Inclusion Criteria:

1. Written informed consent before the enrollment

2. Age ≥18 years old

3. Histologically/cytologically confirmed recurrent or metastatic gastric or
esophagogastric junctional adenocarcinoma

4. Eastern Cooperative Oncology Group (ECOG) performance status 0-2

5. Patients able to swallow food and drugs

6. At least one measurable or evaluable lesion according to RECIST criteria version 1.1

7. Adequate bone, hepatic, and renal function as assessed by the following laboratory
requirements to be conducted within 7 days prior to first administration of study
drugs

- Absolute neutrophil count (ANC) ≥ 1,500/ uL, platelet ≥ 100,000/ uL, haemoglobin
(Hb) ≥ 9.0 g/dl,

- Serum creatinine ≤ 1.5 mg/dL (If serum creatinine is greater than 1.5 mg/dL,
creatinine clearance [Ccr] should be 60 mL/min or greater. Ccr is calculated by
Cockcroft-Gault formula or 24hr urine collection)

- Total bilirubin ≤ 1.5 x upper limit of normal (ULN), AST/ALT levels ≤ 3.0 x ULN
(AST/ALT levels ≤ 5.0 x ULN for patients with liver involvement of their cancer)

8. In patients who received adjuvant or neoadjuvant chemotherapy, completion of systemic
chemotherapy 6 months before the study enrollment, and no previous administration of
platinum derivatives

9. Estimated life expectancy of more than 3 months

Exclusion Criteria:

1. Other histologic types than adenocarcinoma

2. Recurrence within 24 weeks following completion of adjuvant chemotherapy

3. R1 gastrectomy (i.e., microscopic residual disease)

4. History of another malignancy within the last five years from the day of written
informed consent except cured basal cell carcinoma of skin and cured carcinoma in
situ of uterine cervix

5. Radiotherapy within 4 weeks after randomization

6. History of significant neurologic or psychiatric disorders, and presence or history
of CNS metastasis

7. Major surgery within 4 weeks before study entry, or insufficient recovery from major
surgery (except the patients who received only open and closure or biopsy)

8. Other serious illness or medical conditions as follows;

- Any following conditions occurred within 6 months before study entry: myocardial
infarction, severe/unstable angina, bypass surgery for coronary
artery/peripheral artery, congestive heart failure (NYHA class III or IV),
cerebral infarction or transient ischemic attack

- Conduction abnormality such as 2nd degree or greater AV block or severe
arrhythmia that requires medical treatments (right bundle branch block (RBBB) is
eligible, but left bundle branch block (LBBB) is not.)

- Uncontrolled hypertension

- Liver cirrhosis (Child Pugh Class B or greater)

- Interstitial pneumonia, pulmonary fibrosis

- Active viral hepatitis B

- Uncontrolled diabetes mellitus

- Uncontrolled ascites or pleural effusion

- Uncontrolled active infection or sepsis

9. Administration of medications which may have potentially pharmacokinetic interaction
with S-1, cisplatin, and oxaliplatin

- Flucytosine, a fluorinated pyrimidine antifungal agent

- Anti-viral agents, such as sorivudine, and brivudine, or chemical similar drugs

- Warfarin (except, low dose warfarin for the purpose of prophylaxis),
phenprocoumon

- Phenytoin

- Allopurinol

10. Participation to other clinical trials or administration of other investigational
drugs within 30 days before the randomisation

11. Pregnant or lactating women

12. Women or men of child bearing potential not employing adequate contraception during
study treatments or until the 3 months after the end of study treatments

13. Ineligible for the study at the discretion of investigators