A 60-year-old man was admitted to our department for further evaluation of main pancreatic duct dilatation detected on ultrasonography. Endoscopic retrograde cholangiopancreatography showed stenosis of the main pancreatic duct at the junction of the pancreatic head and body. Brush cytology revealed pancreatic ductal carcinoma. Histological examination of the resected pancreas showed a 15-mm in length intraductal growth of carcinoma in situ in the main pancreatic duct, 10mm of which showed microinvasion. There was also atypical hyperplasia at a branch pancreatic duct near the lesion. It was suspected to be an initial stage of pancreatic ductal carcinoma. Intraductal progression type and non-progression type are often suspected in a case showing progression of carcinoma in situ to pancreatic ductal carcinoma, and this case was thought to be intraductal progression.