We encountered a case of ovarian cancer that had positive pleural washing cytology diagnosed by thoracoscopy. A 65-year-old woman was referred to our hospital with a sensation of abdominal fullness. A whole-body examination revealed a solid tumor of the ovary and accumulation of ascites. Neither pleural effusion nor lung metastasis was found at the time. Laparotomy showed extensive dissemination and we classified the stage as IIIc. As the diaphragm was thickened with dissemination, we tried to strip the diaphragm and used thoracoscopy to identify the thoracic area. No tumor or pleural effusion was detected. We performed pleural biopsy and pleural washing cytology. Finally, malignant cells were found in the pleural washing cytology. Pathological findings showed serous papillary adenocarcinoma of the ovary in this stage IV case. We suggest in this report that thoracoscopy is a useful method for making the correct surgical staging.