In a dot pattern, both sufficient strength during the use of a diaper and the ease of tearing can be achieved, while inhibiting sticking of fiber waste of a non-woven fabric in the sealing device. The problems can be solved by the underpants-type disposable diaper characterized by a side seal section 13 is formed in a dot pattern in which a plurality of rows of dot-shaped welded parts 13s, 13b, disposed with intervals in a vertical direction, are disposed in a transverse direction, and non-welded parts 13n, disposed all over in the transverse direction, are disposed on both sides of each welded part 13s, 13b, in the vertical direction； the side seal section 13 has at least partly in the vertical direction a non-densely welded region LD and a densely welded region HD； and each welded part 13b in the densely welded region HD is larger in area than each welded part 13s in the non-densely welded region LD.
