Ectopic ovary is very rare gynecologic abnormality. It divided into accessory ovary and supernumerary ovary. Accessory ovary is connected to an ovary. That can be distinguished from supernumerary ovary whether it is separated from normal ovary. Though accessory ovary is reported more frequently than supernumerary ovary, it is still one of the rare gynecologic condition. Its incidence was reported to be 1:93,000 by Wharton in 1959. Benign or malignant tumor originating from an accessory ovary is extremely rare condition. It is very difficult to diagnosis this before surgery and usually confused with other intraabdominal tumor. We describe the case of a 17-year-old women with chronic pelvic pain. She was diagnosed cystic mass at Lt. adnexa on preoperative abdominal computed tomography. We performed diagnostic laparoscopic operation. That operation disclosed the huge cystic mass was arising in an accessory ovary. And histopathologic diagnosis was mucinous cystic adenoma. Although accessory ovaries are very infrequent, it is necessary to try to detect and identify. Also that may be related to congenital malformation, we should be tested to reveal the presence of other malformation.