 This is an open-label, multicenter, three arm, parallel, randomized, Phase 3 study evaluating the efficacy and safety of S-1 alone compared with S-1 plus CDDP, and S-1 plus CDDP compared with 5-FU plus CDDP in patients with advanced gastric cancer previously untreated with chemotherapy for advanced disease. Patients will be randomly assigned 1:1:1 to S-1 Arm A, S-1CDDP Arm B or 5-FUCDDP Arm C. Patients will be stratified to achieve balanced distribution of patients to each arm according to following stratifications, performance status 0, 1, or 2, the number of metastatic sites 1 vs 1, prior gastrectomy, and center. 