An 86-year-old woman was referred to our hospital after an incidental CT scan of the trunk revealed a mass in the left breast and enlarged axillary lymph nodes. A core needle biopsy(CNB)from a 2 cm mass in the left breast revealed invasive ductal carcinoma, weakly positive result for ER, negative result for PgR, and negative result for HER2. She also had multiple enlarged left supraclavicular lymph nodes and was T2N3cM0, Stage ⅢC on pretreatment evaluation. She was given the S-1 oral drug of choice, starting with 80 mg/day/4-week dosing with a 2-week rest. Eight months after the start of S-1, a partial mastectomy and sentinel lymph node biopsy were performed. Pathological findings showed a pathological complete response(ypTis/ypN0)with only a 2 mm non-invasive carcinoma remnant in the left mammary gland. S-1 is weakly recommended as primary chemotherapy for HER2-negative metastatic recurrent breast cancer, but there are no reports to date of complete response in resection cases. S-1 may be administered to patients with locally advanced breast cancer who cannot tolerate standard drug therapy and may be converted to resection after a successful response.