There is provided a lithium ion secondary battery that includes a positive electrode having high thermal stability and is capable of greatly reducing the possibility of causing thermal runaway even in a nail penetration test. A lithium ion secondary battery including a positive electrode including a lithium composite oxide and a porous film bonded to at least one of a surface of the positive electrode and a surface of a negative electrode, wherein the porous film includes an inorganic oxide filler and a film binder, and the lithium composite oxide is represented by the formula: Lia(Co1-x-yMx1My2)bO2 (wherein element M1 is at least one selected from the group consisting of Mg, Sr, Y, Zr, Ca and Ti, element M2 is at least one selected from the group consisting of Al, Ga, In and Tl, and 0<a≦1.05, 0.005≦x≦0.15, 0≦y≦0.05 and 0.85≦b≦1.1).