The Japanese guidelines for the treatment of gastric cancer recommend nivolumab as third-line chemotherapy for metastatic gastric cancer. We report a case of gastric cancer exhibiting a durable response after the discontinuation of nivolumab due to the early onset of immune-related adverse event (irAE). A 64-year-old man with advanced HER2-positive gastric cancer and distant lymph node metastasis received nivolumab as fourth-line therapy. After two courses of nivolumab, the lymph nodes showed progression. However, the treatment was discontinued because of interstitial pneumonia as an irAE. Disease regression was sustained for approximately 11 months without the readministration of nivolumab.