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-1/CDDP (Arm B) or 5-FU/CDDP (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.

Inclusion Criteria:

• Non-prior chemotherapy treated advanced gastric adenocarcinoma
• Age 18 and over
• Performance status 0, 1, or 2 (ECOG)
• Life expectancy 3 months
• Hematopoietic WBC lower limit of normal-12,000/mm^3 Absolute granulocyte count ≥ 2,000/mm^3 Platelet count ≥ 100,000/mm^3 Hemoglobin ≥ 8.0 g/dL
• Hepatic AST and ALT ≤ 100 U/L ALP ≤ 2 times upper limit of normal (ULN) Bilirubin ≤ 1.5 mg/dL
• Renal Plasma creatinine ≤ ULN Creatinine clearance ≥ 60 mL/min

Exclusion Criteria:

• Interstitial pneumonia, pulmonary fibrosis
• Myocardial infarction within the last 6 months, severe/unstable angina, congestive heart failure
• Intestinal paralysis, intestinal obstruction, uncontrollable diabetes