To achieve both an improvement in standing gather fit and the prevention of falling of the standing gathers on both sides of the intergluteal cleft in an exterior two-piece underpants-type disposable diaper. The present invention solves the problem as a result of side part gathers (60), which are erect from both side parts of an interior body (200), comprising: a main body portion (66) having a base-side portion (60B) and a tip end-side portion (60A)； a collapsing portion (67) that is fixed to the surface of the interior body (200) when both end parts in the front-back direction of the main body portion (66) are in a collapsed state； an unfixed free portion (68) that is positioned between front and back collapsing portions (67)； a gather elastic member (63) that is provided in the front-back direction (LD) in at least the tip end part of the free portion (68)； and a back-side half-collapsing portion (68B) that is continuous with or adjacent to the front side of the back-side collapsing portion (67), and is joined to the base-side portion (60B) when the tip end-side portion (60A) is in a collapsed state, the base-side portion (60B) and a top sheet (30) not being joined in said back-side half-collapsing portion.本發明所要解決的問題在於提供一種短褲型拋棄式尿布，其同時實現外裝二分割型的短褲型拋棄式尿布中的立體皺褶的合身性的提高、及對臀溝兩側的立體皺褶的傾倒的防止等。 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，從內裝體（200）的兩側部立起的側部皺褶（60）具備：主體部分（66），其具有根側部分（60B）和末端側部分（60A）；倒伏部分（67），其是該主體部分（66）的前後方向兩端部在倒伏狀態下固定於內裝體的正面上的部分；不固定的自由部分（68），其位於前後的倒伏部分之間；皺褶彈性部件（63），其沿著前後方向設置於該自由部分（68）的至少末端部；及，後側半倒伏部分（68B），其與後側的倒伏部分（67）的前側連續或相鄰，在該後側半倒伏部分（68B）中，末端側部分（60A）在倒伏狀態下接合於根側部分（60B），並且根側部分（60B）和頂片（30）未接合在一起。
