An 82-year-old, male. He visited his local doctor with a chief complaint of dyspnea on exertion. Anemia was noted, and upper gastrointestinal endoscopy was performed, which revealed an ulcerative lesion in the gastric antrum. A biopsy revealed Group 5, tub2, and HER2 negative, with PD-L1≥5%. cT3N1H1(M1 HEP), cStage ⅣB was diagnosed based on CT scan showing enlarged #8 lymph node and a single liver metastasis in the 2 cm range in S6 of the liver. The patient was deemed unresectable and was started on SOX plus nivolumab therapy. On day 11 after initiation, the patient had Grade 3 diarrhea by CTCAE v5.0, and S-1 was withdrawn for 3 days, but was administered for 2 courses. CT and MRI after chemotherapy showed shrinkage of both the primary tumor and liver metastases; R0 resection was deemed possible, and pyloric gastrectomy, D2 lymph node dissection, and partial hepatic S6 resection were performed. The histological evaluation of response to treatment was Grade 1b, and the patient was in ypStage ⅠA. The patient has been alive without recurrence for 6 months postoperatively while receiving S-1 monotherapy on an outpatient basis.