An 80-year-old woman was admitted to our hospital because of a tumor in her right breast. On physical examination, a 4.0×3.0cm elastic-hard regular mass was palpable, mainly in the upper inner quadrant of the right breast. Mammography revealed a high-density well-circumscribed mass with large calcifications. On ultrasonography, the mass was round in shape with a regular margin and mixed internal echoes. Under the tentative diagnosis of a benign tumor, the patient was closely followed up. However, as the tumor was found to be growing after 4 months, tumor resection was performed. The histological diagnosis was malignant phyllodes tumor with chondro-osseous differentiation and co-existing carcinoma arising from the glandular component. Until now, two years since the surgery, with no additional resection or adjuvant chemotherapy, no clinical recurrence has been observed.