Disclosed herein is a battery cell configured to have a structure in which an electrode assembly, including positive electrodes, negative electrodes, and separators disposed respectively between the positive electrodes and the negative electrodes, is mounted in a battery case made of a laminate sheet including a resin layer and a metal layer, wherein the battery case is made of a one-unit member, which is bent to form an upper case and a lower case, at least one of the upper and lower cases is provided with a receiving part, in which the electrode assembly is mounted, the receiving part having a round corner formed at at least one side edge thereof, and sealed portions, which are formed at the upper case and the lower case by thermal welding, are located at an upper side of the battery case at which at least one electrode terminal is located, a first lateral side of the battery case adjacent to the upper side, and a lower side of the battery case opposite to the upper side, and a second lateral side of the battery case adjacent to the upper side is formed by a bent structure of the battery case.