A 77-year-old man presented to our hospital with a chief complaint of stomachache. He received a diagnosis of unresectable advanced gastric cancer classified as cT3, N+, M1(LYM, HEP, OSS), Stage ⅣB. He underwent first-line chemotherapy with SOX, second-line treatment with PTX plus Ram, and third-line treatment with nivolumab. The primary tumor showed a reduction in size, and liver and lymph node metastases were not detectable. However, after 5 years of chemotherapy, a re- enlargement was observed in the primary gastric lesion without progression of liver and lymph node metastases. Subsequently, conversion surgery was performed. Based on the pathological analysis, the diagnosis was ypT1b2(SM2), N0(0/17), M0, ypStage ⅠA, R0. After nivolumab administration postoperatively for 5 months, chemotherapy was discontinued as there was no recurrence.