An underpants-type disposable diaper, in which a front body part and a back body part are each weld-jointed together on both sides to form side seal sections, thereby forming a waist opening and a right-left pair of leg openings, characterized by: at least an outer surface of the side seal section is formed of a non-woven fabric; the side seal section is formed in a dot pattern in which a plurality of rows of dot-shaped welded parts, disposed with intervals in a vertical direction, are disposed in a transverse direction, and non-welded parts, disposed all over in the transverse direction, are disposed on both sides of each welded part in the vertical direction; the side seal section is set to have a non-densely welded region at least partly in the vertical direction and a densely welded region at least partly in the vertical direction; each welded part in the densely welded region is at least partly larger in area than each welded part in the non-densely welded region; each welded part in the side seal section has the shape of a circle having a diameter of 0.7 to 2.0 mm; each welded part in the densely welded region has an area being 1.5 to 3.5 times as large as each welded part in the non-densely welded region; the number of rows of the welded parts arranged in the non-densely welded region is two; the number of rows of the welded parts arranged in the densely welded region is more than the number of the rows in the non-densely welded region; the non-welded part in the non-densely welded region has a length of 0.05 to 2.0 mm in the vertical direction; and the non-welded part in the densely welded region has a length of 0.05 to 3.0 mm in the vertical direction. at least an outer surface of the side seal section is formed of a non-woven fabric; the side seal section is formed in a dot pattern in which a plurality of rows of dot-shaped welded parts, disposed with intervals in a vertical direction, are disposed in a transverse direction, and non-welded parts, disposed all over in the transverse direction, are disposed on both sides of each welded part in the vertical direction; a plurality of rows of dot-shaped welded parts, disposed with intervals in a vertical direction, are disposed in a transverse direction, and non-welded parts, disposed all over in the transverse direction, are disposed on both sides of each welded part in the vertical direction; the side seal section is set to have a non-densely welded region at least partly in the vertical direction and a densely welded region at least partly in the vertical direction; each welded part in the densely welded region is at least partly larger in area than each welded part in the non-densely welded region; each welded part in the side seal section has the shape of a circle having a diameter of 0.7 to 2.0 mm; each welded part in the densely welded region has an area being 1.5 to 3.5 times as large as each welded part in the non-densely welded region; the number of rows of the welded parts arranged in the non-densely welded region is two; the number of rows of the welded parts arranged in the densely welded region is more than the number of the rows in the non-densely welded region; the non-welded part in the non-densely welded region has a length of 0.05 to 2.0 mm in the vertical direction; and the non-welded part in the densely welded region has a length of 0.05 to 3.0 mm in the vertical direction.