A disposable diaper having a longitudinal direction along a front-rear direction of a wearer and a width direction orthogonal to the longitudinal direction, the disposable diaper including a rear portion to be arranged on the wearer's rear side when worn, a front portion to be arranged on the wearer's front side and a crotch portion located between the rear portion and the front portion, the disposable diaper comprising: an absorbent assembly including a topsheet, a backsheet and an absorbent member that is interposed between the topsheet and the backsheet and that includes an absorbent core; and a pair of leak-proof cuffs provided respectively to the absorbent assembly's both lateral sides along the longitudinal direction, wherein: each of the leak-proof cuffs includes a leak-proof-cuff-forming sheet material and elastic members fixed to the sheet material, and includes an inwardly-oriented portion wherein the sheet material is arranged from a lateral side edge of the absorbent core toward the absorbent member's inner side in the width direction, a fold-back portion where the sheet material is folded back toward the absorbent member's outer side in the width direction, the fold-back portion being located above the absorbent member, and an outwardly-folded portion ranging from the fold-back portion to a free end of the leak-proof cuff; the outwardly-folded portion has a length, in the width direction, that is longer than the length of the inwardly-oriented portion; the plurality of elastic members are fixed in a stretched state to the outwardly-folded portion along the longitudinal direction; the free end of the leak-proof cuff is located widthwisely outside of the lateral side edge of the absorbent core; and in the outwardly-folded portion, a region where the plurality of elastic members are provided is divided, in the outwardly-folded portion's width direction, into two equal parts defined respectively as an inner region and an outer region, the inner region has a higher elongation stress at 70% elongation than the outer region. an absorbent assembly including a topsheet, a backsheet and an absorbent member that is interposed between the topsheet and the backsheet and that includes an absorbent core; and a pair of leak-proof cuffs provided respectively to the absorbent assembly's both lateral sides along the longitudinal direction, wherein: each of the leak-proof cuffs includes a leak-proof-cuff-forming sheet material and elastic members fixed to the sheet material, and includes an inwardly-oriented portion wherein the sheet material is arranged from a lateral side edge of the absorbent core toward the absorbent member's inner side in the width direction, a fold-back portion where the sheet material is folded back toward the absorbent member's outer side in the width direction, the fold-back portion being located above the absorbent member, and an outwardly-folded portion ranging from the fold-back portion to a free end of the leak-proof cuff; the outwardly-folded portion has a length, in the width direction, that is longer than the length of the inwardly-oriented portion; the plurality of elastic members are fixed in a stretched state to the outwardly-folded portion along the longitudinal direction; the free end of the leak-proof cuff is located widthwisely outside of the lateral side edge of the absorbent core; and in the outwardly-folded portion, a region where the plurality of elastic members are provided is divided, in the outwardly-folded portion's width direction, into two equal parts defined respectively as an inner region and an outer region, the inner region has a higher elongation stress at 70% elongation than the outer region. each of the leak-proof cuffs includes a leak-proof-cuff-forming sheet material and elastic members fixed to the sheet material, and includes an inwardly-oriented portion wherein the sheet material is arranged from a lateral side edge of the absorbent core toward the absorbent member's inner side in the width direction, a fold-back portion where the sheet material is folded back toward the absorbent member's outer side in the width direction, the fold-back portion being located above the absorbent member, and an outwardly-folded portion ranging from the fold-back portion to a free end of the leak-proof cuff; an inwardly-oriented portion wherein the sheet material is arranged from a lateral side edge of the absorbent core toward the absorbent member's inner side in the width direction, a fold-back portion where the sheet material is folded back toward the absorbent member's outer side in the width direction, the fold-back portion being located above the absorbent member, and an outwardly-folded portion ranging from the fold-back portion to a free end of the leak-proof cuff; the outwardly-folded portion has a length, in the width direction, that is longer than the length of the inwardly-oriented portion; the plurality of elastic members are fixed in a stretched state to the outwardly-folded portion along the longitudinal direction; the free end of the leak-proof cuff is located widthwisely outside of the lateral side edge of the absorbent core; and in the outwardly-folded portion, a region where the plurality of elastic members are provided is divided, in the outwardly-folded portion's width direction, into two equal parts defined respectively as an inner region and an outer region, the inner region has a higher elongation stress at 70% elongation than the outer region.