Abstract: Providing a membrane-forming solution suitable for producing a separation membrane such as a hollow fiber membrane and a flat membrane. A membrane-forming solution including triacetylcellulose having an acetyl group substitution degree of 2.7 or higher, a good solvent for thermally induced phase separation and a poor solvent for thermally induced phase separation, wherein the good solvent is capable of heat-dissolving the triacetylcellulose (at a solid content concentration of 25 mass %), and the poor solvent is incapable of dissolving the triacetylcellulose up to the heat dissolution temperature of the good solvent, wherein both the good solvent and the poor solvent are included so as to enable phase separation of the heat-dissolved triacetylcellulose solution while the heat-dissolved triacetylcellulose solution is cooled to room temperature (from 20 to 30° C.

Abstract: Porous membranes having a micro phase separation structure and showing a light transmittance at the wavelength of 400 nm of not less than 30% are obtained by the dry phase conversion method comprising drying a coating layer of a dope containing a polymer, a good solvent for the polymer and a poor solvent for the polymer which solvent has a higher boiling point than the good solvent. The polymer includes cellulose derivatives, vinyl-series polymers such as acrylonitrile-series polymers and (meth)acrylic acid ester-series polymers, polysulfone-series polymers, and the like. The porous polymer membranes have a porosity of 10 to 60%, a mean pore size of about 0.002 to 0.35 &mgr;m and a maximum pore size of not greater than 0.4 &mgr;m. These porous membranes shows not only excellent transparency but also high productivity.