A 78-year-old man was diagnosed with HER2-positive advanced gastric cancer, cT3N2M1(LYM: #16a1, 16b1), cStage ⅣB, after being referred for anemia. The lesion was deemed unresectable, and first-line chemotherapy was initiated using S-1, cisplatin(CDDP), and trastuzumab(T-mab). After 2 courses of chemotherapy, the patient developed febrile neutropenia( Grade 3). At this point, the lesion showed partial response(PR), and chemotherapy was continued using oral S-1 as a single agent. After 3 months of S-1 monotherapy, the para-aortic lymph node metastases showed a complete response (CR). S-1 monotherapy was continued without major adverse events for 2 years, and the patient is presently alive and well 6 years after obtaining clinical CR. This may suggest that S-1 is a safe and effective treatment for unresectable advanced gastric cancer in elderly patients.