To prevent deterioration of appearance due to a conspicuous corner of the leg-opening side of the side edge of a buttocks cover portion in a two-part outer-shell-type underpants-type disposable diaper. The abovementioned problem is solved by an underpants-type disposable diaper in which a front outer shell 12F and a rear outer shell 12B are provided separately and the front outer shell 12F and rear outer shell 12B are separated in the longitudinal direction LD at the center of the longitudinal direction LD, wherein the rear outer shell 12B has a buttocks cover portion C extending more toward the longitudinal-direction center than does a side seal portion 12A, and the longitudinal-direction dimension of the side edge of the buttocks cover portion C is 0.9-1.1 times the width-direction dimension W3 from the side edge of the rear outer shell 12B to the width-direction-center-side side edge of the side seal portion 12A.本發明所要解決的問題在於提供一種短褲型拋棄式尿布，其能夠在外裝二分割型的短褲型拋棄式尿布中，防止因臀部罩部的側緣的腿開口側的角比較顯眼而導致外觀惡化。 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，其具備彼此分別之前側外裝體（12F）和後側外裝體（12B），所述前側外裝體（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，其中，後側外裝體（12B）具有從側封部（12A）進一步向前後方向中央側伸出的臀部罩部（C），臀部罩部（C）的側緣的前後方向尺寸為從後側外裝體（12B）的側緣至側封部（12A）的寬度方向中央側的側緣為止的寬度方向尺寸（W3）的0.9～1.1倍。
