PROBLEM TO BE SOLVED: To provide an underpants type disposable diaper which satisfies both sufficient strength during the wearing and easiness for peeling.SOLUTION: In a dot pattern of a side seal part 13, rows of welded parts 13s, 13b on both ends in a lateral direction have an interval expansion part 13d, where an interval in a longitudinal direction is expanded between the adjacent welded parts, provided every a plurality of the welded parts. The interval expansion parts in the rows of the welded parts on both ends in the lateral direction are arranged alternately so that a longitudinal range of the interval expansion part in the row of the welded parts on one end in the lateral direction is not superposed on a longitudinal range of the interval expansion part in the row of the welded parts on the other end in the lateral direction, and that a longitudinal range between the adjacent interval expansion parts in the row of the welded parts on one end in the lateral direction is partially superposed on a longitudinal range between the adjacent interval expansion parts in the row of the welded parts on the other end in the lateral direction. In the longitudinal range of the interval expansion parts, the plurality of welded parts are not arranged side by side in a width direction.
