A-78-year-old woman presented with difficulty swallowing. She was diagnosed as having advanced upper gastric cancer with invasion to the esophagus and the diaphragm. Biopsy examination of the tumor showed MSI-high and HER2-negative. The patient received 4 courses of SOX plus nivolumab in combination. After the chemotherapy, the size of the tumor decreased remarkably. We performed total gastrectomy with D2 lymph node dissection. Pathological examination revealed no residual cancer at the primary tumor location and the regional lymph nodes. The patient has shown no recurrence for 9 months without adjuvant chemotherapy. We conclude that SOX plus nivolumab in combination as first-line chemotherapy is an effective strategy against advanced gastric cancer.