A battery module is constructed of an integral battery case which is constituted by forming a plurality of prismatic cell cases into a unitized body, wherein positive electrode plates and negative electrode plates laminated alternately upon one another with intervening separators are respectively accommodated in each of the cell cases, thereby constituting a plurality of cells, these cells being electrically connected in series. A plurality of such battery modules are arranged in a direction in which the electrode plates within each cell are laminated in a condition with coolant passages formed between the battery modules, and bound tightly together with end plates arranged at both ends in the direction of adjacent arrangement of the battery modules.