A Phase I/II trial of radiotherapy administered concurrently with TS-1 plus cisplatin has been initiated in Japanese patients with clinical resectable type 4 or large type 3 gastric cancer. The aim of this trial is to determine the recommended dose of TS-1 and cisplatin combined with radiotherapy at a fixed dose in the Phase I study, and to evaluate the efficacy and safety in the Phase II study. The primary endpoint for Phase II is the pathological complete response rate, assessed using surgically resected specimens. Secondary endpoints are the response rate, progression-free survival, overall survival, operation transitional rate, R0 resection rate, rate of treatment completion, rate of down-staging and rates of postoperative complications and adverse events. In Phase II, a total of 30 patients will be enrolled in the Osaka Gastrointestinal Cancer Chemotherapy Study Group trial over a period of 6 years.