The case was a woman aged 60 with liposarcoma in the soft tissue of posterior proximal portion of the left thigh. At first the tumor was clinically diagnosed a sort of benign soft tissue tumor. Excision of tumor was performed, but recurrence was observed at operation site. Post-operative irradiation had no effect with recurrence of the tumor. Therefore, amputation of the left thigh was done. However, it gradually became hard and the patient complained of severe pain. Grossly, it was lobulated and dark brown. Pathologically, it was undifferentiated myxoid type according to Stout. Liposarcoma of this type is difficult to distinguish microscopically, therefore, it is often dianosed incorrectly fibrosarcoma, rhabdomyosarcoma, etc.