Paraovarian tumor of borderline malignancy is rare. Only 13 cases in Japan, including the present cases, and 40 cases overseas have been reported. We report two cases of paraovarian tumor of borderline malignancy. Case 1: A 38-year-old woman was found to have a right ovarian cyst. Transvaginal ultrasonography showed a unilocular cystic tumor and a solid part of 8 mm in diameter within the tumor. Malignancy could not be ruled out, therefore the laparotomy was performed. The tumor was paraovarian tumor. Intraoperative rapid histological diagnosis was made and the patient was diagnosed with a serous borderline tumor. There has been no recurrence 1 year after surgery. Case 2: A 30-year-old woman was found to have a right hydrosalpinx, or an ovarian cyst, with a solid part inside. Transvaginal ultrasonography showed the solid part within the tumor was 14 × 9 mm. Laparoscopic surgery was performed and we diagnosed the tumor as paraovarian cyst. Histological diagnosis was mucinous borderline tumor. There has been no recurrence 11 months after surgery.