Radiation therapy is an important modality for treatment of breast cancer, but its use has occasionally resultedin the development of secondary malignancies. A 57-year-old woman was referred to our hospital because of a left axillarytumor. She underwent partial mastectomy and radiation therapy 10 years previously. The tumor was originated from thesoft tissue of the left axilla, and wide resection was carried out. On histological examination, myxofibrosarcoma was diagnosed.Myxofibrosarcoma is a highly rare complication of radiation therapy for breast cancer and patients have a poor prognosis. In thiscase, however, early detection and cureable resection might help patients achieve a better prognosis. The current studyconfirmed the rarity of radiation-induced sarcoma, though it showed the risk increase with time. In the course of observation after radiation therapy, it is necessary to take the secondary sarcoma into consideration and recordthe observations accordingly.