A 72-year-old woman was referred to our hospital because of a large right breast tumor which had grown to 10 cm in diameter. Gross findings showed no dermal or papillary changes. The elastic hard tumor was minimally movable. Mammography revealed a well-circumscribed mass with extensive calcifications. The lesion showed peripheral hypointensity on T1-weighted MRI with central hyperintense areas, suggestive of hemorrhage and was suspected to have expanded to the intercostal muscle. An inadequate specimen was obtained on fine needle aspiration. Since malignancy could not be ruled out, tumor resection was performed. The tumor, expanding into the subcutaneous fat tissue, at 12 cm in diameter had arisen from a small part of the right fifth rib. The operation consisted of local resection of the tumor and reconstruction of the chest wall defect. Pathological analysis revealed grade I chondrosarcoma. We report this case because of its unusual appearance and our difficulties in differentiating it from a breast tumor.