To provide a disposable diaper for pets that fits well around the waist of the pet. A disposable diaper for pets, having: a rear- side waist area (130); a stomach-side waist area (110); a crotch area (120); both end sections (130Am, 110A) in the diaper longitudinal direction; both end sections (100A) 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 receives the attachment section (300); and an expandable/contractible elastic member (600) for a leakage-prevention sheet arranged, in an extended state, in the leakage-prevention sheet (800). The expandable/contractible elastic member (600) for the leakage-prevention sheet is arranged in a prescribed area in the diaper longitudinal direction, between the absorbent core (200) and the end section (100A) in the diaper longitude-intersecting direction. A standing-up section (700) in which the end section in the diaper longitudinal direction stands up is formed as a result of the contraction of the expandable/contractible elastic member (600) for the leakage-prevention sheet.