NOVELTY - Fuel cell system comprises fuel cell stack; fuel gas supply unit; oxidation gas supply unit; fuel cell cooling unit; fuel gas humidifier for humidifying fuel gas that is supplied to the fuel cell stack from the fuel gas supply unit by using the remaining fuel gas that is exhausted from fuel cell stack; oxidation gas humidifier for humidifying oxidation gas using remaining oxidation gas exhausted from fuel cell stack; and control unit.    USE - For converting chemical energy of fuel to electrical energy.    ADVANTAGE - Fuel gas and oxidation gas that are supplied to a fuel cell stack are humidified by water contained in remaining fuel gas and remaining oxidation gas that are exhausted from the fuel cell stack, so an anti-freeze solution can be used as coolant for cooling the fuel cell stack. The fuel cell cooling unit can operate at a lower ambient temperature (e.g. sub-zero temperature).    DETAILED DESCRIPTION - Cell system comprises fuel cell stack (13); fuel gas supply unit for supplying fuel gas to the fuel cell stack; oxidation gas supply unit (17) for supplying oxidation gas to the fuel cell stack; fuel cell cooling unit (19) for circulating coolant through the fuel cell stack to cool the fuel cell stack; fuel gas humidifier (21) for humidifying fuel gas that is supplied to the fuel cell stack from the fuel gas supply unit by using the remaining fuel gas that is exhausted from fuel cell stack; oxidation gas humidifier (23) for humidifying oxidation gas that is supplied to the fuel cell stack from the oxidation gas supply unit by using remaining oxidation gas that is exhausted from fuel cell stack; and control unit for controlling the fuel gas supply unit, oxidation gas supply unit, and fuel cell cooling unit. The fuel gas humidifier comprises a pair of separating plates disposed to face each other such that coolant exhausted from the fuel cell stack flows; and humidifying membrane at a side of the separating plate such that remaining fuel gas exhausted from the fuel cell stack flows on one side of the humidifying membrane and the fuel gas that is supplied to the fuel cell stack from the fuel gas supply unit flows on the other side of the humidifying membrane. Heat contained in the coolant flowing between the pair of separating plates and heat in the remaining fuel gas are transmitted to the fuel gas supplied to the fuel cell stack, and water contained in the remaining fuel gas is supplied to the fuel gas supplied to the fuel cell stack through the humidifying membrane.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic block diagram showing the fuel cell system.    Fuel cell stack (13)    Oxidation gas supply unit (17)    Fuel cell cooling unit (19)    Fuel gas humidifier (21)    Oxidation gas humidifier (23)