NOVELTY - The humidifier (1) has a preheating serpentine (20) mounted in a barrel (10) so that reactant gas is guided into barrel via inlet (21) of serpentine whose coiled section (23) is immersed in humidifying liquid (12) stored in the barrel. A gas dispenser (30) is provided so that a gas chamber (31) is formed in barrel below the dispenser and humidifying liquid is isolated from the chamber. The outlet (22) of serpentine is located in the chamber so that reactant gas guided into the chamber enters the humidifying liquid via the dispenser in the form of tiny bubbles.    USE - For proton exchange membrane fuel cell (PEMFC).    ADVANTAGE - The moisture content of reactant gas is increased within short time and the polymeric membrane in the fuel cell is humidified efficiently, enhancing the power generation efficiency and service life of the fuel cell.    DETAILED DESCRIPTION - The gas humidifier comprises a heat exchanger (40) mounted on the barrel at a predetermined position for guiding the waste heat discharged by the fuel cell to exchange heat with the humidifying liquid in the barrel. The temperature of humidifying liquid is raised. Several thermocouple thermometers (61,62) are mounted to the barrel to contact with the humidifying liquid in the barrel for controlling and indicating the temperature of humidifying liquid. The gas dispenser made of porous material comprises foamed metals, porous metal oxides and porous carbon materials. Several gas baffle plates (50) are mounted in the barrel in front of the gas outlet for intercepting the gas reactant so that super-saturated moisture condenses on the gas baffle plates. The gas baffle plates are provided with fins (51) and inclined at a predetermined angle.    DESCRIPTION OF DRAWING(S) - The figure shows a sectional view of the external gas humidifier.    External gas humidifier (1)    Barrel (10)    Humidifying liquid (12)    Preheating serpentine (20)    Inlet (21)    Outlet (22)    Coiled section (23)    Gas dispenser (30)    Gas chamber (31)    Heat exchanger (40)    Gas baffle plate (50)    Fin (51)    Thermocouple thermometers (61,62)