A pull-on absorbent article having a waist opening and a pair of leg openings, comprising: a rectangular front panel adapted to be worn around a wearer's front; a rectangular rear panel adapted to be worn around a wearer's rear; and an absorbent assembly fixed to the front panel and the rear panel so as to bridge them, wherein each of the front panel and the rear panel includes a body portion which includes side seals located at both lateral sides of the panels; and an extension portion which is extended from the body portion toward a crotch portion and which has no side seals at both lateral sides of the panels, the absorbent assembly includes an absorbent member oblong in longitudinal direction and side sheet portions located outward of both side edges in the longitudinal direction of the absorbent member, the side sheet portion includes a stretchable region having stretchability due to an elastic member fixed to the side sheet portion in a stretched state, the stretchable region being located at the crotch portion and on the front panel and the rear panel, the stretchable region is formed in such a manner that a length from a center line in the longitudinal direction of the article to an end part of the stretchable region located on the side of the front panel is larger than a length from the center line in the longitudinal direction of the article to an end part of the stretchable region located on the side of the rear panel, and an entire range of a portion of the stretchable region located on the front panel is not joined onto the front panel and at least a part of the stretchable region located on the rear panel is joined onto the rear panel. a rectangular front panel adapted to be worn around a wearer's front; a rectangular rear panel adapted to be worn around a wearer's rear; and an absorbent assembly fixed to the front panel and the rear panel so as to bridge them, wherein each of the front panel and the rear panel includes a body portion which includes side seals located at both lateral sides of the panels; and an extension portion which is extended from the body portion toward a crotch portion and which has no side seals at both lateral sides of the panels, a body portion which includes side seals located at both lateral sides of the panels; and an extension portion which is extended from the body portion toward a crotch portion and which has no side seals at both lateral sides of the panels, the absorbent assembly includes an absorbent member oblong in longitudinal direction and side sheet portions located outward of both side edges in the longitudinal direction of the absorbent member, the side sheet portion includes a stretchable region having stretchability due to an elastic member fixed to the side sheet portion in a stretched state, the stretchable region being located at the crotch portion and on the front panel and the rear panel, the stretchable region is formed in such a manner that a length from a center line in the longitudinal direction of the article to an end part of the stretchable region located on the side of the front panel is larger than a length from the center line in the longitudinal direction of the article to an end part of the stretchable region located on the side of the rear panel, and an entire range of a portion of the stretchable region located on the front panel is not joined onto the front panel and at least a part of the stretchable region located on the rear panel is joined onto the rear panel.