A case of intraductal papillary mucinous tumor of the pancreas with complete absence of the ventral pancreatic duct of Wirsung is presented. A 74-year-old Japanese man was admitted to our hospital because of elevated serum amylase concentration. Abdominal computed tomography (CT) scanning revealed diffuse dilatation of the main pancreatic duct and a diffuse and uncircumscribed area with heterogeneous density in the pancreas head. Endoscopic retrograde cholangiopancreatography revealed that the main pancreatic duct was connected with an accessory papilla and was diffusely dilated, without any irregularity of the duct wall being observed in the entire length of the duct. The common bile duct was detected only by cannulation through Vater's papilla, and no pancreatic duct or its communicating branch was found. Some branches, directed to the dorsal portion of the pancreas head, were found arising from the accessory pancreatic duct. Intraductal ultrasound examination performed through the accessory papilla and the common bile duct revealed a small tumor with a heterogeneous echo level in the pancreas head. From these findings, intraductal papillary-mucinous tumor (IPMT) occurring in the pancreas head was diagnosed, and pylorus-preserving pancreaticoduodenectomy was performed. The resected specimen revealed IPMT in the pancreas head. A roentgenographic study of the resected specimen revealed a defect caused by the tumor located in the pancreatic duct connected with the accessory papilla and showed that there was complete absence of the pancreatic duct connected with Vater's papilla. Surgical resection enabled us to completely analyze the duct system of pancreas divisum. Although it is not known whether there is a relationship between the pathogenesis of IPMT and embryological anomaly of the pancreatic duct system, this case may provide an insight into the pathogenesis of IPMT.