Disclosed is a highly safe battery separator, in particular a separator for a lithium ion secondary battery, which reduces internal resistance, achieves good ionic conductivity, prevents passing of electrode active materials, and also prevents electrical short circuit by controlling deposition of lithium metal (dendrite). Also disclosed is a means for stably producing the battery separator with high productivity. Specifically disclosed are: a battery separator which is composed of a porous polyolefin sheet that is formed from a group of polyolefin nanofilaments that have an average filament diameter of less than 1 μm and a filament size distribution of 0.2 or less; and a means for producing the battery separator.