A 70-year-old man was referred to our hospital with bowel obstruction because of rectal cancer. High anterior resection of rectum and lymph node dissection was performed. The rectal cancer was in stage III, and the patient selected no adjuvant chemotherapy. At 1-year follow-up, the CEA level was 17. 6 ng/mL, and CT revealed multiple lung metastases and paraaortic and parailiac lymph node metastases. S-1, 100 mg/body, was administered for 4 weeks followed by 2 drug-free weeks. After 3 courses, the CEA level was 4. 5 ng/mL, and metastatic lesions were remarkably reduced in the CT findings. After 10 courses, the CEA level was hovering around 6 ng/mL, and CT showed no recurrent foci. The effect of S-1 treatment was PR, and no severe side effect was observed throughout the treatment.