A multilayer microporous membrane containing a thermoplastic resin, comprising a coarse structure layer with a higher open pore ratio and a fine structure layer with a lower open pore ratio, wherein said coarse structure layer is present at least in one membrane surface having a thickness of not less than 5.0 .mu.m, a thickness of said fine structure layer is not less than 50% of the whole membrane thickness, and said coarse structure layer and said fine structure layer are formed in one-piece.