Plastic films are prevented from being peeled off from each other at a joining portion. A composite film has superposed external and internal films and is superposed with an additional body. The films and the body are joined with each other to form a first joining portion between the films, and a second joining portion between the internal film and the body. The first joining portion has a first joining width and includes first outer and inner edges. The second joining portion has a second joining width and includes second outer and inner edges. The first or second joining width is adjusted such that the first joining width is less than the second joining width, with the first inner edge formed outwardly of the second inner edge.