An object of the present invention is to provide a thin battery separator having an excellent nitrate group trapping performance and a high piercing strength. The inventive separator for a secondary battery, which can accomplish such an object, is made of a resin composition mainly containing a polyolefin having a hydrophilic functional group, wherein the resin composition has a content (ratio to a total resin amount) of a low-density polyethylene of 20 mass % or less, a piercing strength of 5 to 100 N, a Metsuke of 20 to 75 g/m2, a thickness of 15 to 150 μm, and an unneutralized hydrophilic functional group amount of 1×10−3 to 5×10−2 mol/m2.