A membrane humidifier for a fuel cell, of the present invention, comprises: a middle case in which a plurality of hollow fiber membranes are accommodated; a cap case coupled to the middle case; potting parts formed at the ends of the plurality of hollow fiber membranes; an assembly member disposed between the cap case and the end of the middle case so as to provide air tight coupling therebetween; and a protrusion part extending toward the edges of the potting parts from the inside of the cap case so as to provide air tight coupling between the cap case and the potting parts.