Disclosed is a battery pack including a battery module array having battery modules arranged in two or more rows in a state in which the battery modules are erected in an upside-down fashion, each of the battery modules being formed by stacking battery cells or unit modules, each having two or more battery cells mounted therein, in a lateral direction with respect to electrode terminals of the battery cells, a front support member and a rear support member that are a pair of side support members to support the front and rear of the battery module array, respectively, lower end support members coupled to lower ends of the side support members, and mounting members coupled to an upper end of the battery module array and fastened to an external device such that an upper part of the battery pack is mounted to the external device.