NOVELTY - The cooling system has a cooling water tank (20) provided in a cooling water circular flow path (17) between the downstream of a fuel cell (11) and a circulation pump (19). The capacity of cooling water tank is changed with the negative pressure generated by the flow of air off-gas from air off-gas emission flow path (16) so that differential pressure at inlet side and discharge side of circulation pump is increased and cooling capacity of circulation pump with respect to fuel cell is heightened.    USE - Cooling system used for continuous cooling of fuel cell used in vehicle.    ADVANTAGE - Enables rapid reduction of temperature that rises due to heat generation in fuel cell since flow volume of cooling water supplied to the cooling flow path of fuel cell by the circulation pump is increased. Improves cooling capacity of circulation pump with respect to fuel cell since pressure of the cooling water between downstream of fuel cell and circulation pump is lowered and differential pressure at the inlet side and discharge side of circulation pump is increased. Ensures continuous and appropriate cooling of the fuel cell since cooling water tank is provided in the cooling water circular flow path and capability of the circulation pump is supplemented by the simple structure of changing the capacity of the cooling water tank using the negative pressure made by air off-gas. The diaphragm in cooling water is operated easily and capacity of cooling water in cooling water tank is changed rapidly since diaphragm is provided in the cylinder and operated with negative pressure generated by flow of air off-gas from the air off-gas emission flow path. The negative pressure is effectively produced in the cooling water tank by the air off-gas flowing through the edge part of orifice to which the opening is restricted. Improves handling of leakage prevention since negative pressure is generated by the flow of air off-gas flowing through the air off-gas emission flow path.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the cooling system of a fuel cell. (Drawing includes non-English language text).    Fuel cell (11)    Air off-gas emission flow path (16)    Cooling water circular flow path (17)    Circulation pump (19)    Cooling water tank (20)