A disposable diaper for pets comprising: a back-side waist area, an abdomen-side waist area, and a crotch area between the back-side waist area and the abdomen-side waist area, a diaper longitudinal direction in which the back-side waist area, the crotch area and the abdomen-side waist area contiguously extend when the disposable diaper is not worn by a pet, and a diaper transverse direction crossing the diaper longitudinal direction, a back-side waist area end in the diaper longitudinal direction, an abdomen-side waist area end in the diaper longitudinal direction, and both ends in the diaper transverse direction, a back-side flap formed in the back-side waist area, an abdomen-side flap formed in the abdomen-side waist area, a tail insertion opening formed in a prescribed region in the diaper longitudinal direction, an absorbent core formed on one side of the tail insertion opening in the diaper longitudinal direction and disposed in a prescribed region extending over the crotch area and the abdomen-side waist area, a fastening part having a prescribed length and provided in the abdomen-side flap, and a fastening region provided in the back-side waist area and configured to receive the fastening part, wherein: when the disposable diaper is put on the pet, the crotch area and the abdomen-side waist area cover a crotch and an abdomen of the pet, while the back-side waist area is closely fitted to a back of the pet, and the fastening part is fastened to the fastening region, and while the disposable diaper is worn by the pet, the weight of the absorbent core after excretion is received in the longitudinal direction of the fastening part, and an identification region is configured such that an abnormality of the back-side flap can be identified based on visual information of the identification region from outside the disposable diaper, characterised in that the identification region comprises a back-side flap identification region formed in the back-side flap and an abdomen-side flap identification region formed in the abdomen-side flap. a back-side waist area, an abdomen-side waist area, and a crotch area between the back-side waist area and the abdomen-side waist area, a diaper longitudinal direction in which the back-side waist area, the crotch area and the abdomen-side waist area contiguously extend when the disposable diaper is not worn by a pet, and a diaper transverse direction crossing the diaper longitudinal direction, a back-side waist area end in the diaper longitudinal direction, an abdomen-side waist area end in the diaper longitudinal direction, and both ends in the diaper transverse direction, a back-side flap formed in the back-side waist area, an abdomen-side flap formed in the abdomen-side waist area, a tail insertion opening formed in a prescribed region in the diaper longitudinal direction, an absorbent core formed on one side of the tail insertion opening in the diaper longitudinal direction and disposed in a prescribed region extending over the crotch area and the abdomen-side waist area, a fastening part having a prescribed length and provided in the abdomen-side flap, and a fastening region provided in the back-side waist area and configured to receive the fastening part, wherein: when the disposable diaper is put on the pet, the crotch area and the abdomen-side waist area cover a crotch and an abdomen of the pet, while the back-side waist area is closely fitted to a back of the pet, and the fastening part is fastened to the fastening region, and when the disposable diaper is put on the pet, the crotch area and the abdomen-side waist area cover a crotch and an abdomen of the pet, while the back-side waist area is closely fitted to a back of the pet, and the fastening part is fastened to the fastening region, and