The present invention provides a battery module, which comprises: (a) a battery cell stacked body, which has a plurality of battery cells or unit modules that are connected serially and/or in parallel, stacked together and standing sideways, and which has a busbar on the front surface for connecting the electrode terminals of battery cells to external input/output terminals; (b) voltage detection members, which are electrically connected to the electrode terminal connection portions of the battery cells that are respectively located on the front and the rear surfaces of the battery cell stacked body, and which have connection terminals on the end portion for detecting the voltage of the battery cells or the unit modules; (c) an upper portion case, which has a structure encasing one side of the end portion of the battery cell stacked body and partially encasing the upper and lower portions thereof, and which has a mounting portion for inserting and mounting the voltage detection members; and (d) a lower portion case, which has a structure encasing the other side of the end portion of the battery cell stacked body and partially encasing the upper and lower portions thereof, and which has external input/output terminals on the front surface.