Provided is an underpants-type disposable diaper comprising a plurality of long and thin elastic stretchable members (19) that extend along the width direction and in which stretchable areas (A2, A3) adjacent to the central side of a side seal section (12A) in the width direction in exterior bodies (12F, 12B) are arranged with an interval therebetween in the front-back direction between two sheet layers (12S, 12H). Both ends of the elastic stretchable members in the width direction are configured as fixed ends (19f) that are fixed to the two sheet layers. The two sheet layers are welded and joined at a location between elastic stretchable members adjacent in the front-back direction in order to form joined sheet sections (20) in areas between the fixed ends in the stretchable areas that are not fixed to the two sheet layers between the fixed ends. The end of the side seal section in the stretchable areas does not have a joined sheet section protruding from the side seal section.本發明所欲解決的問題在於防止源自於片接合部所造成的側密封部的橫裂。 為了解決上述問題，本發明提供一種內褲型拋棄式尿布，其在外裝體12F、12B中，伸縮區域A2、A3具有複數根細長狀的彈性伸縮構件19，該伸縮區域A2、A3鄰接在側密封部12A的寬度方向中央側，該彈性伸縮構件19是在前後方向隔開間隔地設置在2片的片層12S、12H之間，並且沿著寬度方向延伸；該彈性伸縮構件19，其寬度方向的兩端部會被固定於2片的片層12S、12H上而成為固定端部19f，並且在該等固定端部19f之間，並未固定於2片的片層12S、12H上；在伸縮區域A2、A3中的固定端部19f之間的區域中，在於前後方向鄰接的彈性伸縮構件19之間的部位，2片的片層12S、12H熔接接合而形成片接合部20；並且，在伸縮區域A2、A3中的側密封部12A側的端部，不具有自側密封部12A突出的片接合部20。
