An energy storage device ( 1, 100) comprising: a negative electrode including a negative composite layer on a negative electrode collector foil and a negative composite layer non-forming part along a side of the negative electrode collector foil; a positive electrode including a positive composite layer on a positive electrode collector foil and a positive composite layer non-forming part along a side of the positive electrode collector foil; and a separator including an insulating layer on its surface opposing the positive electrode, the separator being layered between the negative electrode and the positive electrode, wherein the negative composite layer non-forming part and the positive composite layer non-forming part are disposed in opposite directions to each other, a part of the negative composite layer non-forming part is connected to a negative electrode current collector, a part of the positive composite layer non-forming part is connected to a positive electrode current collector, an end S1 on a side of the negative electrode current collector of the separator is projected more than an end P1 on the side of the negative electrode current collector of the positive composite layer, an end S2 on a side of the positive electrode current collector of the separator is projected more than an end P2 on the side of the positive electrode current collector of the positive composite layer, and a distance W2 from the end P2 of the positive composite layer to the end S2 of the separator is smaller than a distance W1 from the end P1 of the positive composite layer to the end S1 of the separator, and wherein the positive electrode has a thickness of D, the positive composite layer has a thickness of d, and the distance W2 is not less than 0.5(d 2 /D 2)W1 and not more than 2(d 2 /D 2)W1. a negative electrode including a negative composite layer on a negative electrode collector foil and a negative composite layer non-forming part along a side of the negative electrode collector foil; a positive electrode including a positive composite layer on a positive electrode collector foil and a positive composite layer non-forming part along a side of the positive electrode collector foil; and a separator including an insulating layer on its surface opposing the positive electrode, the separator being layered between the negative electrode and the positive electrode, wherein the negative composite layer non-forming part and the positive composite layer non-forming part are disposed in opposite directions to each other, a part of the negative composite layer non-forming part is connected to a negative electrode current collector, a part of the positive composite layer non-forming part is connected to a positive electrode current collector, an end S1 on a side of the negative electrode current collector of the separator is projected more than an end P1 on the side of the negative electrode current collector of the positive composite layer, an end S2 on a side of the positive electrode current collector of the separator is projected more than an end P2 on the side of the positive electrode current collector of the positive composite layer, and a distance W2 from the end P2 of the positive composite layer to the end S2 of the separator is smaller than a distance W1 from the end P1 of the positive composite layer to the end S1 of the separator, and wherein the positive electrode has a thickness of D, the positive composite layer has a thickness of d, and the distance W2 is not less than 0.5(d 2 /D 2)W1 and not more than 2(d 2 /D 2)W1.