We treated a patient with gastric cancer considered to be unresectable due to peritoneal metastasis, who responded remarkably to treatment with TS-1. The patient was a 62-year-old male. His diagnosis was gastric cancer, for which he underwent surgery on February 22, 2001. Laparotomy disclosed many nodules measuring 2-3 mm in diameter in the abdominal cavity, so rapid pathological tests were conducted during the operation. The test results indicated peritoneal metastasis from gastric cancer. Therefore, simple laparotomy was employed as the best option. On day 13 after surgery, oral administration of TS-1, bid., at a daily dose of 120 mg was commenced. In our outpatient clinic, he was given 3 courses, each comprising 4 weeks' medication and 2 weeks' discontinuation. Subsequently, upper digestive tract endoscopy was performed but only scars in the gastric vestibular area were observed. Biopsy could not detect any malignant findings. Medication was discontinued due to the patient's preference and he died of gastric cancer 10 months after operation.