A positive electrode capable of achieving both of high volumetric energy density and high volumetric power density and a lithium ion secondary battery using the same are provided. A lithium ion secondary battery includes a positive electrode including a current collector with a positive active material mixture layer applied on both faces thereof, the positive active material mixture layer including active material particles, conductive additive particles and a binder. The active material particles used have a value D of an average particle diameter D50 of the active material particles in the range from 1 to 10 μm. The ratio b/a of the volume fraction b of the vacancy volume in the positive active material mixture layer to the volume fraction a of the active material particles in the positive active material mixture layer is in the range of −0.01D+0.57≦b/a≦−0.01D+0.97.