A 46-yr-old woman was admitted to our hospital with mild epigastric pain. Ultrasonography and computed tomography revealed an extremely thickened gallbladder wall. Endoscopic retrograde cholangiopancreatography demonstrated that the main pancreatic duct joined the nondilated common bile duct at the outer point of the duodenal wall (P-C type of pancreaticobiliary maljunction), and the cystic duct joined the common channel directly. The intraoperative amylase levels of the bile juices both in the common bile duct and the cystic duct were high. A cholecystectomy was performed. The wall of the gallbladder was markedly thick, yellowish, elastic, and soft. Histologically, Rokitansky-Aschoff sinus proliferation, hypertrophy of smooth muscles, and fibrosis were seen. The diagnosis was a generalized type of adenomyomatosis. The pathogenesis of the adenomyomatosis was believed to result from chronic stimulation as a result of pancreatic juice reflux. The etiology of this unusual type of junction was considered to be the result of the combination of pancreaticobiliary maljunction and an anomaly of lower junction of the cystic duct.