A disposable pull-on diaper having a vertical direction and a body-cross direction, and including a front panel defining a front waist region, a back panel defining a back waist region and a crotch panel defining a crotch region and provided with a bodily-fluid absorbent core extending from the front waist region to the crotch region in the vertical direction, the diaper is characterized in that: when the diaper is in the closed configuration, both lateral portions opposed to each other in the body-cross direction of the back panel are joined to respective lateral portions opposed to each other in the body-cross direction of the front panel along a series of seams and the back panel includes an upper end edge, an lower end edge and a buttocks-covering portion extending from a lower end edge of the seams to the lower end edge of the back panel in the vertical direction between lateral edges of the back panel so as to extend downward in the vertical direction beyond the front panel, wherein none of linear elastic members extending in the body-cross direction under tension is present in the buttocks-covering portion; wherein the back panel includes a main portion and the buttocks-covering portion, the main portion being formed of an interior sheet and an exterior sheet overlapping with each other and a plurality of elastic members extending in parallel under tension in the body-cross direction and interposed between these interior and exterior sheets (31, 32), in which main portion both the lateral portions are joined to the lateral portions of the front panel and wherein, of said sheets (31, 32) lying interior and exterior sides of the diaper including the elastic members interposed therebetween, at least one of the interior sheet and the exterior sheet extends downward from the main portion in the vertical direction so as to form the buttocks-covering portion; and as viewed in the vertical direction, the crotch panel has a front end portion joined to the front panel and a back end portion joined to the buttocks-covering portion so that a mass of the absorbent core is supported by the front panel and the buttocks-covering portion, and the crotch panel extends upward in the vertical direction beyond the back end portion joined to the buttocks-covering portion and includes a lower joining portion adapted to be bonded to the buttocks-covering portion and an upper joining portion adapted to be joined to the main portion and wherein a distance between the lower joining portion and the upper joining portion on the crotch panel is longer than a distance between the lower joining portion and the upper joining portion on the back panel so that a sag is formed between the lower joining portion and the upper joining portion of the crotch panel. when the diaper is in the closed configuration, both lateral portions opposed to each other in the body-cross direction of the back panel are joined to respective lateral portions opposed to each other in the body-cross direction of the front panel along a series of seams and the back panel includes an upper end edge, an lower end edge and a buttocks-covering portion extending from a lower end edge of the seams to the lower end edge of the back panel in the vertical direction between lateral edges of the back panel so as to extend downward in the vertical direction beyond the front panel, wherein none of linear elastic members extending in the body-cross direction under tension is present in the buttocks-covering portion; wherein the back panel includes a main portion and the buttocks-covering portion, the main portion being formed of an interior sheet and an exterior sheet overlapping with each other and a plurality of elastic members extending in parallel under tension in the body-cross direction and interposed between these interior and exterior sheets (31, 32), in which main portion both the lateral portions are joined to the lateral portions of the front panel and wherein, of said sheets (31, 32) lying interior and exterior sides of the diaper including the elastic members interposed therebetween, at least one of the interior sheet and the exterior sheet extends downward from the main portion in the vertical direction so as to form the buttocks-covering portion; and as viewed in the vertical direction, the crotch panel has a front end portion joined to the front panel and a back end portion joined to the buttocks-covering portion so that a mass of the absorbent core is supported by the front panel and the buttocks-covering portion, and the crotch panel extends upward in the vertical direction beyond the back end portion joined to the buttocks-covering portion and includes a lower joining portion adapted to be bonded to the buttocks-covering portion and an upper joining portion adapted to be joined to the main portion and wherein a distance between the lower joining portion and the upper joining portion on the crotch panel is longer than a distance between the lower joining portion and the upper joining portion on the back panel so that a sag is formed between the lower joining portion and the upper joining portion of the crotch panel.