An energy storage device including a spiral electrode group in which a first electrode plate and a second electrode plate having polarity reverse to that of the first electrode plate are spirally wound with a separator interposed therebetween, wherein the second electrode plate is opposed to an inner circumference and an outer circumference of the first electrode plate, portions of the separator are reinforced, the reinforced portions of the separator include a first reinforced portion formed between a winding-start end of the first electrode plate and the second electrode plate located on a radially outer side of the winding-start end, and a second reinforced portion formed between the winding-start end of the first electrode plate and the second electrode plate located on a radially inner side of the winding-start end, and the first reinforced portion and the second reinforced portion are arranged apart from each other.