The patient was a 72-year-old woman who had advanced carcinoma of the stomach. She presented massive ascites due to peritonitis carcinomatosa. The cytology of ascites was class V and the CEA level of ascites was elevated. Since renal function was low, we injected 300 mg of carboplatin intraperitoneally 2 times in 8 weeks. The amount of ascites was significantly diminished and the CEA level of ascites was decreased. The result suggested the effectiveness of intraperitoneal injection of carboplatin for the therapy of peritoneal metastasis of gastric cancer.