A middle- or large-sized battery module comprising: a battery cell stack, wherein the battery cell stack comprises a plurality of unit modules each including plate-shaped battery cells having electrode terminals formed at the upper and lower ends thereof, each battery cell comprising an electrode assembly mounted in a pouch-shaped battery case, the plurality of battery cells or unit modules (200, 201) being electrically connected with each other, wherein each unit module comprises two or more of the battery cells constructed in a stacked structure in which the electrode terminals of the battery cells are connected in series with each other and electrode terminal connections are bent such that the battery cells are stacked, and a pair of high-strength cell covers (210, 220) for surrounding the outer surfaces of the battery cells excluding the electrode terminals of the battery cells when the cell covers are coupled with each other, the high strength cell covers restraining the repetitive expansion and contraction change of the battery cells during their charge and discharge; a first module case constructed in a structure to entirely surround one side end of the battery cell stack and to partially surround upper and lower ends of the battery cell stack, the first module case being provided at a front part thereof with external input and output terminals; a second module case coupled with the first module case, the second module case being constructed in a structure to entirely surround the other side end of the battery cell stack and to partially surround the upper and lower ends of the battery cell stack, the second module case being provided at a front part thereof with bus bars for connecting electrode terminals (320, 321) of the battery cell stack to the external input and output terminals; wherein the battery cells or unit modules, which have opposite side edges defined as a lateral direction, are erected in the lateral direction, and wherein the first and second module cases respectively have a plurality of mounting grooves in which edges of the battery cells or unit modules are inserted; a sensing member mounted to the first module case or the second module case; and a battery management system mounted to the first module case or the second module case, while being connected to the sensing member, for monitoring and controlling the operation of the battery module, wherein the upper end of each bus bar is formed in the shape of a depression such that the external input and output terminals are inserted into the depressions of the bus bars when the first and second module cases (400, 500) are coupled with each other. a battery cell stack, wherein the battery cell stack comprises a plurality of unit modules each including plate-shaped battery cells having electrode terminals formed at the upper and lower ends thereof, each battery cell comprising an electrode assembly mounted in a pouch-shaped battery case, the plurality of battery cells or unit modules (200, 201) being electrically connected with each other, wherein each unit module comprises two or more of the battery cells constructed in a stacked structure in which the electrode terminals of the battery cells are connected in series with each other and electrode terminal connections are bent such that the battery cells are stacked, and a pair of high-strength cell covers (210, 220) for surrounding the outer surfaces of the battery cells excluding the electrode terminals of the battery cells when the cell covers are coupled with each other, the high strength cell covers restraining the repetitive expansion and contraction change of the battery cells during their charge and discharge; a first module case constructed in a structure to entirely surround one side end of the battery cell stack and to partially surround upper and lower ends of the battery cell stack, the first module case being provided at a front part thereof with external input and output terminals; a second module case coupled with the first module case, the second module case being constructed in a structure to entirely surround the other side end of the battery cell stack and to partially surround the upper and lower ends of the battery cell stack, the second module case being provided at a front part thereof with bus bars for connecting electrode terminals (320, 321) of the battery cell stack to the external input and output terminals; wherein the battery cells or unit modules, which have opposite side edges defined as a lateral direction, are erected in the lateral direction, and wherein the first and second module cases respectively have a plurality of mounting grooves in which edges of the battery cells or unit modules are inserted; a sensing member mounted to the first module case or the second module case; and a battery management system mounted to the first module case or the second module case, while being connected to the sensing member, for monitoring and controlling the operation of the battery module, wherein the upper end of each bus bar is formed in the shape of a depression such that the external input and output terminals are inserted into the depressions of the bus bars when the first and second module cases (400, 500) are coupled with each other.