A disposable diaper for pets, comprising: an abdomen-side waist area, a back-side waist area, and a crotch area between the abdomen-side waist area and the back-side waist area, a diaper longitudinal direction in which the abdomen-side waist area, the crotch area and the back-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, two ends (110A, 130A) in the diaper longitudinal direction, and two ends in the diaper transverse direction, 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, an absorbent-core non-arrangement region in which the absorbent core is not disposed, a flap formed in the abdomen-side waist area and having an end on the back side, a fastening part having a fixed part disposed on the flap and a free part protruding from the flap, an abdomen-side waist stretchable elastic member disposed in a stretched state in the abdomen-side waist area, and a fastening region provided in the back-side waist area and configured to receive the fastening part, wherein: the fixed part of the fastening part is overlapped with the abdomen-side waist stretchable elastic member, 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 the weight of the absorbent core after excretion is received in the longitudinal direction of the fastening part while the disposable diaper is worn by the pet, characterised in that the abdomen-side waist stretchable elastic member is disposed on an inside surface (100Z1) side of the disposable diaper beyond the fixed part of the fastening part. an abdomen-side waist area, a back-side waist area, and a crotch area between the abdomen-side waist area and the back-side waist area, a diaper longitudinal direction in which the abdomen-side waist area, the crotch area and the back-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, two ends (110A, 130A) in the diaper longitudinal direction, and two ends in the diaper transverse direction, 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, an absorbent-core non-arrangement region in which the absorbent core is not disposed, a flap formed in the abdomen-side waist area and having an end on the back side, a fastening part having a fixed part disposed on the flap and a free part protruding from the flap, an abdomen-side waist stretchable elastic member disposed in a stretched state in the abdomen-side waist area, and a fastening region provided in the back-side waist area and configured to receive the fastening part, wherein: the fixed part of the fastening part is overlapped with the abdomen-side waist stretchable elastic member, 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 the weight of the absorbent core after excretion is received in the longitudinal direction of the fastening part while the disposable diaper is worn by the pet, characterised in that the abdomen-side waist stretchable elastic member is disposed on an inside surface (100Z1) side of the disposable diaper beyond the fixed part of the fastening part. the fixed part of the fastening part is overlapped with the abdomen-side waist stretchable elastic member, 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 the weight of the absorbent core after excretion is received in the longitudinal direction of the fastening part while the disposable diaper is worn by the pet, characterised in that the abdomen-side waist stretchable elastic member is disposed on an inside surface (100Z1) side of the disposable diaper beyond the fixed part of the fastening part.