The patient was a male in his early 60s. Diabetes had aggravated 6 months earlier, and the patient was referred to our hospital for close examination. On contrast CT, enhanced mass shadows filling the lumen of the main pancreatic duct, which was dilated throughout the pancreas, were observed, and the mass was diagnosed as an adenocarcinoma on EUS-FNA. Based on these findings, main-duct IPMN was suspected and total pancreatectomy was performed. On macroscopic observation of the resected specimen, outgrowth of a solid tumor was observed in the main pancreatic duct, whereas only low-level mucus retention was noted in the pancreatic duct. Histopathological examination revealed a papillary/tubular tumor growth, suggesting interstitial infiltration throughout the pancreas. On immunostaining, the tumor was partially positive for MUC5AC, based on which the patient was diagnosed with an intraductal pancreatic mallignant tumor, with difficulty in differentiating between IPMC and ITPC. Clinicopathologically, many aspects regarding ITPN remain unclear. Further accumulation of such cases and investigation of the tumor pathology are necessary.