PROBLEM TO BE SOLVED: To improve fitness to buttocks.SOLUTION: In an underpants type disposable diaper 1, an intermediate part is provided in a pattern passing a hip obliquely lower area which is an area between an edge 29 of a leg opening in an exterior body 20 of a back body B and a side edge of an interior body 10, and located in the hip obliquely lower area. The intermediate part is a free part 26f which is not fixed to the exterior body 20, and movable according to the swell of the buttocks. In a back side curved elastic member 26, one end side and the other end side of the free part 26f are fixed part 26b to the exterior body 20. The width-direction movable area of the free part 26f of the back side curved elastic member 26 is extended at least from the vicinity of the edge 29 of the leg opening in the exterior body 20 of the back body B to the vicinity of the side of the side edge of the interior body 10.
