The present invention provides a humidifier, which is used as an auxiliary humidifier of a fuel cell together with an existing gas-to-gas humidifier to improve humidification performance in a low efficiency region of the gas-to-gas humidifier, thus increasing the output of the fuel cell and providing high efficiency operation. Particularly, the humidifier comprises: 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; 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; and a pumping pipe including a porous material disposed therein and electrodes for applying a voltage to both ends of the porous material and disposed between the inlet assembly and the outlet assembly such that the water is transferred through a capillary path in the porous material with a pumping force by electroosmosis when the electrodes apply a voltage.