Ovarian choriocarcinoma is a rare neoplasm that can be either gestational or non-gestational in origin. In the presence of adnexal mass and elevated levels of human chorionic gonadotropin (hCG) in the serum, this condition mimics an ectopic pregnancy. Here we report a case of histopathologically diagnosed non-gestational ovarian choriocarcinoma, which was preoperatively suspected of ectopic pregnancy. A 30-year-old woman, gravida 2 para 2, visited her primary clinic complaining of amenorrhea with genital spotting. During two weeks of observation, her urine hCG test was positive although there was no gestational sac (GS) detected in the uterus. At gestational age of the 8th week and the 1st day from the last menstrual period, she was transferred to our hospital, suspected of ectopic pregnancy or abortion. Examination at our hospital revealed increased serum hCG level (2269.5 mIU/ml) although GS was not detected in either the uterus or adnexa, nor was ascites. By the following day, serum hCG level further increased to 2512.6 mIU/ml. We strongly suspected ectopic pregnancy and performed a diagnostic laparoscopy. Intraoperative observation revealed no lesion in fallopian tubes and a 2 cm mass in the right ovary without ascites. We made an incision into the mass and found chorionic villi like tissue filled. Therefore we suspected ovarian pregnancy and performed a partial oophorectomy. However, histopathological analysis disclosed the diagnosis of choriocarcinoma. Computed tomography (CT) of the chest, abdomen, and pelvis revealed no metastasis to be evident. Intrauterine curettage and laparoscopic right salpingo-oophorectomy dully performed showed no evidence of residual tumor inside the uterus or right adnexa. The patient was given four courses of combination chemotherapy consisting of methotrexate, etoposide, and actinomycin D. Because this patient presented with choriocarcinoma without any other component of germ cell tumor, there was a possibility of gestational choriocarcinoma arising from a term pregnancy two years before or non-gestational choriocarcinoma originated from the ovary. DNA polymorphisms regarding the length of short tandem repeat (STR) in representative 16 genes was analyzed between the patient blood and the tumor tissue to determine whether the mass was gestational or non-gestational in origin. An identical pattern of STRs of 16 genes was found in the blood sample and the tumor sample, suggesting that the tumor was of non-gestational origin. When ovarian pregnancy was suspected from the clinical course, the pathological diagnosis should be definitely made with the diagnosis of ovarian choriocarcinoma in mild, followed by the treatment adapted to the diagnosis. [Adv Obstet Gynecol, 68 (4) : 381-388, 2016 (H28.10)]