A 69-year-old woman was referred on suspicion of multiple bone metastases. She had undergone modified radical mastectomy for right breast cancer at age 40 years. Positron emission tomography revealed multiple bone metastases, and serum levels of CEA, CA15-3, and NCCST439 were elevated. The diagnosis was metastatic breast cancer. Pathological re-examination confirmed that the tumor cells were positive for estrogen receptor and progesterone receptor, and negative for HER2. Endocrine therapy with letrozole was initiated, followed by toremifene citrate as second-line therapy and exemestane as third-line therapy. Seventeen months later, she developed trouble walking because of lower-extremity edema, which was caused by a synovial cyst of the left hip joint. Although no other metastases were detected, S-1 was introduced (2 weeks followed by 1 week of rest) due to severe decline in quality of life. Six months after initiation of S-1, the synovial cyst disappeared and lower-extremity edema was improved. At 11 months after initiation of S-1, serum CA15-3 level had fallen to approximately within normal limits. As of now, although the tumor marker level has been rising gradually, no other metastases except for bone metastases have been detected and treatment is continuing.