Metal cell tanks including secondary cells respectively are connected to one another by connection members. For example, a connection member has: an insulation portion interposed between adjacent cell tanks for keeping the cell tanks electrically insulated from each other; an one-end holding portion extending from one end of the insulation portion for holding one end portion of one of the adjacent cell tanks; and an opposite-end holding portion extending from the other end of the insulation portion for holding the other end portion of the other of the adjacent cell tanks.