A 57-year-old woman suffering from lower abdominal malaise was diagnosed with cStage IV advanced gastric cancer because of simultaneous liver metastasis and peritoneal dissemination. Because she was regarded as inoperable, she was administered combined immunochemotherapy comprising S-1/paclitaxel(PTX) and Krestin(PSK). One course of therapy lasted for 3 weeks as follows: 80 mg/m2 of S-1 on days 1-14 orally, 50 mg/m2 of PTX on days 1 and 8 intravenously, and 3g/day of PSK on days 1-21 orally. After 20 courses of this treatment, the metastatic liver tumor and peritoneal dissemination had disappeared on the abdominal computed tomography scan. At that time, curative resection was considered possible and she underwent total gastrectomy (Roux-en Y) with D2 lymphadenectomy. Histopathological examination revealed pStage IB [pT2 (mp), pN0] and the histological effects of the main tumor were diagnosed as grade 2. She is alive and free from recurrent disease for more than 6 years, and remains in a good condition. This combination of immunochemotherapy using S-1/PTX/PSK can be safe and effective, and is an initial treatment option for unresectable advanced gastric cancer.