This study is to compare and evaluate the efficacy and safety of the treatment with ONO-4578 in combination with nivolumab and chemotherapy with those of the treatment with placebo in combination with nivolumab and chemotherapy in chemotherapy-naïve participants with HER2- negative unresectable advanced or recurrent gastric cancer (including esophagogastric junction cancer).

Inclusion Criteria:

• Patients with gastric or esophagogastric junction cancer histologically diagnosed as adenocarcinoma
• Patients have not been treated with systemic chemotherapy as first-line therapy
• Patients who can provide tumor tissue samples

Exclusion Criteria:

• Patients unable to take oral medicines
• Patients with HER2-positive
• Patients with contraindications to nivolumab, oxaliplatin, S-1, or capecitabine
• Patients who have a history of severe drug-related adverse reactions caused by non- steroidal anti-inflammatory drugs (NSAIDs) or who have complications caused by NSAIDs requiring treatment
• Patients with concurrent autoimmune disease or a history of chronic or recurrent autoimmune disease, such as rheumatoid arthritis
• Patients with headache and/or nausea associated with brain metastasis