This report describes a patient with unresectable advanced gastric cancer who was successfully treated with potentially curative conversion surgery after chemotherapy with S-1 plus oxaliplatin(SOX). An 82-year-old man was diagnosed with type 5 gastric cancer(por1, HER2-negative)with multiple granular mucosal necroses that had metastasized throughout his body. Computed tomography revealed multiple lymph node metastases, tumor thrombosis in the splenic and portal veins, and peritoneal dissemination. After 9 courses of first-line chemotherapy with SOX, there was no tumor thrombosis in the splenic and portal veins or peritoneal dissemination, and the primary tumor and lymph node metastases were markedly reduced in size, indicative of a partial response(PR). The patient subsequently underwent total gastrectomy as curative conversion surgery. The histological diagnosis was ypT2N0M0, ypStage I B, and the primary lesion was categorized as Grade 2 gastric cancer. At present, 1 year after surgery, the patient remains alive without tumor recurrence.