A 65-year-old male was referred to our hospital 2 years ago for a multilocular cyst accompanied with a protein plug in the pancreas tail. He was diagnosed as having branch duct-type intraductal papillary mucinous neoplasm and was followed-up. Two years later, endoscopic ultrasonography revealed a hypoechoic lesion, 10mm in diameter, near the cyst-like lesion. Finally, he was diagnosed with small pancreatic adenocarcinoma concomitant with intraductal papillary mucinous neoplasm and underwent radical distal pancreatectomy with splenectomy. Resected specimen revealed that the protein plug in the main pancreatic duct had caused distal pancreatic duct dilatation, resembling a multilocular cyst and pancreatic duct stenosis with inflammatory changes and fibrosis around the pancreatic parenchyma. Here, we report a rare case of protein plugs in the pancreatic duct mimicking pancreatic cancer concomitant with branch-type intraductal papillary mucinous neoplasm.