The present invention relates to a device for separating a bag defecated from a defecation device and replacing it with a new defecation bag to allow the dog to receive defecation while the dog is active in the house or on a walk outside the house, and the entrance is a pet dog. A bowel movement (10) that is in contact with the anus of the body and has a plurality of fastening members 23 fixedly coupled to the upper outer circumferential surface； A cap 12 screwed to the lower circumference of the bowel movement 10 to seal the lower portion of the bowel movement 10； A disposable defecation bag 15 fixedly inserted into the defecation container 10； The elastic waistband 30 is fastened to the waist of the pet dog, and one end is integrally coupled to the waistband 30, and a coupling member 33 is coupled to the other end to be coupled to the fastening member 23. In the defecation device for a pet dog, comprising a plurality of elastic fixing bands 32, a coupling groove 18 is formed along the upper outer circumferential surface of the defecation container 10, and an upper part of the defecation container 10 A plurality of fastening grooves 19 bent to a predetermined length vertically and horizontally are formed at regular intervals on the inner circumferential surface, and the disc-shaped sealing member 11 is coupled to the lower end of the bowel container 10 by interference fitting, , The coupling hole 41 is inserted into the upper side of the bowel movement 10, the coupling protrusions 42 are formed along the inner circumferential surface on one side of the lower side of the coupling hole 41 to the coupling groove 18 A fastening member 40 that is coupled and is fastened by being inserted into the fastening groove 19 by a fastening projection 43 formed at a predetermined interval on the other lower side inside the joining hole 41, and having a flange 44 protruding outward from the lower side 40 ), the upper portion of the defecation bag 15 is inserted into and fixed to t
