 The purpose of study is to evaluate the efficacy and safety of postoperative adjuvant chemotherapy with Nivolumab in combination with tegafur-gimeracil-oteracil potassium S-1 therapy or capecitabine oxaliplatin CapeOX therapy, in comparison with placebo in combination with S-1 therapy or CapeOX therapy, in pStage III gastric cancer including esophagogastric junction cancer after D2 or more extensive lymph node dissection. 