The invention provides a separator for nonaqueous electrolyte secondary batteries, which is high in puncture strength, can achieve thin-film structure and is high in air porosity. The nonaqueous electrolyte secondary battery includes an electrode assembly 3 including a positive electrode 20, a negative electrode 30, and a separator 40, and a nonaqueous electrolyte. The separator includes a porousresin sheet having at least a three-layer structure consisting of an A-layer 41, a B-layer 42, and a C-layer 43 stacked in that order. The average thermal expansion coefficient of each of the A-layerand the C-layer at a temperature of 0 DEG C to 50 DEG C. is 100 ppm/K or more less than the average thermal expansion coefficient of the B-layer at a temperature of 0 DEG C to 50 DEG C.