High-grade pancreatic intraepithelial neoplasia (PanIN-3) is recognized as a precursor lesion of invasive ductal carcinoma (IDC). However, histological evidence that PanIN-3 invades beyond the basement membrane of pancreatic ductal epithelium, that is, the moment PanIN-3 becomes IDC, has not been captured yet. This may be because PanINs which are microscopic papillary or flat lesion rarely develop clinical symptoms and are not detectable on imaging examination. On the other hand, most IDCs were found in the advanced stage with massive invasion. In this report, PanIN-3 obstructed several branch pancreatic ducts and subsequently caused pancreatitis which developed clinical symptom and was detectable as a pancreatic mass in imaging studies. A 65-year-old woman was referred to our institution for further examination of her repeated pancreatitis. Abdominal ultrasound revealed a low echoic mass of 13 mm in diameter in the pancreatic body without upstream dilatation of the main pancreatic duct (MPD). Endoscopic retrograde pancreatography showed a strictured segment of 2 mm in length in the MPD at the pancreatic body. Cytological examination of pancreatic juice revealed adenocarcinoma and distal pancreatectomy was performed. A resected specimen revealed a whitish mass of 15 mm in diameter in the pancreatic body, which was identified as pancreatitis by histological examination. Papillary growth of PanIN-3 was seen mainly in the branch ducts. Each PanIN-3 was located separately in the branch ducts with normal epithelia in the MPD between them. In three adjacent branch ducts, PanIN-3 was observed to be invading microscopically beyond the basement membrane.