Disclosed is a humidifier for a fuel cell having an enhanced humidifying performance and being able to prevent an instantaneous decrease of the output of a car which might occur at the time of high-speed driving. The humidifier comprises a membrane housing with first and second ends; a bundle of hollow fiber membranes provided in an inner space of the membrane housing; a humidity retainer provided in the inner space of the membrane housing; a first cover mounted on the first end of the membrane housing, the first cover including an inlet for introducing unreacted gas of high-humidity discharged from a stack; and a second cover mounted on the second end of the membrane housing, the second cover including an outlet for discharging the unreacted gas used for humidification.