Disclosed is a membrane humidifier for a fuel cell, which uniformly humidifies entire hollow fiber membranes from an outer side to a central portion of an interior of the membrane humidifier to improve the distribution of wet air and dry air, thereby improving a humidification performance. The membrane humidifier for the fuel cell includes a hollow upper case including first wet air inlet apertures and first wet air outlet apertures and a membrane module assembly including a plurality of unit membrane modules received lengthwise within the upper case along the flow direction of dry air.