A disposable diaper with a front, a back, lateral sides and a liquid-permeable topsheet has been patented. The liquid-impermeable backsheet and an absorbent core is sandwiched between the sheets. Flexible flaps located on the outside of the absorbent core prevent excrement leakage. The flaps extend from a front longitudinal end to a rear longitudinal end of a body of the diaper. Each flexible flap is comprised of a support portion that extends upward with respect to a surface of the diaper, an inner branched portion and an entire length extending inwardly from an upper part of the support portion parallel to the backsheet. An outer branched portion, the entire length of which extends outwardly from the upper part of the support portion is parallel to the backsheet. At least one of the inner and outer branched portions is directly attached at opposite longitudinal ends from the front and rear longitudinal ends of the body of the diaper. At least one elastic member is incorporated into both the inner-branched portion and the outer-branched portion to form elastic gathers in these two portions and a pocket formed by an inner surface of the support portion and the inner branched portion serves to impede the leakage of excretions from the diaper where the flexible flap is formed with a T-shape in cross-section as defined by the support portion and the inner and outer branched portions.