Fan of radiator (8)    Bypass valve to control flow rate of bypass portion (9)    Cooling water (10)    Bypass portion (11)    Voltage sensor (12)    Temperature sensor (13)    Current sensor (14)    Controller (15)