PROBLEM TO BE SOLVED: To improve a fitting property by suppressing a positional deviation of an open side in a one-side-opening underpants-type diaper.SOLUTION: A one-side-opening underpants type diaper 1 includes a front waist part 30, a back waist part 20, and an absorbent body 10. One-side end of the back waist part 20 in the lateral direction and one-side end of the front waist part 30 in the lateral direction are joined by a first joining art 1b. The other side of the back waist part 20 in the lateral direction includes an elastic area X stretching in the lateral direction, and a locking part 41 being locked to the front waist part 30 when being worn. In a state where the locking part 41 is locked to the front waist part 30 such that, in the lateral direction, an end 41er of the locking part 41, in the elastic area X side is located at an end 10er of the absorbent body 10 in an opposite side of the first joining part 1b, the elastic area X is located at a non-skin side of an end 300 in the other side of the front waist part 30 in the lateral direction and has a part overlapped with the other-side end 300 of the front waist part 30.SELECTED DRAWING: Figure 9【課題】片開きパンツ型のおむつにおいて、オープン側の位置ずれを抑制してフィット性を良好にすることにある。【解決手段】前胴回り部３０と、後胴回り部２０と、吸収性本体１０と、を備えた片開きパンツ型のおむつ１であって、横方向における後胴回り部２０の一方側の端部と、横方向における前胴回り部３０の一方側の端部とは、第１接合部１ｂによって接合されており、横方向における後胴回り部２０の他方側には、横方向に伸縮する弾性領域Ｘが設けられると共に、着用時に前胴回り部３０に係止可能な係止部４１が設けられており、横方向において、係止部４１の、弾性領域Ｘの側の端４１ｅｒが、吸収性本体１０の、第１接合部１ｂとは反対側の端１０ｅｒに位置するように、前胴回り部３０に係止部４１を係止した状態において、弾性領域Ｘは、横方向における前胴回り部３０の他方側の端部３００の非肌側に位置し、かつ、前胴回り部３０の他方側の端部３００と重なり部分を有する。【選択図】図９
