Disclosed herein is a battery pack including: a plurality of cylindrical battery cells arranged in a plurality of rows and a plurality of layers; a lower case for supporting the battery cells; and an upper cover welded to the lower case in the state of covering the battery cells supported by the lower case; wherein the end parts of the upper cover and the lower case are opposed to and abutted on each other, and are welded to each other by ultrasonic welding.