A case of a breast tumor which showed the appearance of invasive ductal carcinoma and chondrosarcoma is reported. The patient was a 36-year-old female. Simple mastectomy and lymph node dissection were performed for the tumor. Local recurrence and metastases to the contralateral breast, lymph nodes, subcutaneous tissue, and bones arose after the operation. She died of the disease 2 years and 3 months after the operation. It is thought that this tumor had formed as a result of metaplasia of the carcinomatous tissue, since a histological transition between the carcinoma and sarcoma was demonstrated.