A 62-year-old woman underwent upper endoscopy in January 2009 to reveal the presence of an extrinsic compression measuring approximately 3 cm in the anterior wall of the gastric antrum. Further examinations suggested that it was caused by peritoneal cancer of an unknown origin; thus, staging laparoscopy was performed in May 2009. Multiple white nodules of varying sizes were found scattered throughout the right upper quadrant of the abdomen and the right abdomen. Based on a biopsy of the greater omentum, the patient was diagnosed with papillary serous adenocarcinoma. As no abnormalities were observed in the uterus and ovary, it was suspected that the patient had primary peritoneal cancer. Hence, in July 2009, the patient underwent resection of the greater omentum, gastric pylorus, gall bladder, and right hemicolon where the tumors were localized, as well as bilateral adnexectomy. Based on intraoperative findings and postoperative histology, the patient was diagnosed with high-grade primary peritoneal serous adenocarcinoma and received paclitaxel and carboplatin therapy. Subsequent follow-up examinations, including positron emission tomography-computed tomography(PET-CT), indicated repeated recurrences in the mesentery, the pelvic floor, and around the remnant stomach. After identifying these recurrences, the patient was treated with platinum-based drugs, experiencing repeated response and cessation cycles. Since September 2019, the patient has received olaparib therapy. PET-CT examination performed in September 2020 indicated that the patient remained in complete remission.