[Problem] To maintain, in a wide state, the spaces in a ventilation channel formed when a waist expansion member contracts. [Solution] This disposable diaper has formed therein, on one or both of a front part (1) and a rear part (2), a cushion part (40). The cushion part (40) includes: an inner sheet (41) an outer sheet (42) one or a plurality of waist expansion members (43) and a cushion forming sheet (44). In a state where the waist expansion member (43) is stretched in the width direction, the member is secured between the inner sheet (41) and the outer sheet (42). The cushion forming sheet (44) overlaps one or both of the inner sheet (41) and the outer sheet (42), and is joined thereto so that in the width direction joining portions (51) and non-joining portions (52) are alternatingly continuous. The joining portions (51) are formed by joining-lines (53) which are straight and extend in the longitudinal direction and at least two columns of which are side by side. The space (second space (D2)) between the joining-line columns in one joining portion is narrower than the space (first space (D1)) between adjacent joining portions.Le but de linvention est dassurer, dans une large mesure, le maintien des espaces formés dans un canal daération lorsquun élément élargisseur de taille se resserre. A cet effet, la couche-culotte jetable selon linvention comporte, sur une partie avant (1) et/ou une partie arrière (2), une partie coussinet (40). La partie coussinet (40) comprend : une feuille intérieure (41) une feuille extérieure (42) un ou plusieurs éléments élargisseurs de taille (43) et une feuille formant coussinet (44). Lorsque lélément élargisseur de taille (43) est étiré dans le sens de la largeur, ledit élément est maintenu entre la feuille intérieure (41) et la feuille extérieure (42). La feuille formant coussinet (44) recouvre la feuille intérieure (41) et/ou la feuille extérieure (42), et est reliée à celles-ci, des parties de jonction (51) et des parties nassur
