An energy storage device includes: a flattened electrode assembly formed by winding electrodes such that a hollow portion is formed, the electrode assembly including a pair of curved portions opposed manner in a major axis direction and a pair of flat portions opposed in a minor axis direction; and a case storing the electrode assembly therein, wherein assuming a thickness of the flat portion in the minor axis direction as A, a thickness of the curved portion in a radial direction as B, and a thickness of the hollow portion in the minor axis direction as W, the electrode assembly satisfies A+(W/2)≤B in a state where the electrode assembly is discharged.