This disposable diaper (1) is provided with a front sheet (2) which forms a skin-facing surface, an outer sheet (3) which forms a non-skin-facing surface, and an absorbent body (4) disposed between both sheets. The absorbent body (4) has an absorbent core (40) that is longer in a lengthwise direction (X) corresponding to the front-back direction of a wearer, and the absorbent core (40) contains a water-absorbing polymer. The absorbent core (40) has a large-area region (40a) and a small-area region (40b) which have different surface areas and which are located to the front and the back respectively when an absorbent-body crotch region (40c) is positioned in a narrowed portion. In the absorbent core (40), a slit (7) extending in the lengthwise direction (X) is formed from the absorbent-body crotch region (40c) to the large-area region (40a). The slit (7) intersects the center line (CL) of the outer sheet (3), and at least 80% of the whole length of the slit (7) is located on the large-area region (40a) side of the center line (CL). The width (W2) of the slit (7) in the large-area region (40a) is larger than the width (W1) thereof in the absorbent-body crotch region (40c).La présente invention concerne une couche jetable (1) qui est dotée dune feuille avant (2) qui forme une surface tournée vers la peau, dune feuille extérieure (3) qui forme une surface non tournée vers la peau, et dun corps absorbant (4) disposé entre les deux feuilles. Le corps absorbant (4) a un cœur absorbant (40) qui est plus long dans le sens de la longueur (X) correspondant à la direction avant-arrière dun utilisateur, et le cœur absorbant (40) contient un polymère absorbant leau. Le cœur absorbant l(40) a une région de grande aire (40a) et une région de petite aire (40b) dont les aires sont différentes et qui sétendent dans lavant et larrière, respectivement, lorsquune région dentrejambe de corps absorbant (40) est appliquée à une zone contrainte. Dans le cœur absorbant (40), une fente (7) séte
