We report a case of stomach and pancreas cancers that showed marked responses to combination chemotherapy consisting of S-1, paclitaxel (PTX), and lentinan (LNT). A 67-year-old Japanese man was referred to our hospital in July 2005, diagnosed with advanced gastric cancer. Subsequent examination revealed the existence of cancers in the stomach and pancreas, with lymph nodes and peritoneal metastasis and ascites. The patient received combined chemotherapy (one course comprised 3 weeks) with S-1 (100 mg/body, day 1-14 followed by withdrawal for 1 week), PTX (50 mg/m2, day 1 and day 8), and LNT (2 mg/m2, day 1, day 8 and day 15). After completion of 4 courses, the patient achieved partial response (PR), with complete disappearance of the primary gastric tumor and ascites. He maintained in PR for 17 months. We analyzed Th1/ Th2 ratio and LNT binding rate to monocytes by flow cytometry. Combination chemotherapy with S-1/PTX/LNT can be an effective treatment for unresectable advanced gastric carcinoma.