The electrode assembly of a lithium ion secondary battery is obtained by alternately laminating sheet-like positive electrodes (11) and sheet-like negative electrodes (12), with sac-like separators 13being interposed therebetween. First main surfaces 11a of the positive electrodes (11) have a smaller area than first main surfaces (12a) of the negative electrodes (12). Second main surfaces (11b) of the positive electrodes (11) have a smaller area than second main surfaces (12b) of the negative electrodes (12). The first main surfaces (11a, 12a) of the positive electrodes (11) and the negativeelectrodes (12) face each other, with the sac-like separators (13) being interposed therebetween; and the second main surfaces (11b, 12b) of the positive electrodes (11) and the negative electrodes (12) face each other, with the sac-like separators (13) being interposed therebetween.