A humidifier for a fuel cell is provided. The humidifier is configured to humidify discharged gas from a cathode of the fuel cell and dry air supplied via an air compressor using membrane, and supply humidified air to the cathode. The humidifier includes: a membrane module having bundles of hollow fiber membranes disposed therein and manifolds each of which is connected to both sides of the membrane module, configured to inject the discharge gas and the dry air into an interior of the membrane module, and discharge the discharged gas from which moisture is removed and the humidified air from the interior of the membrane module. At least one bypass line is inserted into interiors of the manifolds and is connected to the interior of the membrane module, and is selectively connected to a supplying path of the dry air supplied from the air compressor.