A lithium ion secondary battery includes a housing. An electrode group and an electrolytic solution are in the housing. The electrode group is immersed in the electrolytic solution. The electrode group includes a positive electrode including a positive electrode active material layer and a negative electrode including a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is disposed on a surface of the negative electrode current collector. A metal piece is electrically connected to the negative electrode current collector. The metal piece is disposed at a position at which at least a part of the metal piece is immersed in the electrolytic solution. An oxidation-reduction potential of the metal piece is within an overdischarging voltage range and is lower than an oxidation-reduction potential of the negative electrode current collector.