A 74-year-old man was admitted to a nearby clinic complaining of high fever. Abdominal CT showed a 10 mm diameter cystic mass in the head of pancreas and dilation of the pancreatic duct. Endoscopy revealed a fistula filled with mucin in the posterior wall of the duodenum. The patient was referred to our institution for a surgical resection. Endoscopic ultrasonography revealed dilation of the pancreatic duct and also mural nodules in the pancreatic duct, ERP demonstrated a fistula from the pancreatic duct to the duodenum. Biopsied specimen from the papillary nodule in the pancreatic duct showed adenoma. We performed pancreaticoduodenectomy for main-duct IPMN penetrating to the duodenum. Pathological findings showed a non-invasive type of IPMC. Furthermore, a cancer invasion to the duodenum was not detected. These findings suggest that the increased pressure within the pancreatic duct caused a fistula to the duodenum.