The patient was an 82-year-old woman. Three years ago, she was aware of a tumor in her left breast that was gradually increasing in size and bleeding, prompting her to visit our hospital. Ultrasonography showed an irregular and low- echoic mass with a skin ulcer of about 2×2 cm in size in the left breast A area. Skin biopsy examination indicated adenocarcinoma, estrogen receptor-positive, progesterone receptor-positive, and human epidermal growth factor receptor type 2-negative. An overall examination did not show a distant metastasis. We diagnosed left breast cancer (luminal A type) with T4bN0M0, stage IIIB. We administered tamoxifen at a dose of 20 mg/day. After 8 months of tamoxifen treatment, no signs of an apparent mass were observed by palpation and ultrasonography. The effect of tamoxifen was considered a clinical complete response (CR). The patient did not experience any adverse events during treatment. Although CR was maintained for nearly 6 years by continuation of hormone therapy, tumor growth was observed after 6 years and 9 months. We performed partial resection of the left breast under local anesthesia. At present, 3 months after the operation, our patient is alive with no recurrence. We conclude that hormone (tamoxifen) therapy is a useful treatment for elderly postmenopausal woman with estrogen receptor-positive advanced breast cancer.