A battery pack comprising: (a) a battery module assembly having a structure in which two or more battery modules, each of which comprises a plurality of unit modules, each of which comprises two or more battery cells, mounted in a cartridge in a state in which the unit modules are connected in series and/or parallel to each other, are arranged in a lateral direction such that the battery modules are disposed in tight contact with each other or adjacent to each other in a state in which the battery modules are electrically connected to each other; (b) a pair of a front support member and a rear support member to cover a front and rear of outermost battery modules of the battery module assembly; (c) at least one upper end connection member to connect the front support member and the rear support member to each other at an upper end of the battery module assembly to support an upper part of the battery module assembly; and (d) at least one lower end connection member to connect the front support member and the rear support member to each other at a lower end of the battery module assembly to support a lower part of the battery module assembly, wherein the cartridge is provided at a lower end thereof with a fastening hole, via which the at least one lower end connection member is mounted, wherein an insulation member, having a bracket shape in longitudinal vertical section, is mounted between cathode terminals and anode terminals of the unit modules at a top of each of the battery modules to prevent contact between the cathode terminals and the anode terminals of the unit modules, and the at least one upper end connection member is mounted in a depressed groove of the insulation member. (a) a battery module assembly having a structure in which two or more battery modules, each of which comprises a plurality of unit modules, each of which comprises two or more battery cells, mounted in a cartridge in a state in which the unit modules are connected in series and/or parallel to each other, are arranged in a lateral direction such that the battery modules are disposed in tight contact with each other or adjacent to each other in a state in which the battery modules are electrically connected to each other; (b) a pair of a front support member and a rear support member to cover a front and rear of outermost battery modules of the battery module assembly; (c) at least one upper end connection member to connect the front support member and the rear support member to each other at an upper end of the battery module assembly to support an upper part of the battery module assembly; and (d) at least one lower end connection member to connect the front support member and the rear support member to each other at a lower end of the battery module assembly to support a lower part of the battery module assembly, wherein the cartridge is provided at a lower end thereof with a fastening hole, via which the at least one lower end connection member is mounted, wherein an insulation member, having a bracket shape in longitudinal vertical section, is mounted between cathode terminals and anode terminals of the unit modules at a top of each of the battery modules to prevent contact between the cathode terminals and the anode terminals of the unit modules, and the at least one upper end connection member is mounted in a depressed groove of the insulation member.