Disclosed is a battery separator consisting essentially of a nonwoven fabric having a substantially unilayered structure, wherein an apparent total surface area of fibers per a surface density of the nonwoven fabric is 20 m.sup.2 or more, a thickness of the nonwoven fabric is 0.1 mm or less, a uniformity index of the nonwoven fabric is 0.15 or less, and the nonwoven fabric contains fine fibers having a fiber diameter of 4 .mu.m or less.