Disclosed herein is a battery pack constructed in a structure in which a plurality of secondary battery cells are electrically connected to one another via a connection member while the secondary battery cells are mounted in a receiving part of a pack case having no partition, wherein the connection member is located between the battery cells arranged in the longitudinal direction or in both the longitudinal direction and the lateral direction, the connection member is connected, in a mechanical coupling manner, to a lower electrode terminal of the front battery cell in the longitudinal direction and/or to an upper electrode terminal of the rear battery cell in the longitudinal direction, and the connection member is elastically pressed while the connection member is located between the battery cells.