Disclosed herein is a battery pack wherein ultrasonic welding is performed at side walls extending from an upper case and a lower case, each of the side walls having a predetermined thickness, and a film-shaped label is formed at the upper surface and/or the lower surface of the case, whereby the coupling force due to the ultrasonic welding is very large although the thickness is very small, the structural stability against being dropped or external impacts is high, and interface area of the coupling regions is large, with the result that the attachment of any label for increasing the coupling force and preventing the introduction of foreign matters is not necessary.