A 12-year-old spayed dog of mixed-breed with a 6-month history of progressive dyschezia was surgically treated successfully. Abdominal palpation, radiography, ultrasonography, and rectal endoscopy revealed the presence of a mass expending from the posterodorsal part of the abdomen to the pelvic cavity, which pressed against the rectum toward the abdominal wall. We tried removing the mass through an abdominal median incision, but found the removal difficult since the mass was attached to the posterior part of the pelvic cavity. We changed our approach, and reached the site from the dorsum of the anus. This time, the mass could be detached with the fingers, and we removed it through the abdominal surgical wound. Pathologically, the mass was dianosed as a leiomyoma. At the present time, 12 months after surgery, there is no evidence of recurrence nor any problem with defecation.