A stretchable/contractible sheet having an up-down direction and a left-right direction intersecting each other, the stretchable/contractible sheet, comprising: a first sheet; a second sheet; a plurality of welded portions joining the first sheet and the second sheet; a plurality of elastic members being capable of stretching/contracting in the left-right direction, the plurality of elastic members arranged with a space in the up-down direction and between the first sheet and the second sheet that are joined by the plurality of welded portions, and each of the elastic members being sandwiched between two welded portions adjacent in the up-down direction with contracting in the left-right direction, and being attached to the first sheet and the second sheet; a first welded-portion row including a plurality of the welded portions arranged side-by-side in the up-down direction; and a second welded-portion row including a plurality of the welded portions arranged side-by-side in the up-down direction, and located adjacent to the first welded-portion row on a one side in the left-right direction, the first welded-portion row including a first convex portion (61P1) that projects toward the one side in the left-right direction, and a second convex portion (61P2) located adjacent to the first convex portion (61P1) in the up-down direction, the second welded-portion row including a third convex portion (61P3) that projects toward another side in the left-right direction, in a natural state as defined in the description, an other-side end of the third convex portion (61P3) being located on the other side with respect to a straight line connecting a one-side end (61P1A) of the first convex portion (61P1) and a one-side end (61P2B) of the second convex portion (61P2), in a state where the stretchable/contractible sheet is stretched in the left-right direction, the other-side end of the third convex portion (61P3) being located on the one side with respect to the straight line connecting the one-side end (61P1A) of the first convex portion (61P1) and the one-side end (61P2B) of the second convex portion (61P2). a first sheet; a second sheet; a plurality of welded portions joining the first sheet and the second sheet; a plurality of elastic members being capable of stretching/contracting in the left-right direction, the plurality of elastic members arranged with a space in the up-down direction and between the first sheet and the second sheet that are joined by the plurality of welded portions, and each of the elastic members being sandwiched between two welded portions adjacent in the up-down direction with contracting in the left-right direction, and being attached to the first sheet and the second sheet; the plurality of elastic members arranged with a space in the up-down direction and between the first sheet and the second sheet that are joined by the plurality of welded portions, and each of the elastic members being sandwiched between two welded portions adjacent in the up-down direction with contracting in the left-right direction, and being attached to the first sheet and the second sheet; a first welded-portion row including a plurality of the welded portions arranged side-by-side in the up-down direction; and a second welded-portion row including a plurality of the welded portions arranged side-by-side in the up-down direction, and located adjacent to the first welded-portion row on a one side in the left-right direction, the first welded-portion row including a first convex portion (61P1) that projects toward the one side in the left-right direction, and a second convex portion (61P2) located adjacent to the first convex portion (61P1) in the up-down direction, the second welded-portion row including a third convex portion (61P3) that projects toward another side in the left-right direction, the first welded-portion row including a first convex portion (61P1) that projects toward the one side in the left-right direction, and a second convex portion (61P2) located adjacent to the first convex portion (61P1) in the up-down direction, a first convex portion (61P1) that projects toward the one side in the left-right direction, and a second convex portion (61P2) located adjacent to the first convex portion (61P1) in the up-down direction, the second welded-portion row including a third convex portion (61P3) that projects toward another side in the left-right direction, in a natural state as defined in the description, an other-side end of the third convex portion (61P3) being located on the other side with respect to a straight line connecting a one-side end (61P1A) of the first convex portion (61P1) and a one-side end (61P2B) of the second convex portion (61P2), in a state where the stretchable/contractible sheet is stretched in the left-right direction, the other-side end of the third convex portion (61P3) being located on the one side with respect to the straight line connecting the one-side end (61P1A) of the first convex portion (61P1) and the one-side end (61P2B) of the second convex portion (61P2).