An underpants-type disposable diaper having a vertical direction and a horizontal direction that intersect each other, comprising: an absorbent main body having a lengthwise direction and being folded in two at a predetermined location in the lengthwise direction as well as having the lengthwise direction arranged along the vertical direction; an abdomen side waist part overlapping and being joined to a non-skin side face of an end part on one side in the lengthwise direction of the absorbent main body; and a back side waist part overlapping and being joined to a non-skin side face of an end part on another side in the lengthwise direction of the absorbent main body; wherein the absorbent main body includes an absorbent core that absorbs liquid and a sheet-like flap part that extends horizontally outward with respect to the absorbent core, the flap part includes on each of the one side and the other side in the lengthwise direction, a Z-shape folded portion that is fixed in a state folded along the horizontal direction in a Z-shape, portions located horizontally outward with respect to the absorbent core in the abdomen side waist part and the back side waist part respectively have the Z-shape folded portions of the flap part fixed, the diaper further comprises a barrier cuff that can stand from a skin side face and is supported by a support that is provided to the skin side face of the flap part, the flap part includes a middle portion that is not fixed in a folded state at a location between the Z-shape folded portion on the one side and the Z-shape folded portion on the other side, and a maximum length (L10fM) from the support to a horizontal tip end part of the middle portion, when the middle portion is unfolded and in a horizontally spread state, is greater than a maximum length (LLSG) from the support to a horizontal tip end part (LSGe) of the barrier cuff. an absorbent main body having a lengthwise direction and being folded in two at a predetermined location in the lengthwise direction as well as having the lengthwise direction arranged along the vertical direction; an abdomen side waist part overlapping and being joined to a non-skin side face of an end part on one side in the lengthwise direction of the absorbent main body; and a back side waist part overlapping and being joined to a non-skin side face of an end part on another side in the lengthwise direction of the absorbent main body; wherein the absorbent main body includes an absorbent core that absorbs liquid and a sheet-like flap part that extends horizontally outward with respect to the absorbent core, the flap part includes on each of the one side and the other side in the lengthwise direction, a Z-shape folded portion that is fixed in a state folded along the horizontal direction in a Z-shape, portions located horizontally outward with respect to the absorbent core in the abdomen side waist part and the back side waist part respectively have the Z-shape folded portions of the flap part fixed, the diaper further comprises a barrier cuff that can stand from a skin side face and is supported by a support that is provided to the skin side face of the flap part, the flap part includes a middle portion that is not fixed in a folded state at a location between the Z-shape folded portion on the one side and the Z-shape folded portion on the other side, and a maximum length (L10fM) from the support to a horizontal tip end part of the middle portion, when the middle portion is unfolded and in a horizontally spread state, is greater than a maximum length (LLSG) from the support to a horizontal tip end part (LSGe) of the barrier cuff.