We report the case of a 60-year-old male patient who was diagnosed with metastasis from primary lung cancer to the breast. The patient presented with a mass in the right breast. Mammography, ultrasound, and magnetic-resonance imaging each suggested primary breast cancer. A core-needle biopsy of the right breast mass indicated poorly differentiated adenocarcinoma. A basic chest X-ray showed a shadow in the left upper lung. Thoraco-abdominal computed tomography revealed a mass with a diameter of 90 mm in the left superior region, the shape of which was indicative of primary lung cancer. A lung biopsy confirmed poorly differentiated adenocarcinoma. We diagnosed primary lung cancer with metastases to the bone, brain and right breast (cT2N3M1, stage IV) by imaging and histopathology. He was administered carboplatin (area under the curve 6 mg / ml) and paclitaxel (200 mg / m(2)) tri-weekly, and underwent gamma-knife treatment for the brain metastasis. The treatments reduced the primary tumor and the metastases. However, after completion of the fifth treatment cycle, he developed disseminated intravascular coagulation from septic shock, and died on the eleventh day after completing the fifth cycle of treatment. Although metastasis to the mammary gland is uncommon, especially among males, metastasis to the mammary gland should be considered when a mammary mass does not exhibit the typical characteristics of breast cancer. A correct diagnosis of metastasis to the mammary gland from lung cancer makes it possible to select the most appropriate treatment method.