A 73-year-old male was admitted for unhealed wound. Eight months ago, the patient had been operated for excision of soft tissue mass on left distal thigh area in previous hospital and after 4 months from first operation, had been reoperated because of recurrence. The pathologic diagnosis of previous operation was simple cyst. In operating finding, the mass invaded the vastus lateralis fascia and had irregular margin and adhesion. We carried out simple excision with retaining 5 cm of free margin from the mass. The pathologic diagnosis of our hospital was malignant fibrous histiocytoma, and then the patient was performed radiation therapy. In 1 year follow-up, there was no significant finding either increasing mass size or metastasis. We misdiagnosed as simple cyst and then performed simple excision, however finally pathologic diagnosis confirmed as malignant fibrous histiocytoma. It is considered to operate a mass that preoperative proper evaluation and diagnosis are required.