<p>To provide a substantially nondefective porous polytetrafluoroethylene (PTFE) membrane. A composite porous PTFE membrane is produced by paste extruding a PTFE produced by emulsion polymerization, semi-firing the extrudate to give a PTFE film, stretching longitudinally the film at a temperature below the melting point of the fired PTFE, and stretching transversally two sheets of the above-stretched film superposed one on another.</p>
