This study will conduct a phase II study of triple combination with oxaliplatin, irinotecan, and S-1 as the first-line chemotherapy in patients with advanced gastric cancer.

It is widely accepted that the efficacy of chemotherapy for patients with inoperable, advanced, and metastatic gastric cancer is better than that compared to best supportive care. In general, combination chemotherapies are more efficient than monotherapy and so it is reasonable to give combination chemotherapy to patients with good performance status.

Especially dual combination chemotherapy with fluoropyrimidine and platinum has shown the objective response rate of 25-48% in patients with newly diagnosed advanced metastatic gastric cancer and several studies about triple combination with oxaliplatin, irinotecan, and fluoropyrimidine have shown the response rate of 53~75% but also higher rate of hematologic adverse events. So the investigators had conducted the phase I study of these three drugs with modification of dosage and schedule and will conduct a phase II study with recommended dose of triple chemotherapy from this phase I study.

Inclusion Criteria:

• Pathologically confirmed advanced, recurrent or metastatic adenocarcinoma of stomach (stage IV by primary tumor, regional nodes, metastasis(TNM) staging system)
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2
• More than 3 months expected life span
• Measurable lesion by RECIST criteria version 1.1
• Palliative chemotherapy naive
• Adequate organ functions
• Participants must sign an informed consent indicating that they are aware of the investigational nature of the study in keeping with the policy of the hospital

Exclusion Criteria:

• Positive Her2 status on participants' cancer tissue.
• Any prior 2 years or concurrent malignancy other than non-melanoma skin cancer, in situ cancer of uterine cervix, or papillary or follicular thyroid cancer.
• Participants who had received radiation therapy for target lesions 4 weeks before study enrollment
• Participants who had received major surgery 4 weeks before study enrollment
• Participants with active infection, severe heart disease, uncontrollable hypertension or diabetes mellitus, myocardial infarction during the preceding 12 months, pregnancy, or breast feeding
• Participants with central nervous system(CNS) metastases
• Participants with peripheral sensory neuropathies with impaired functional activities
• Participants with gastrointestinal obstruction or bleeding inducing mal-absorption of oral chemotherapeutic agents.