A lithium-ion secondary battery capable of securing safety at a time of battery abnormality and restricting a drop in a high rate discharge property is provided. A lithium-ion secondary battery 1 has an electrode group 5 formed by winding a positive electrode plate 2 in which a positive electrode mixture including a positive electrode active material is formed at a collector and a negative electrode plate 3 in which a negative electrode mixture including a negative electrode active material is formed at a collector via a porous separator 4. A flame retardant is mixed to the positive electrode mixture of the positive electrode plate 2. The mode of pore diameters formed at the positive electrode mixture, which is measured by a mercury porosimetry, is set to a range of from 0.5 to 2.0 μm. The moving path for lithium-ions and at the same time the moving path for electrons are secured at a charge/discharge time.