The constitution of an outer covering (3) includes a laminate of a stretchable sheet (30) containing elastic fibers and non-elastic fibers, and a non- stretchable sheet containing no elastic fibers. The thickness of a side seal part (4) is greater in comparison with the thickness of sections other than the side seal part (4), in a front side part (F) and a rear side part (R), respectively, which constitute the part. At an outside end surface (40) of the side seal part (4) there is formed a smooth area (35) in which the outside end surface (40) is smooth, elastic fibers being present in the smooth area (35).
