Phase Ib/II Study to Evaluate the Safety and Efficacy of Nivolumab in Combination With Paclitaxel in Epstein-Barr Virus(EBV)-Related, or Microsatellite Instability-High (MSI-H), or Programmed Cell Death Ligand 1 (PD-L1) Positive Advanced Gastric Cancer

Inclusion Criteria: 1. Has provided digned written informed Consent 2. Is male or female ≥19 years of age 3. Has a histologically or cytologically confirmed diagnosis of advanced gastric adenocarcinoma 4. Has documented EBV-related, MSI-high, or PD-L1 positive tumor in primary or metastatic tumor tissue 5. Has a life expectancy of at least 3 months 6. Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 7. Has measurable or evaluable disease as determined by RECIST 1.1. 8. Is able to swallow and retain orally administered medication 9. Has an adequate baseline organ function defined as: * White blood cells ≥3000/mm3 and neutrophils ≥1500/mm3 * Platelets ≥100000/mm3 * Hemoglobin ≥9.0 g/dL * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤3.0 × upper limit of normal (ULN) of the study site (or ≤5.0 × ULN in patients with liver metastases) * Total bilirubin ≤2.0 × ULN * Creatinine≤1.5 × ULN or creatinine clearance (either measured value or estimated value using the Cockcroft-Gault equation) \>60ml/min. Exclusion Criteria: 1. Has HER2-positive or indeterminate gastric cancer 2. Have multiple cancers 3. Have a current or past history of severe hypersensitivity to any other antibody products 4. Have concurrent autoimmune disease or a history of chronic or recurrent autoimmune disease 5. Have a current or past history of interstitial lung disease or pulmonary fibrosis diagnosed based on imaging (preferably CT) or clinical findings 6. Have brain or meninx metastases. Patients may be randomized for the study if they are asymptomatic and require no treatment. 7. Have pericardial fluid, pleural effusion, or ascites requiring treatment 8. Have a history of uncontrollable or significant cardiovascular disease 9. Have systemic infection requiring treatment 10. Are contraindicated for paclitaxel 11. Has had prior treatment with: \- Require or, within 28 days before treatment, have received systemic corticosteroids or immunosuppressants 12. Have undergone surgery (any surgery involving general anesthesia) within 28 days before study treatment 13. Have received radiotherapy for gastric cancer within 28 days before treatment or radiotherapy for bone metastases within 14 days before treatment 14. Have a positive test result for human immunodeficiency virus-1 (HIV-1) antibody, 15. Hepatitis B surface protein (HBs) antigen and HBV titer \>2000 IU/ml (10,000 copy/ml), or hepatitis C virus (HCV) antibody positive result 16. Are pregnant or breastfeeding, or possibly pregnant 17. Has any unresolved ≥Grade 2 (per CTCAE v4.0) toxicity from previous anti-cancer therapy at the time of enrollment such as neuropathy, except alopecia or anemia 18. Have previously received nivolumab, anti-programmed cell death-1 (PD-1) antibody, anti-PD-L1 antibody, anti-programmed cell death-ligand 2 (PD-L2) antibody, anti-CD137 antibody, anti-cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) antibody, or other therapeutic antibodies or pharmacotherapies for the regulation of T-cells 19. Are incapable of providing consent for specific reasons, such as concurrent dementia 20. Are otherwise inappropriate for this study in the investigator's or subinvestigator's opinion.