A 63-year-old man who had been aware for 5 years of a tumor near his left nipple was admitted complaining of rapid tumor enlargement. He received standard mastectomy under the diagnosis of breast sarcoma. The regional lymph nodes were not involved and the excised tumor was 12.0 cm in diameter it showed a grayish-white cut surface and a sharp border. Histologically we made a diagnosis of stromal sarcoma of the breast showing diffuse proliferation of spindle shaped cells with high cellularity, moderate atypia and scattered mitosis. Postoperatively, he underwent chemotherapy with cyclophosphamide. Two years and five months after the operation, he is alive and well without any sign of recurrence.