PURPOSE: To obtain a dried semipermeable membrane free from deformation without giving surface tension to the semipermeable membrane in the case of volatilization of a solvent by bringing the wet semipermeable membrane into contact with a dry medium such as carbon dioxide and nitrous oxide in an ultracritical state and thereafter heating it is critical temp. and above.

CONSTITUTION: A wet semipermeable membrane consisting of polyamide, polyimide and polysulfone, etc., is used. Water and an organic solvent are contained in this wet semipermeable membrane as a solvent. A dry medium selected from among carbon dioxide, nitrous oxide and low fluorohydrocarbon is brought into contact with this wet semipermeable membrane in the ultracritical state thereof. Then the dry medium is heated and made to a mixed state of a liquid phase and a vapor phase and furthermore heated and made to a gaseous state via the neighborhood of a critical point and removed from the semipermeable membrane together with the solvent. By using such a drying method, surface tension is not almost added to the semipermeable membrane in the case of volatilization of the solvent and thereby deformation and destruction of the surface of the membrane are not caused.