Patent ID: 8231999

Claim:
A separator comprising: a base material layer made of a polyolefin resin including a plurality of through-holes; and a functional resin layer which is made of a resin different from said polyolefin resin and includes a plurality of through-holes creating a porous interconnected structure in which through-holes of the base material layer are interconnected to the through-holes of the functional resin layer, wherein a diameter of a narrowest portion of through-holes of said functional resin layer is larger than a diameter of a narrowest portion of through-holes of said base material layer, wherein all of the diameters of the through-holes of said functional resin layer range from 0.015 μm or more to 20 μm or less and the number of said through-holes of said functional resin layer whose diameters range from 0.02 μm or more to 10 μm or less is equal to 50% or more of the whole number of through-holes of said functional resin layer.