NOVELTY - A lithium ion secondary battery consists of a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte. The positive electrode has positive mix layers containing positive-electrode active material on at least one surface of an electrical power collector. The positive mix layer has a volume density (Vc') of 62 vol.% or more and capacity per unit area of 2 mAh/cm2 or more. The negative electrode has negative mix layers containing negative electrode active material on at least one surface of the electrical power collector.    USE - Lithium ion secondary battery is used for portable electronic device and hybrid vehicle.    ADVANTAGE - The lithium ion secondary battery has high capacity and excellent charging/discharging characteristics.    DETAILED DESCRIPTION - A lithium ion secondary battery consists of a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte. The positive electrode has positive mix layers containing positive-electrode active material on at least one surface of an electrical power collector. The positive mix layer has a volume density (Vc') of 62 vol.% or more and capacity per unit area of 2 mAh/cm2 or more. The negative electrode has negative mix layers containing negative electrode active material on at least one surface of the electrical power collector. The negative mix layer has a volume density (Va') of 62 vol.% or more and capacity per unit area of 2.5 mAh/cm2 or more. The separator has air permeability of 200 seconds/100 ml or less, calculated by Gurley test. When a counter electrode is lithium and a charge voltage is 4.2 V, then the first time charging capacity (C' (in mAh/g)) per 1 g positive electrode active material, and volume density (Vc') satisfies a relation: (C'2/Vc') less than or equal to 200 (a). When a counter electrode is lithium and a discharge voltage is 0.01 V, then the first time discharge capacity (A in (mAh/g)) per 1 g negative electrode active material, and volume density (Va') satisfies a relation: (A2/Va') less than or equal to 1700 (b). The average pore diameter (d in (nm)) of the separator, discharge capacity (A in (mAh/g)) and volume density (Va') satisfies a relation: (d/(A2/Va')) greater than or equal to 0.039 (c). The ratio ((A2/Va')/(C'/Vc')) of discharge capacity (A in (mAh/g)) and volume density (Va') to charging capacity (C' (in mAh/g)) and volume density (Vc') is 30 or less.