Disclosed is a method and device for producing a membrane module used in a membrane humidifier for humidifying air in a polymer fuel cell, in which a small amount of hollow fiber membranes are disposed in the center of the membrane module and a larger amount of hollow fiber membranes are disposed toward the edge based on the design of a potting cap. Dry air introduced from an air blower into the membrane can be distributed to the edge where the larger amount of hollow fiber membranes are disposed, thereby maintaining uniform humidification performance over the entire hollow fiber membranes.