We report a rare case of pneumothorax caused by metastatic carcinoma of the breast, in a 69-year-old woman who was admitted to our hospital with severe chest pain. Four years previously, she had undergone modified radical mastectomy for a left breast tumor. Chest X-ray examination and computed tomography (CT) scan on current admission revealed right pneumothorax and bilateral pulmonary tumors. Although operation is not usually indicated in such circumstances, the patient had persistent air leakage for 7 days, despite receiving effective closed cather drainage, making right thoracotomy necessary. During the operation, an open bronchopleural fistula in the metastatic tumor of the upper lobe, infiltrating close to the visceral pleura, was observed. Wedge resection, including the necrotic tumor, was thus performed. Microscopic examination of the resected specimen showed poorly differentiated adenocarcinoma, consistent with metastasis from breast carcinoma. This is the second reported case of pneumothorax caused by metastatic carcinoma of the breast.