While mature cystic teratoma of the ovary is the most common ovarian tumor in young women, immature teratoma is a very rare tumor, representing only 1% of all ovarian cancers. In the three cases presented here, young women who were suspected to have mature cystic teratoma, based on CT scan and MRI, were ultimately diagnosed with immature teratoma Ic (b) G1 after laparoscopic operation. They underwent salpingo-oophorectomy of the affected side only and have shown no sign of recurrence during follow-up. We sometimes encounter patients with immature teratoma who have no findings pointing to malignancy on CT or MRI. Generally, if the components of immature nerve cells that represent immature teratoma are very few, it is difficult to diagnose the entity as immature teratoma with imaging evaluations such as CT or MRI. In many hospitals, laparoscopic surgery is selected for patients with ovarian mature teratoma. Therefore, it is essential to attempt as much as possible not to disseminate the fluid content of the tumor into the intra-abdominal space during laparoscopic operation, because in rare cases the tumor turns out not to be benign mature teratoma.