A disposable diaper having a front, a back and lateral sides and further including a liquid-permeable topsheet (1), a liquid-impermeable backsheet (2), an absorbent core (3) sandwiched between said sheets, and excrement leakage preventing flexible flaps (7) located outwardly with respect to said absorbent core (3) and extending from a front longitudinal end to a rear longitudinal end of a body of the diaper, each flexible flap (7) comprising: (a) a support portion (8) extending upwardly with respect to a surface of the diaper, (b) an inner branched portion (9), an entire length of which extends inwardly from an upper part of said support portion generally parallel to the backsheet (2), (c) an outer branched portion (10), an entire length of which extends outwardly from the upper part of said support portion generally parallel to the backsheet (2), wherein at least one of the inner and outer branched portions is directly attached at opposite longitudinal ends thereof to the front and rear longitudinal ends of the body of the diaper, (d) at least one elastic member (11) incorporated into both said inner branched portion (9) and said outer branched portion (10) so as to form elastic gathers in these two portions, (e) a pocket formed by an inner surface of the support portion (8) and the inner branched portion which serves to impede the leakage of excretions from the diaper; wherein said flexible flap is formed with a T-shape in cross-section as defined by the support portion and the inner and outer branched portions.