A cell includes an electrode plate group which is formed by laminating a positive electrode plate and a negative electrode plate with a separator interposed between them, and includes leads protruding toward directions opposite to each other from one side of the positive electrode plate and the negative electrode plate. Collectors are joined to the leads on both sides of the electrode plate group, and include connection protrusions formed so as to protrude outside. A bag-shape battery case containing the electrode plate group is joined to the collectors such that only the connection protrusions of the collectors are protruded outside the bag-shaped battery case. A battery module is constituted by placing a plurality of the cells, connected together, into a prismatic battery case.