The patient was a 75-year-old man who was admitted because of diarrhea and anemia. Endoscopic examination revealed advanced sigmoid colon cancer. Serum CEA levels were markedly elevated. In July 2007, surgery was performed, but the sigmoid colon cancer was unresectable. After surgery, the patient was treated with chemotherapy and concurrent radiotherapy. The chemotherapy consisted of oral UFT (420 mg/body/day)and Leucovorin (75 mg/body/day) administered for 6 weeks. Radiotherapy at 2 Gy/day was administered 30 times (total dose 60 Gy). The tumor decreased slightly in size and serum CEA levels also decreased. The patient refused surgery as an additional therapy. In August 2007, we started combination chemotherapy using oral S-1 (100 mg/body/day, day 1-14) and intravenous CPT-11 (140 mg/body/day, day 1 and 15) as one course for 4 weeks. After 4 courses, serum CEA levels were normal, the sigmoid colon cancer was not found by endoscopy and a biopsy specimen revealed no malignant cells. Moreover, after 8 courses, the tumor disappeared, as confirmed by computed tomography (CT) and positron emission tomography-CT, representing a complete response. Chemoradiotherapy using UFT and Leucovorin, and chemotherapy consisting of S-1 and CPT-11 as an additional therapy may be effective for treating unresectable advanced sigmoid colon cancer.