A 51-year-old female patient visited our department with a complaint of pain in the left breast. She was found to have Stage Ⅳ breast cancer with liver metastasis. The biopsy-based historical diagnosis was triple negative breast cancer(TNBC). Epirubicin plus cyclophosphamide therapy(EC therapy)plus weekly paclitaxel therapy(weekly PTX)was started for the unresectable advanced breast cancer, but infiltration of an armor-like tumor was observed in the chest wall. It was judged that drug resistance had occurred; hence, the treatment was switched to S-1. Subsequently, almost all the chest wall tumors disappeared after 2 months. However, we did not control the disease, and the patient died. We report about the positioning of S-1 with regard to TNBC, including a literature review.