The present invention is to provide an absorbent article for pet animal improved so as to restrict a possibility that the liquid absorbent core might be indirectly caught by the rear legs when it is tried to pass the liquid absorbent core between the both rear legs of the pet animal, thereby facilitating the article to be put on the pet animal's body. A diaper (10) as an example of the absorbent article having a longitudinal direction (Y), a transverse direction (X) being orthogonal thereto, a body-contact surface facing pet animal's body and a non-body-contact surface lying on the side opposite thereto and including an absorbent panel (19) having a ventral region (11), a dorsal region (12) and an intermediate region (13) defined between the ventral region (11) and the dorsal region (12) as viewed in the longitudinal direction (Y) wherein the absorbent panel (19) includes a liquid-permeable topsheet (interior sheet) (15), a liquid-impermeable backsheet (exterior sheet) (16) and a liquid absorbent core (34) lying between these interior and exterior sheets. The absorbent core is arranged in a limited area extending from a zone of the intermediate region (13) defined adjacent to a slit (22) for formation of an opening into which the pet animal's tail is inserted into the ventral region (11) and has a first end (38) lying in the ventral region (11) and a second end (40) lying in the intermediate region (13) opposed to each other in the longitudinal direction. A dimension in the transverse direction (X) of the ventral region (11) of the absorbent core (34) gradually narrows from the side of the second end (40) toward the first end (38).