PURPOSE: To provide a porous support membrane excellent in heat resistance and mechanical strength, constituted so as not to have fine pores having a pore size of 1μm or more on either one of the surfaces of a membrane mainly formed of an aromatic polyamide.

CONSTITUTION: As aromatic polyamide, for example, one shown by the general formula is dissolved in an amide solvent to form a solution which is, in turn, immersed in a coagulation bath to be coagulated after discharged from a nozzle and the product is washed with water, subjected to heat treatment and dried to be preserved in water. As the amide solvent, N, N-dimethylacetamide is used and the concn. thereof is adjusted to 3W35wt%. As the coagulation bath, water is used and, as a support membrane, ones with various configurations are used according to its application. The thickness of the thin membrane is 0.01W10μm and there is no pores having a pore size of 1μm or more on either one of the surfaces thereof. The air permeation amount thereof is within a range of 1W1× 10-6 cc(STP)/cm2.sec.cmHg at 20°C.