A disposable diaper having a front waist region , a rear waist region and a crotch region located between the front and rear waist regions and includes a vertically long absorbent chassis extending toward the front and rear waist regions centering on the crotch region, wherein: the absorbent chassis includes an absorbent structure and a pair of side flaps extending outward in the lateral direction (X) from both side edges of the absorbent structure; each of the side flaps is formed of layered sheet materials and include a distal edge being spaced apart in the lateral direction from the side edge of the absorbent structure and extending in the vertical direction (Y), a cuff branch line defined between the side edge of the absorbent structure and the respective distal edge so as to extend in the vertical direction (Y), a leg-opening elasticized area extending between the cuff branch line and the distal edge and a leakage-barrier cuff branched from the cuff branch line and extending in a direction intersecting with the leg-opening elasticized area ; the leakage-barrier cuff has a free edge parallelly-spaced in the lateral direction from the cuff branch line so as to extend in the vertical direction; a distance (L1) from the cuff branch line to the distal edge is larger than a distance (L2) from the cuff branch line to the free edge ; the number of sheet material layers included in the leg-opening elasticized area is larger than the number of the sheet material layers included in the leakage-barrier cuff ; the leakage-barrier cuff is formed separately from the leg-opening elasticized area ; each of the leakage-barrier cuffs includes two layers of the sheet material and at least one cuff-elastic member joined under tension in the vertical direction between two layers of the sheet material; and at least one of the two layers of the sheet material extends into the leg-opening elasticized area so as to be included therein. the absorbent chassis includes an absorbent structure and a pair of side flaps extending outward in the lateral direction (X) from both side edges of the absorbent structure; each of the side flaps is formed of layered sheet materials and include a distal edge being spaced apart in the lateral direction from the side edge of the absorbent structure and extending in the vertical direction (Y), a cuff branch line defined between the side edge of the absorbent structure and the respective distal edge so as to extend in the vertical direction (Y), a leg-opening elasticized area extending between the cuff branch line and the distal edge and a leakage-barrier cuff branched from the cuff branch line and extending in a direction intersecting with the leg-opening elasticized area ; the leakage-barrier cuff has a free edge parallelly-spaced in the lateral direction from the cuff branch line so as to extend in the vertical direction; a distance (L1) from the cuff branch line to the distal edge is larger than a distance (L2) from the cuff branch line to the free edge ; the number of sheet material layers included in the leg-opening elasticized area is larger than the number of the sheet material layers included in the leakage-barrier cuff ; the leakage-barrier cuff is formed separately from the leg-opening elasticized area ; each of the leakage-barrier cuffs includes two layers of the sheet material and at least one cuff-elastic member joined under tension in the vertical direction between two layers of the sheet material; and at least one of the two layers of the sheet material extends into the leg-opening elasticized area so as to be included therein. <EOS>