We report a case of malignant Schwannoma in the retroperitoneal space and discuss the clinical features briefly. A 77-year-old male visited our hospital in August, 1994 with the chief complaint of bloody stool. Digital rectal examination disclosed a large, elastic and soft mass in the prostatic region, presented identically by ultrasonography and computed tomography. Histological examination of the mass by transperineal biopsy revealed benign cellular Schwannoma. The patient underwent radical prostatectomy including the tumor because the tumor and the prostate could not be separated. Histological examination of this specimen also revealed benign Schwannoma with normal prostatic tissue. However, 2 months after the operation an abdominal CT scan revealed multiple metastasis in the liver, which was later diagnosed as malignant Schwannoma histologically. He is still alive with hepatic metastasis 15 months postoperatively.