The patient was a 53-year-old woman who underwent colonoscopy for anal pain and melena. We diagnosed her with Stage I (T2N0M0) anal canal squamous cell carcinoma by biopsy specimen and CT scan. We recommended chemo-radiotherapy because she hoped to keep her anus. For this patient, we planned an S-1 administration at a dose of 120 mg/ body/day for consecutive 14 days followed by 7 days of rest period with whole pelvis and bilateral inguinal radiation (total 45 Gy/25 Fr). Then we added a booster radiation (14 Gy/7 Fr) to a local area for 5 days followed by 2 days of rest period. After 2 weeks of chemo-radiotherapy, we could not detect any tumors by colonoscopy. We diagnosed it as a pathological complete response for biopsy specimen.