We report the case of a 72-year-old male with Stage IV gastric cancer accompanied by multiple liver metastases and carcinomatous ascites which responded to chemotherapy using TS-1. Treatment of the patient with daily oral administration of 120 mg TS-1 for 4 weeks resulted in complete regression of the liver metastases and ascites, as well as a 35% reduction in the size of the primary lesion. After 2 cycles, the primary tumor size was reduced to 55% and serum CA19-9 and CA125 levels were decreased to the normal ranges. This regimen was effective without any adverse effects, and improved the patient's QOL, for 6 months before progression of liver lesions. The patient received chemotherapy at our outpatient clinic for 10 months after the first treatment, after which he died of peritonitis carcinomatosa. The current case suggests that TS-1 may have a potent therapeutic efficacy in advanced gastric cancer patients.