A case of extravesical malacoplakia, the first case in Japan, is described in detail. The patient was a 61-year-old woman with a right flank mass. Radiologically, the mass was thought to be of the renal origin. Surgically, however, the tumor was found attached not only to the cortical surface but extended to the retroperitoneum and psoas muscle. Pathological examination confirmed the lesion to be malacoplakia characterized by the presence of von Hansemann cells and Michaelis-Gutmann bodies.