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, two ends 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 fastening part having a prescribed length and provided on the abdomen-side waist area, a fastening region provided in the back-side waist area and configured to receive the fastening part, and a stretchable elastic member arranged in the diaper longitudinal direction, 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, 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, and part of the crotch area is erected by contraction of the stretchable elastic member and forms a first erected section, a fixing part for fixing contraction force of the stretchable elastic member onto the absorbent-core non-arrangement region, wherein the stretchable elastic member extends between a first end thereof in the abdomen side waist area and a second end thereof in the crotch area, characterized by the second end of the stretchable elastic member being spaced from the tail insertion opening in the diaper longitudinal direction towards the abdomen side waist area, and wherein the absorbent-core non-arrangement region between the fixing part and the absorbent core is erected by contraction of the stretchable elastic member and forms the first erected section, such that the first erected section is formed between an end of the absorbent core in the crotch area in the diaper longitudinal direction and the tail insertion opening. 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, two ends 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 fastening part having a prescribed length and provided on the abdomen-side waist area, a fastening region provided in the back-side waist area and configured to receive the fastening part, and a stretchable elastic member arranged in the diaper longitudinal direction, 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, 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, and part of the crotch area is erected by contraction of the stretchable elastic member and forms a first erected section, 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, 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, and part of the crotch area is erected by contraction of the stretchable elastic member and forms a first erected section, a fixing part for fixing contraction force of the stretchable elastic member onto the absorbent-core non-arrangement region, wherein the stretchable elastic member extends between a first end thereof in the abdomen side waist area and a second end thereof in the crotch area, characterized by the second end of the stretchable elastic member being spaced from the tail insertion opening in the diaper longitudinal direction towards the abdomen side waist area, and wherein the absorbent-core non-arrangement region between the fixing part and the absorbent core is erected by contraction of the stretchable elastic member and forms the first erected section, such that the first erected section is formed between an end of the absorbent core in the crotch area in the diaper longitudinal direction and the tail insertion opening.