A humidifier for a fuel cell system is provided to improve humidification efficiency of a hollow fiber improving a gas flow inside a membrane module into which tube side air and moist air are introduced. The humidifier includes a membrane module in which hollow fibers are fixed inside a case in a bundle form and a first manifold and a second manifold each assembled at both ends of the case. An inside of the membrane module includes a tube side air supply tube which forms a gas flow in the same direction as the hollow fiber and one side of the tube side air supply tube includes at least one moist air pumping part configured to suction moist air flowing in an exterior of the tube side air supply tube by a flow velocity and a flow pressure of tube side air.