The present invention relates to a fuel cell membrane humidifier which can seal the gap between an inner case and a mid-case constituting a cartridge to prevent exhaust gas from being bypassed. The disclosed fuel cell membrane humidifier comprises: a mid-case which has a partition wall partitioning the inner space thereof; a first cap which is fastened to one side of the mid-case; a second cap which is fastened to the other side of the mid-case; an inner case which is disposed in the mid-case and in which a plurality of hollow fiber membranes are accommodated; a rib which is formed to protrude from one side of the inner case and comes into contact with one surface of the partition wall; a hook which is formed to protrude from the other side of the inner case and comes into contact with the other surface of the partition wall; a first gasket which is disposed between the first cap and the inner case and transmits the fastening force between the first cap and the mid-case to the rib; and a second gasket which is disposed between the second cap and the inner case and transmits the fastening force between the second cap and the mid-case to the hook.