Disclosed is a fuel cell membrane humidifier capable of improving humidification efficiency by preventing the differential pressure loss in a humidification module. The disclosed fuel cell membrane humidifier performs water exchange between a first fluid and a second fluid and comprises: a mid-case; a second fluid inlet through which the second fluid flows into the mid-case; a second fluid outlet through which the second fluid is discharged to the outside; a partition wall that divides the inner space of the mid-case into a first space and a second space; and at least one cartridge arranged in the mid-case and accommodating a plurality of hollow fiber membranes. The cross-sectional area of the second fluid inlet is greater than or equal to the total area of windows that constitute first mesh hole portions arranged in a mesh shape in the at least one cartridge for fluid communication with the first space.