A 6 3-year-old man with a huge pancreatic tumor was referred to our hospital. Abdominal computed tomography revealed a heterogeneously enhanced encapsulated mass, 14 cm in diameter, in the pancreas head. The tumor thrombus extended to the bifurcation of the portal vein. The tumor, which had invaded the descending duodenum, was diagnosed as a probable case of acinar cell carcinoma, based on the biopsy results. Prior to resection, we prepared an ileocecal vein-umbilical vein bypass. Initially, we planned to perform a pancreatoduodenectomy, however, a total pancreatectomy had to be performed due to the atrophy of the residual pancreas tail. Since the tumor thrombus was visible, floating up from the portal vein wall at the upper level of pancreas, we dissected the portal vein at this level. The thrombus was extracted after securing the main tract and both (right and left) branches of the portal vein with vessel tape. About 5 cm of portal vein was resected and reconstructed. Since patients who undergo resection of acinar cell carcinoma have a better prognosis and long-term survival is often reported for cases of resected tumor thrombus of the portal vein, it is advisable to resect acinar cell carcinomas even in cases as advanced as reported here.