We report a rare case of advanced ovarian mucinous adenocarcinoma in a pregnant woman. A 28-year-old pregnant Japanese woman was diagnosed with an ovarian tumor 8 cm in diameter at a local hospital. She was sent to a private hospital at 25 weeks of gestation because of the growing ovarian tumor. Advanced ovarian carcinoma with widespread intraabdominal dissemination was detected by laparotomy at the hospital and she was referred to our hospital for further management. At 27 weeks of gestation, she underwent cesarean section, followed by abdominal total hysterectomy, and bilateral salpingo-oophorectomy. A girl weighing 879 g was delivered, with Apgar scores of 4 and 6 at 1 and 5 min, respectively. The pathological diagnosis of the tumor was mucinous cystadenocarcinoma grade 2. Although chemotherapy was not effective for her and she died of the disease 4 months after the surgery, her baby grew well and weighed 3750 g 3 months after delivery. For the treatment of such patients, we believe we should choose operative therapy as early as possible after the maturation of the fetus, although there are several reports of successful treatment with the administration of chemotherapy during pregnancy. To determine a better approach for such patients, multidisciplinary staff meetings, including gynecological oncologists, obstetricians, neonatologists, psychologists, and the patient are important.