An object of the present invention is to provide a separator for a nonaqueous secondary battery having excellent heat resistance and excellent ion permeation as well as shut-down properties. The first invention is a separator for a nonaqueous secondary battery, comprising a porous composite membrane which includes a substrate comprising a polyolefin microporous membrane and a heat-resistant porous layer comprising a heat-resistant resin, the heat-resistant porous layer being formed on at least one surface of the substrate, wherein the separator is characterized in that the sheet resistance (A) of the substrate, the Gurley value (B) of the substrate, the sheet resistance (C) of the porous composite membrane, and the Gurley value (D) of the porous composite membrane satisfy a specific relationship. The second invention is characterized in that the heat-resistant porous layer has an average pore size of 0.1 to 0.2 μm as measured by mercury intrusion porosimetry. The third invention is characterized in that the heat-resistant resin fibrils have an average fibril diameter of 10 to 80 nm, and the pores in the heat-resistant porous layer have an average pore size of 50 to 250 nm.