A lithium ion secondary battery that includes an electrode assembly having a plate-shaped positive electrode and a plate-shaped negative electrode stacked with a separator interposed therebetween, and an outer package made of a laminate film housing the electrode assembly. The lithium ion secondary battery has a polygonal shape having at least one set of a width and a length in a plan view thereof, and C/(A×B) is 0.50 (N/mm2) to 1.20 (N/mm2), where A is a thickness of the lithium ion secondary battery, B is a width of the lithium ion secondary battery, and C is a three-point bending strength of the lithium ion secondary battery.