The present invention provides a nonaqueous electrolyte secondary battery separator which is excellent in cycle characteristic. The nonaqueous electrolyte secondary battery separator includes a polyolefin porous film in which a difference between (i) a porosity calculated from a continuous image which is formed so as to extend from a surface to an inside of the polyolefin porous film in a thickness direction and in which a void part and a resin part are shown at respective two gray levels and (ii) a porosity calculated from a thickness, a weight per unit area, and a real density of the polyolefin porous film is 4% to 20%.