To provide a convenient and effective method for suppressing the penetration of dendrite over the microporous film mainly containing the base portion, which occupies the most part of the entire separator (total area), rather than the peculiar concept (resulting in a difficult measure), in which only the pore structure of the rib portion is densified or contracted for suppressing dendrite from penetrating through the rib portion. A separator for a lead-acid battery, containing a microporous film obtained in such a manner that a raw material composition mainly containing a polyolefin resin, silica powder, and a plasticizer is melt-kneaded and formed into a film, from which the plasticizer is entirely or partially removed, the raw material composition containing glass flakes having an average particle diameter of from 20 to 800 μm and an average thickness of 0.2 to 8 μm and having no self-film formability in an amount of from 2 to 15% by weight based on a total amount of the silica powder and the glass flakes, the glass flakes in the microporous film being disposed in such a manner that a plane direction thereof is substantially oriented in a plane direction of the microporous film, a value of (the content of the glass flakes in the microporous film)/(the average thickness of the glass flakes in the microporous film) being 1 or more.