Disclosed herein is a battery pack having a plurality of unit cells electrically connected to each other to provide high-output, large-capacity electricity, the battery pack including 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, 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 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 battery cells and the first plate-shaped connection part, and a pack case to cover the outsides of the unit cells and the connection members.