A 58-year-old Japanese male patient visited our hospital for evaluation of an elastic hard mass, measuring 80 x 50 mm, in the right axillary area. Incisional biopsy for suspected malignancy was performed, and histopathologic examination by hematoxylin-eosin (H&E) staining yielded a diagnosis of poorly differentiated adenocarcinoma metastatic from an unknown primary. As the tumor was immunohistochemically positive for both ER and PgR, metastatic breast cancer was strongly suspected. Ultrasonography, CT, and MRI revealed no evidence of tumors in the bilateral mammary glands. Detailed examination of the head and neck region, lung, and upper and lower gastrointestinal tract also revealed no evidence of a primary tumor. After chemotherapy, the patient underwent tumor resection with axillary lymph node dissection. On the basis of the histological features of H&E-stained specimens and immunohistochemistry of the resected tumor, this case was diagnosed as breast cancer of unknown origin in a male. The tumor could have been an axillary lymph node metastasis from an occult breast carcinoma, or primary cancer arising in an accessory mammary gland.