[Problem] To enhance fit around the legs, and particularly to improve fit in the groin region during sitting or walking, in an exterior two-piece-type underpants-type disposable diaper. [Solution] The above problem is solved by an underpants-type disposable diaper in which a front-side exterior body (12F) and a rear-side exterior body (12B) are separated in the front-rear direction (LD) at an intermediate point in the front-rear direction (LD), and an interior body (200) extends in the front-rear direction (LD) from the front-side exterior body (12F) to the rear-side exterior body (12B) and is joined to each of the front-side exterior body (12F) and the rear-side exterior body (12B), wherein inside/outside joining parts (201, 202) for joining the interior body (200) and the front-side exterior body (12F) are provided so as to reach both width-direction end parts on a waist side, the width of the inside/outside joining parts (201, 202) decreases in stepwise fashion or continuously toward a crotch side of the front-rear exterior body (12F), and both width-direction end parts on the crotch side of the front-side exterior body (12F) have non-joined portions (203) in which the interior body (200) and the front-side exterior body (12F) are not joined.本發明所要解決的問題在於提供一種短褲型拋棄式尿布，其能夠實現外裝二分割型的短褲型拋棄式尿布中的腿部周圍的合身性的提高，特別是改善就坐或步行時的腹股部的合身性等。 上述問題藉由下述方案得到了解決，一種短褲型拋棄式尿布，前側外裝體（12F）和後側外裝體（12B）在前後方向（LD）中間處在前後方向（LD）上分離，內裝體（200）在前後方向（LD）上從前側外裝體（12F）一直延伸到後側外裝體（12B），且分別接合於前側外裝體（12F）和後側外裝體（12B），其中，將內裝體（200）和前側外裝體（12F）接合在一起的內外接合部（201、202），在腰側以遍及寬度方向兩端部之方式設置，且內外接合部（201、202）的寬度隨著朝向前側外裝體（12F）的胯間側的緣而階段性地或連續地減小，在前側外裝體（12F）的胯間側的寬度方向兩端部，具有內裝體（200）和前側外裝體（12F）未接合在一起的非接合部分（203）。
