There is provided a half-open underpants-shaped diaper (1) including: a front waist portion (30)； a back waist portion (20)； and an absorbent main body (10). A one-side end portion of the back waist portion (20) on a one side in the lateral direction is joined by a first joining portion (1b) to a one-side end portion of the front waist portion (30) on the one side in the lateral direction. The back waist portion (20) includes an elastic region (X) and a fastening portion (41) on an other side in the lateral direction, the elastic region (X) stretching/contracting in the lateral direction, the fastening portion (41) being capable of being fastened to the front waist portion (30) when putting on the diaper (1). While the fastening portion (41) being fastened to the front waist portion (30) so that an end of the fastening portion (41) on a side closer to the elastic region (X) is positioned in the lateral direction on an end of the absorbent main body (10) opposite to the first joining portion (1b), the elastic region (X) being located on a non-skin side of an other-side end portion (300) of the front waist portion (30) on the other side in the lateral direction, and having a stacking part in which the elastic region is stacked on the other-side end portion (300) of the front waist portion (30) .
