TS-1 is an antitumor drug including 5-chloro-2,4 dihydroxypyridine (CDHP), which inhibits dihydriopyrimidine dehydrogenase (DPD) activity selectively in metabolism of 5-FU. However, TS-1 therapy tends to increase adverse events for patients with impaired renal function due to excessively high blood concentration of 5-FU, because CDHP is mainly excreted into the urine. In a 67-year-old male with advanced gastric cancer, renal dysfunction occurred during TS-1 administration as its adverse event. We studied the pharmacokinetics of 5-FU, which were analyzed on the T1/2 value and the AUC (0-infinity) of 5-FU with a single and consecutive TS-1 administration, and estimated an optimal TS-1 administration regimen for this patient. The regimen is 60 mg/body/day given in one divided dose for 28 days consecutively followed by 14 days rest. This regimen enabled a continuation of TS-1 treatment for the patient. In conclusion, individual dose adjustment using pharmacokinetic study of 5-FU might be beneficial to patients with impaired renal function.