A 52-year-old woman with abdominal pain and a feeling of incomplete evacuation visited a local clinic. Enlargement of the right ovary was detected, and the patient was referred to the gynecological department of our hospital. CT and MRI revealed a round-shaped mass, 8 cm in diameter, with cystic and solid components in the Douglas pouch. The patient underwent a laparotomy under the diagnosis of ovarian cancer. Intraoperatively, both the ovaries appeared normal and the tumor strongly adhered to the rectum and uterus. An exploratory laparotomy was performed; the tumor was identified as unresectable, and the patient was referred to our department after the surgery. PET-CT revealed nodules in the liver and peritoneum, in addition to the main tumor. Gastrointestinal endoscopy and immunohistochemical examination of a needle biopsy of the main tumor did not lead to the identification of the primary lesion. Thus, debulking surgery was performed to alleviate the patient's complaints. Histologically, the tumor was diagnosed as a primary peritoneal clear cell carcinoma. One month after surgery, multiple liver metastases and swelling of the peritoneal lymph nodes occurred. Six courses of dose-dense TC therapy were administered, and the patient achieved a complete response. At 8 months after surgery, the patient is still alive without tumor recurrence.