A 78-year-old obese woman with breast cancer underwent breast-conserving surgery and axillary lymph node dissection. Due to the prior exposure to long-term taxan chemotherapy for her recurrent gastric cancer, the patient did not undergo adjuvant chemotherapy and began to receive radiotherapy to both the conserved breast and supraclavicular region on the 39th day after operation. Two aspiration therapies were done to the enlarging seroma only at the initial phase of the radiotherapy. No further aspiration therapies were done to the seroma during and after radiotherapy for more than 3 months despite the undoubtable seroma formation. High degree of tension due to large seroma formation, extended from the axilla to deep into the breast parenchyma, made the patient request us to heal the long-lasting seroma. Five aspiration therapies and one simultaneous minocycline intrathecal injection therapy did not bring about wound healing. To heal the persistent seroma, capsulectomy was done to the encapsulated lesion 7 months after the operation. Resected capsule was 110 × 45 mm in size and had smooth inner surface. Pathological study showed the seroma capsule mainly consisting of fibrous tissue with some inflammatory changes. Postoperative course was uneventful, and wound healing was promptly obtained after capsulectomy. Breast surgeons and radiation oncologists should note this type of unfavorable radiation-induced adverse event after breast-conserving therapy.