Breast cancer in males is comparatively rare. A 41-year-old man visited our hospital with a complaint of left breast tumor. Initial examination showed remarkably diffuse metastatic lesions in the lung field and small metastatic lesions in the surrounding skin. Modified radical mastectomy, including the surrounding metastatic skin lesions, and incisional biopsy of the lung were performed. Estrogen receptor was positive in both the primary breast cancer and metastatic cancer lesions in the lung. Postoperative medication of tamoxifen citrate, an estrogen receptor blocking agent, in a dose of 30 mg/day, was given together with fluorouracil, BCG, cyclophosphamide, and methotrexate. No evident change could be seen in the lung field six months after the operation. The metastatic lesions in the lung disappeared two years after the operation when the dose of tamoxifen citrate was increased to 60 mg/day. At this writing, thirty months after the operation, the patient is in good health.