We report the case of an elderly, advanced breast cancer patient with multiple bone metastases. Breast reduction surgery was useful for this patient. The patient was an 81-year-old woman who had a breast lump. A core needle biopsy for breast cancer led to a diagnosis of invasive ductal carcinoma. The mucinous carcinoma was estrogen receptor (ER) nd progesterone receptor (PgR) positive and HER2/neu negative. Due to patient complications, it was not possible to treat with chemotherapy. The patient was administrated aromatase inhibitors (AI) and zoledronic acid hydrate. However, the AI treatment was not effective, and so she was administered toremifene. Toremifene treatment was effective for 6 months, after which she received fulvestrant. Fulvestrant treatment maintained stable disease (SD)for 14 months. After 14 months of fulvestrant treatment, serum concentrations of the tumor markers CA15-3, CEA, and BCA225 increased. We therefore decided to perform surgical breast reduction surgery. The pathological diagnosis from the surgically resected specimen was mucinous carcinoma, positive for ER and HER2, and negative for PgR. After surgery, serum concentrations of the tumor markers decreased. Following surgery, the patient was administrated lapatinib plus denosumab plus fulvestrant. The patient remains well, without bone metastases, 2 years and 6 months after surgery.