A 79 year-old man was admitted to our hospital because of upper abdominal pain and nausea. A mobile tumor was palpable in the right upper abdomen. Abdominal ultrasonography, computed tomography and celiac angiography revealed a gallbladder tumor. Endoscopic retrograde cholangiopancreatography revealed a fistula 1.5 cm oral to the orifice of the papilla of Vater, dilatation of the common bile duct, and a filling defect in the gallbladder. Pancreatoduodenectomy associated with reconstruction using Imanaga's method was performed under a pre-operative diagnosis of gallbladder carcinoma with choledochoduodenal fistula. The gallbladder contained a tumor and two bilirubin stones impacted in the orifice of the duodenal papilla. Histological studies confirmed that the gallbladder tumor was a mucinous adenocarcinoma and had not infiltrated the bile duct. We speculated that choledochoduodenal fistula stimulated the development of cancer due to chronic irritation from pancreatic juice reflux.