Fifty-six-year old male patient visited our institution for low back pain for 2 months. The clinician of surgical department diagnosed as abscess or lymphocele on the basis of the findings of abdomen CT and lumbar MRI. The patient visited department of orthopaedic surgery after 6 months later from initial symptom. The back mass was grown larger on the CT and MRI. High grade pleomorphic sarcoma (malignant fibrous histiocytoma) was confirmed by aspiration biopsy. We performed massive excision of the mass with adjuvant chemotherapy and radiotherapy. No recurrence has been found during 1-year follow-up. Malignant fibrous histiocytoma can be mistaken for simple cyst or abscess on its early stage and can bring about bad prognosis because it grows rapidly. If the margin of the cystic lesion is irregular or thick, and the clinical findings are is not suitable for infection, malignant tumor should be suspected.