A 38-year-old premenopausal woman with a right axillary mass was told that she had an atheroma. The mass enlarged. An infectious atheroma was suspected, and incision and drainage were performed. A mass 4 cm in diameter was palpated in the right axillary region. Ultrasonography showed a mass 6 cm in diameter, and magnetic resonance imaging revealed a hypervascular mass at the same site. Stage IIIB triple-negative of accessory breast cancer (T4bN1M0) was diagnosed. The patient received four courses of FEC 100 (5-fluorouracil, epirubicin, and cyclophosphamide) as preoperative chemotherapy, but progressive disease was diagnosed, and the regimen was switched to weekly paclitaxel. The tumor became ulcerated and friable, and the hemoglobin level fell to 6 g/dL. Emergency surgery was thus performed. Postoperatively, the patient received six courses of chemotherapy with paclitaxel plus gemcitabine. Accessory breast cancer is extremely rare, but should be borne in mind when an axillary mass is encountered. In our patient, progressive disease had developed during the preoperative chemotherapy for accessory breast cancer. Although treatment was switched to a different regimen, bleeding was detected, and the patient underwent a semi-emergency surgery.