PURPOSE: To produce a porous membrane of a polytetrafluoroethylene-based resin excellent in mechanical performances (strength and elongation and puncture pressure).

CONSTITUTION: In this method for producing a porous membrane of polytetrafluoroethylene-based resin, comprising molding an uniform mixture obtained by mixing a dispersion of the polytetrafluoroethylene-based resin with a fiber forming polymer and then heat-treating the resultant moldings at a temperature not lower than melting point of the resin and removing the fiber- forming polymer, preliminary heat treatment is carried out at ≥160°C and a temperature lower than melting point of the resin prior to the heat treatment.