A peritoneal loose body is reported to develop because of torsion and separation of the epiploic appendages. The condition is usually symptomless and may be incidentally during abdominal surgery or autopsy. It usually occurs in middle-aged and elderly adults and is very rare in children. In this paper, we report a case of a peritoneal loose body in the pelvic cavity of a 10-year old-girl who presented with urinary frequency and left lower abdominal discomfort. A second plain X-ray film of the abdomen, obtained before surgery, in a different view than the first, revealed that the calcified mass had migrated to a lower position. The mass was laparoscopically resected, and histological examination revealed it to be a fibrotic nodule with central liquefaction and calcification.