A lithium ion secondary battery includes a flat-shaped electrode wound body. Positive and negative terminals are connected respectively to the body and placed to partially protrude from a battery lid. Assuming that the body length in a winding axial direction is W, the body size in a direction perpendicular to the axial direction and parallel to the flat surfaces is H, and a distance between a farthest position in a connecting range of one of the terminals, in which the terminal is connected to an uncoated portion, from the lid and the body edge located on the lid side is X, the values W, H, and X are determined so that a point defined in a plane (X/H, H/W) by values (X/H) and (H/W) is located in a triangle defined by joining three points, (0.50, 0.70), (0.32, 0.40), and (0.70, 0.40) in the same plane.