A case of retroperitoneal abscess with an unknown origin is presented. A 60-year-old woman visited our hospital complaining of fever and right flank pain. Abdominal computed tomography and ultrasound examination revealed retroperitoneal abscess posterior to right kidney. Percutaneous drainage and administration of antibiotics were performed. Then, both symptoms were improved. The fluid culture proved to be anaerobic bacteria, Peptostreptococcus species and Porphyromonas asaccharolytica. She had no history of surgery or diabetes mellitus. The origin of this abscess was unknown.