NONAQUEOUS ELECTROLYTE SECONDARY BATTERY SEPARATOR

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%. 本发明提供种循环特性优异的非水电解液二次电池用间隔件。该非水电解液二次电池用间隔件包含聚烯烃多孔膜，根据从上述聚烯烃多孔膜表面朝向内部厚度方向所形成的由空隙部分和多孔膜部分被二值化而成的连续图像计算出的空隙率与根据厚度、重量基重和真密度计算出的空隙率之差为4～20％。