The patient was a 59-year-old woman with advanced double cancers of the stomach and endometrium with peritoneal metastasis. Abdominal computed tomography revealed that the endometrial cancer was more advanced than the gastric cancer; therefore, the peritoneal metastasis was diagnosed as arising from the endometrial cancer. Treatment of the endometrial cancer with cytoreductive surgery followed by adjuvant chemotherapy was performed first. She underwent total hysterectomy, bilateral salpingo-oophorectomy, and omentectomy. Disseminated nodules were found throughout her abdomen. The histopathological findings indicated carcinosarcoma of the uterus, pT3bNXM1, Stage Ⅳb. One month after surgery, she received 6 courses of adjuvant chemotherapy with paclitaxel plus carboplatin. After the adjuvant chemotherapy, abdominal computed tomography revealed that both the ascites and the disseminated nodules had disappeared. Therefore, a second-look surgery for the endometrial cancer and definitive surgery for the gastric cancer were planned. At the laparotomy, no disseminated nodules were found, so distal gastrectomy and D2 lymphadenectomy were performed. The histopathological findings were pT4aN1M0P0Cy0, Stage ⅢA. She received adjuvant chemotherapy with S-1 for 1 year, and has been alive with no evidence of recurrence for 2 years and 7 months after the initial surgery.