The prognosis for advanced pancreatic cancer with peritoneal dissemination is extremely poor, and no effective standard therapy has been established. We present a case of a very old patient whose QOL improved shortly after administration of only S-1 to treat advanced pancreatic cancer with peritoneal dissemination. A 85-year-old man presented to our hospital with anorexia and loss of weight. CT scanning showed severe ascites and a low-density area 2 cm wide at the tail of the pancreas. Ascitic cytology revealed adenocarcinoma and carcinomatous peritonitis due to pancreatic cancer. Considering his general condition due to old age, the regimen for oral S-1 (80 mg/body/day) was set at 4 consecutive weeks of administration followed by a 2-week rest period. His abdominal circumference decreased and his appetite improved by 14 days following commencement of the therapy. The blood examination one month following commencement showed a significant decrease in the tumor marker. There was no adverse drug reaction. Six months later CT scanning showed that the ascites had disappeared and that the low-density area at the tail of the pancreas had become less obvious. The tumor marker and biochemical parameters were within standard ranges. Twelve months since the therapy began: there still has been no adverse drug reaction and his QOL has been good. He is receiving the therapy as an outpatient. This case suggests that S-1 is a safe and effective drug for very elderly patients.