An electric energy storage device comprising: a battery element retained in a multilayered outer package body having an internal layer formed of a thermoplastic resin which can be heat-sealed and at least a metal layer; a tabular internal lead connected to the battery element in the outer package body; a tabular external lead arranged outside the outer package body so as to face the internal lead; a connection terminal electrically connecting the internal lead and the external lead together; an internal insulating member provided between the outer package body and the internal lead along an internal surface of the outer package body; and an external insulating member provided between the outer package body and the external lead along an external surface of the outer package body so as to face the internal insulating member, and wherein the connection terminal includes a penetration shaft passing all the way through the external lead, the external insulating member, the outer package body, the internal insulating member, and the internal lead, and flanges integrally formed at both ends of the penetration shaft, the flanges at both ends of the connection terminal depress and sandwich the external insulating member, the outer package body, and the internal insulating member between the tabular external lead and internal lead, and the external insulating member and/or the internal insulating member is pressed in between the penetration shaft of the connection terminal and the metal layer of the outer package body, and the internal insulating member also functions as internal-layer protecting means for suppressing any damage of the internal layer of the multilayered outer package body as the internal insulating member covers an end portion of the internal lead in the vicinity of the internal layer of the multilayered outer package body across a lengthwise direction of the internal lead. a battery element retained in a multilayered outer package body having an internal layer formed of a thermoplastic resin which can be heat-sealed and at least a metal layer; a tabular internal lead connected to the battery element in the outer package body; a tabular external lead arranged outside the outer package body so as to face the internal lead; a connection terminal electrically connecting the internal lead and the external lead together; an internal insulating member provided between the outer package body and the internal lead along an internal surface of the outer package body; and an external insulating member provided between the outer package body and the external lead along an external surface of the outer package body so as to face the internal insulating member, and wherein the connection terminal includes a penetration shaft passing all the way through the external lead, the external insulating member, the outer package body, the internal insulating member, and the internal lead, and flanges integrally formed at both ends of the penetration shaft, the flanges at both ends of the connection terminal depress and sandwich the external insulating member, the outer package body, and the internal insulating member between the tabular external lead and internal lead, and the external insulating member and/or the internal insulating member is pressed in between the penetration shaft of the connection terminal and the metal layer of the outer package body, and the internal insulating member also functions as internal-layer protecting means for suppressing any damage of the internal layer of the multilayered outer package body as the internal insulating member covers an end portion of the internal lead in the vicinity of the internal layer of the multilayered outer package body across a lengthwise direction of the internal lead.