To provide a disposable diaper for pets that fits well around the waist of the pet. A disposable diaper for pets, comprising: a back-side waist area (130); a stomach-side waist area (110); a crotch area (120); both end sections (130A, 110A) in the diaper longitudinal direction; both end sections (120A) in a diaper longitude-intersecting direction; a tail insertion opening (190); an absorbent core (200); an area (170) not having the absorbent core arranged therein; an attachment section (300) provided in the stomach-side waist area (110); an attachment area (900) that accepts the attachment section (300); an expandable/contractible elastic member (400) for legs, arranged, in the extended state, in a prescribed area in the diaper longitudinal direction, between the absorbent core (200) and an end section (100A) in the diaper longitude-intersecting direction; a leg gather (410A) formed by the contraction of the expandable/contractible elastic member (400) for legs; and a standing-up section (700) in which the end sections (130A, 110A) in the diaper longitudinal direction stand up as a result of the contraction of the expandable/contractible elastic member (400) for legs.