A 50-year-old post-menopausal woman with sudden lower abdominal pain was transported to hospital by ambulance. Abdominal symptoms and computed tomography(CT)suggested a diagnosis of acute pan-peritonitis due to gastrointestinal perforation, and emergency surgery was performed. The intraoperative findings led to a diagnosis of a ruptured cyst in the left ovary, a portion of which was observed to be partially solid, and therefore, ovarian cancer was suspected. Accordingly, a unilateral(left)salpingo-oophorectomy and intraperitoneal drainage were performed with assistance from a gynecologist. Following a diagnosis of ovarian clear cell adenocarcinoma based on histopathological examination, the patient underwent further debulking surgery at a later date, followed by postoperative chemotherapy. In acute pan-peritonitis associated with a large quantity of ascites in women, the rupture of ovarian tumors should be considered as a possible etiology. The therapeutic strategy for ovarian cancer is determined according to post-operative staging, even during emergency surgery. It is therefore important not only to repair the rupture, but also, if possible, to perform a diagnosticbiopsy or resection.