Disclosed is a porous film formed of a polyolefin resin comprising an ethylene-α-olefin copolymer (A) which comprises structural units originating from ethylene and structural units originating from one or more sorts of monomers selected from α-olefins having 4-8 carbon atoms and which satisfies the requirements (I) the intrinsic viscosity [η] is 9.0 to 15.0 dl/g; (II) the melting point Tm is not lower than 115° C. but lower than 130° C.; (III) the content of cold-xylene-soluble components included in the ethylene-α-olefin copolymer (A) is 3% by weight or less; and (IV) Tm≦0.54×[η]+114. A battery separator including the porous film and a method for the preparation of the porous film are also disclosed.