PURPOSE: To control the cell temperature to predetermined level against the load variation by comparing the temperature of fuel cell with a setting level to obtain a pressure level then comparing said pressure level with the pressure in a cooling water tank and increasing/decreasing the opening of a regulation valve at the steam discharge port.

CONSTITUTION: Cooling water 5 in a tank 4 is circulated by a circulation pump 8 through cooling flow paths 6, 7 into a fuel cell 1 to perform cooling. Here, the temperature of cell 1 is detected through a detector 12 and compared with a setting level 11 through a controller 14 to obtain a setting pressure 15 which is compared with the level of a pressure sensor 16 in a tank 4 through a controller 18 thus to increase/decrease the opening of a regulation valve 10 provided at the steam discharge port 9. Consequently, when varying the temperature in accordance to the pressure variation of cooling water 5 in the cooling flow paths 6, 7, the temperature of the cell can be maintained at predetermined level with quickened response while following after the temperature variation under load variation.