A-39-year-old man presented to a nearby clinic owing to long-term diarrhea and painful defecation. He was diagnosed with a well-differentiated tubular adenocarcinoma of the rectosigmoid and underwent a laparotomy at another hospital. At surgery, the tumor was seen to deeply invade into the urinary bladder with the presence of intra-abdominal abscess. Loop sigmoid colostomy was performed due to possible invasion into the pelvic wall and peritoneal dissemination. The patient was referred to the our hospital for systemic chemotherapy. After 32 courses of FOLFIRI, 10 courses of CapeOX + Bmab, and 34 courses of LV5FU2 + Bmab, radiographic examination revealed complete response (CR) of the rectal tumor, and low anterior resection of the rectum was carried out. Since pathological examination showed no viable cancer cells in any specimen, the patient was considered to have achieved a CR from a pathological standpoint.