A membrane humidifier for a fuel cell, the membrane humidifier comprising: a middle case accommodating a plurality of hollow fiber membranes; and a cap case coupled to the middle case; a cartridge disposed in the middle case, wherein the hollow fiber membranes are accommodated in the cartridge; and a potting unit provided at an end of the cartridge, wherein the potting unit is formed at ends of the plurality of hollow fiber membranes and is configured to bind the hollow fiber membranes and to fill gaps between the hollow fiber membranes, characterized in that: the membrane humidifier further comprises an assembly member disposed between the cap case and an end of the middle case, the assembly member being configured to perform hermetic coupling therebetween; the cap case includes a projection portion extending from an inside of the cap case toward an edge of the potting unit, the projection portion being configured to perform hermetic coupling between the cap case and the potting unit; the assembly member is made of a soft material that has lower hardness than the cap case and, when assembled, is inserted into a gap between the cap case and the end of the middle case and then pressed by the cap case and the end of the middle case so as to perform hermetic coupling therebetween; the potting unit is pressed by the projection portion and the edge of the potting unit is shrunk by a predetermined depth by the projection portion, whereby the hermetic coupling between the cap case and the potting unit is achieved; and an outer surface of the potting unit and an outer surface of the cartridge are spaced apart from an inner surface of the middle case. a middle case accommodating a plurality of hollow fiber membranes; and a cap case coupled to the middle case; a cartridge disposed in the middle case, wherein the hollow fiber membranes are accommodated in the cartridge; and a potting unit provided at an end of the cartridge, wherein the potting unit is formed at ends of the plurality of hollow fiber membranes and is configured to bind the hollow fiber membranes and to fill gaps between the hollow fiber membranes, characterized in that: the membrane humidifier further comprises an assembly member disposed between the cap case and an end of the middle case, the assembly member being configured to perform hermetic coupling therebetween; the cap case includes a projection portion extending from an inside of the cap case toward an edge of the potting unit, the projection portion being configured to perform hermetic coupling between the cap case and the potting unit; the assembly member is made of a soft material that has lower hardness than the cap case and, when assembled, is inserted into a gap between the cap case and the end of the middle case and then pressed by the cap case and the end of the middle case so as to perform hermetic coupling therebetween; the potting unit is pressed by the projection portion and the edge of the potting unit is shrunk by a predetermined depth by the projection portion, whereby the hermetic coupling between the cap case and the potting unit is achieved; and an outer surface of the potting unit and an outer surface of the cartridge are spaced apart from an inner surface of the middle case.