PURPOSE: To dry only the seal part of a wet semi-permeable membrane within a short time, by irradiating microwave with specific frequency to a part to be sealed by a resin at the terminal end of a separation membrane.

CONSTITUTION: Microwave is introduced into a dryer 1 made of a metal such as aluminum or stainless steel through a waveguide 3 and a stirrer 4 is attached to the ceiling of the dryer in order to obtain uniform heating. A polysulfone hollow fiber with a diameter of 20μm is immersed in water or an aqueous glycerine solution for 30min and, while the terminal end part 5 to be dried is left uncovered, the other part thereof is covered with an aluminum foil 6 and an earth 7 is attached to the aluminum foil 6 in order to avoid abnormal concn. of microwave. This hollow fiber is placed on a teflon plate 8 to irradiate microwave of 2,450MHz and 100W for 10W20min. When the terminal end parts thereof are sealed by a urethane resin, the ultrafiltration capacity of the membrane reaches 1,000W1,300ml/hr.mmHg.m2.