A humidifier includes an inlet assembly provided in an air exhaust pipe, through which water-supersaturated exhaust gas is discharged from a fuel cell stack, to supply water condensed from the exhaust gas to the fuel cell stack and an outlet assembly provided in an air supply pipe, through which a reactant gas to be humidified and supplied to the fuel cell stack passes, to supply the water transferred through a pumping pipe to the air supply pipe such that the reactant gas is humidified. A pumping pipe including a porous material disposed therein along with electrodes are disposed between the inlet assembly and the outlet assembly so that the water is transferred through a capillary path in the porous material via a pumping force by electroosmosis when the electrodes apply a voltage.