An 80-year-old man was admitted to our hospital with the chief complaint of urinary retention. On physical examination, a large tumor was recognized in pelvic cavity. Serum levels of prostatic acid phosphatase and gamma-seminoprotein were elevated. Anterior pelvic exenteration was performed because the tumor was huge and malignancy could not be ruled out preoperatively. The tumor weight was 660 g (11 x 8 x 7 cm) and had a multicystic structure macroscopically. Immunohistochemical study by the prostatic specific antigen stain showed positive staining in the epithelial cells of the tumor, which suggest that the tumor was of prostatic origin.