The purpose of study is to evaluate the efficacy and safety of ONO-4538 with chemotherapy in unresectable advanced or recurrent gastric cancer (including esophagogastric junction cancer) not previously treated with the first-line therapy. Part 1 is intended to evaluate the tolerability, safety, and efficacy of ONO-4538 in combination with SOX therapy (Tegafur / gimeracil / oteracil potassium + Oxaliplatin) or CapeOX therapy (Capecitabine + Oxaliplatin). In part 2, the investigator or the subinvestigator will choose a chemotherapy (SOX or CapeOX therapy), taking into account the condition of each subject. Part 2 is planned to evaluate the efficacy and safety of ONO-4538 + chemotherapy in comparison with placebo + chemotherapy.