A disposable diaper to be fit around the waist of the pet, having: a rear-side waist area a stomach-side waist area a crotch area end sections in the diaper longitudinal direction end sections in a diaper longitude-intersecting direction a tail insertion opening an absorbent core an area not having the absorbent core arranged therein an attachment section in the stomach-side waist area an attachment area that receives the attachment section and an expandable/contractible elastic member for a leakage-prevention sheet arranged, in an extended state, in the leakage-prevention sheet. The expandable/contractible elastic member is arranged in a prescribed area in the diaper longitudinal direction, between the absorbent core and the end section in the diaper longitude-intersecting direction. A standing-up section 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 for the leakage-prevention sheet.
