A connection plate in form of flat plate is welded to be connected to portion that are close to each other, i.e., a connection-electrode portion of one of two adjacent cells and a bottom of a cell case of the other cell, while being bridged between the connection-electrode portion and the bottom of the cell case, thereby forming an inexpensive connecting structure between cells, that reduces the electric resistance, by simple processes. A battery module having a solid structure impervious to vibration or shock is formed by accommodating cylindrical cells in cell-accommodating portions, each of which has a square shape seen from the front, of a holder case made of synthetic resin with their cell axes arranged in parallel, and by closing both sides of the holder case with cover members having radiation holes. Then, a battery pack is formed by attaching outer plates to a plurality of such battery modules that are arranged and integrating them.