A lithium ion secondary battery according to an embodiment of this disclosure includes: a positive electrode mixture layer provided on a main plane of a positive electrode current collector; a negative electrode mixture layer provided on a main plane of a negative electrode current collector; and an insulator covering a region of a part of a surface of a gradually-decreasing portion included in the positive electrode mixture layer. The portion has thickness gradually decreasing toward a terminal of the positive electrode mixture layer; the surface of the portion has a tangent line in contact with the surface in at least two contact points, and has a depressed part between any adjacent two contact points on the tangent line; and an end of the insulator is positioned between the contact points closest to and farthest from the terminal of the positive electrode mixture layer along the tangent line.