After failure of first-line chemotherapy with fluoropyrimidines and platinum compounds for advanced gastric cancer, second-line chemotherapy with ramucirumab plus paclitaxel, which elicits a durable response, and third-line or later chemotherapy with nivolumab have been shown to lead to a more favorable prognosis in advanced gastric cancer patients. As new and more effective drugs are now available, sequential chemotherapy would contribute to prolonged survival. From this point of view, the patient’s disease course should be frequently monitored in order to adapt treatment regimens. This review summarizes the points to note in regard to radiological assessment, and discusses the integration of prognostic factors, tumor markers, and clinical symptoms that need to be taken into account to change treatment at an appropriate timing.