A battery pack having a plurality of unit cells electrically connected to each other to provide high-output, large-capacity electricity, the battery pack comprising: two or more unit cells each of which has electrode terminals formed at opposite ends thereof, the unit cells being disposed in a lateral direction so that the electrode terminals are oriented in the same directions; connection members (200, 200'), each of which comprises a first conductive plate-shaped connection part connected to the electrode terminals of the unit cells by welding and a second plate-shaped connection part coupled to the outside of the first conductive plate-shaped connection part, the second plate-shaped connection part being provided with through holes, into which a welding rod is inserted to weld the unit cells and the first conductive plate-shaped connection part; and a pack case to cover the outsides of the unit cells and the connection members (200, 200'), wherein the first conductive plate-shaped connection part and the second plate-shaped connection part are coupled to each other by cream soldering, and a solder paste is disposed between the first conductive plate-shaped connection part and the second plate-shaped connection part to achieve the cream soldering, and the solder paste is provided with openings formed at positions corresponding to the through holes of the second plate-shaped connection part, wherein an input and output terminal, through which the battery pack is connectable to an external device, is formed at an end of the second plate-shaped connection part being bent toward an outermost one of the unit cells, wherein the first conductive plate-shaped connection part has a thickness equivalent to 5 to 50% of the thickness of the second plate-shaped connection part, wherein the second plate-shaped connection part has a thickness of 0.5 to 20 mm, and wherein the first conductive plate-shaped connection part has a thickness the same as the thickness of a case of each of the unit cells. two or more unit cells each of which has electrode terminals formed at opposite ends thereof, the unit cells being disposed in a lateral direction so that the electrode terminals are oriented in the same directions; connection members (200, 200'), each of which comprises a first conductive plate-shaped connection part connected to the electrode terminals of the unit cells by welding and a second plate-shaped connection part coupled to the outside of the first conductive plate-shaped connection part, the second plate-shaped connection part being provided with through holes, into which a welding rod is inserted to weld the unit cells and the first conductive plate-shaped connection part; and a pack case to cover the outsides of the unit cells and the connection members (200, 200'), wherein the first conductive plate-shaped connection part and the second plate-shaped connection part are coupled to each other by cream soldering, and a solder paste is disposed between the first conductive plate-shaped connection part and the second plate-shaped connection part to achieve the cream soldering, and the solder paste is provided with openings formed at positions corresponding to the through holes of the second plate-shaped connection part, wherein an input and output terminal, through which the battery pack is connectable to an external device, is formed at an end of the second plate-shaped connection part being bent toward an outermost one of the unit cells, wherein the first conductive plate-shaped connection part has a thickness equivalent to 5 to 50% of the thickness of the second plate-shaped connection part, wherein the second plate-shaped connection part has a thickness of 0.5 to 20 mm, and wherein the first conductive plate-shaped connection part has a thickness the same as the thickness of a case of each of the unit cells.