[Introduction] The struma ovarii is rare ovarian tumor. Almost 95% of it is indicated as the benign tumor. Therefore, it is important to be diagnosed preoperatively. A case is presented, that is diagnosed successfully as a stuma ovarii preopretively. [Patient] A 36 years old G1P1 woman was referred to our hospital for the operation of an ovarian tumor. She had no family history or past illness. The ovarian tumor was her right side and 7cm in a diameter using ultrasound test. Using pelvic MR image test, some regions of the tumor show T1W high and T2W low intensity. Other regions show T1W low and T2W high intensity. Although there was a possibility of mucinous malignant tumor as a differential diagnosis, we mostly diagnosed as a struma ovarii. We planed to laparoscopic tumorectomy. If the histological examination shows malignancy, we planed to treat about itã€€secondary. At the time of laparoscopy, a small amount of ascites was observed. Right ovary was as large as 7cm and adhered to retro peritoneum. Laparoscopic tumorectomy was performed and histological examination revealed as a struma ovarii without malignancy. [Conclusions] It is important to diagnose struma ovarii carefully using image study, because it is benign tumor, however sonetimes difficult to diagnose preoperatively. MR image might be useful for preoperative diagnosis.