Pancreatic acinar cell carcinoma is rare, and its incidence is less than 1% of all the malignant pancreatic tumors. Little is reported on effectiveness of chemotherapy. We report a 64-year-old male patient with pancreatic acinar cell carcinoma and a giant metastatic liver tumor, which responded to combination chemotherapy with gemcitabine(GEM)and peroral S-1 administration. The patient had upper abdominal pain and hypervascular tumors in liver(15 cm in diameter)and pancreas tail (3 cm in diameter), which were detected by an enhanced abdominal computed tomography(CT)scan, and was admitted for further examination. Abdominal angiography, FDG-positron emission tomography(PET), and liver tumor biopsy led to a diagnosis of pancreatic acinar cell carcinoma in the pancreas tail with liver metastasis. The patient was then treated with combination chemotherapy, which consisted of intravenous infusion of GEM and peroral administration of S-1, and the metastatic liver tumor was markedly reduced(partial response in RECIST). Although the prognosis of patients with unresectable pancreatic acinar cell cancers is generally unfavorable, it is suggested that the GEM/S- 1 combination chemotherapy is effective for these patients' treatment.