LITHIUM ION BATTERY

A lithium ion battery () of the invention includes a battery main body () which includes one or more power generation elements configured by laminating a positive electrode layer, an electrolyte layer, and a negative electrode layer, in this order; an outer package () which includes at least a heat-fusion resin layer () and a barrier layer (), and in which the battery main body () is sealed; and a pair of electrode terminals (), each of which is electrically connected to the battery main body () and at least a part of which is exposed to the outside of the outer package (). The outer package () includes an accommodation portion () which accommodates the battery main body (), a joint portion () where the heat-fusion resin layers () positioned on a peripheral portion of the accommodation portion () are joined with each other directly or through the electrode terminal (), and an extruded resin portion () which is formed by extrusion of a part of the heat-fusion resin layer () of the joint portion () from the joint portion () to the accommodation portion () side, and a difference (Lmax−Lmin) between a maximum length Lmax and a minimum length Lmin of the extruded resin portion () is equal to or greater than 0.0 mm and equal to or smaller than 1.0 mm.