A 25 year-old woman experienced a sudden onset of epigastralgia with nausea, and consulted our hospital. Because the abdominal pain did not subside with medication, she was hospitalized. On physical examination she had a slight tenderness of the right upper abdominal quadrant. Laboratory studies disclosed increases in the serum alkaline phosphatase, glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, and serum amylase levels. Abdominal ultrasonography, computed tomography, and endoscopic retrograde cholangiopancreatography revealed choledocholithiasis and a pancreatic duct which originated from the common bile duct. A common bile duct stone was removed with a basket catheter after an endoscopic sphincterotomy was performed. Since an anomalous union of a pancreatobiliary duct is a high risk factor of gallbladder cancer, laparoscopic cholecystectomy was perfomed. The post-operative course was uneventful and she was discharged on the twentieth post-operative day. In a microscopical examination of the resected specimen, a pyloric type gastric mucosa was clearly evident in the submucosa, while the remaining gallbladder demonstrated chronic cholecystitis. Some cases of heterotopic gastric mucosa in the gallbladder come from metaplasia, and metaplasia is also one of the most important factors in the carcinogenesis of gallbladder cancer. In conclusion, the present case is the first report of gastric mucosa with an anomalous union of the pancreatobiliary duct. Heterotopic gastric mucosa in the gallbladder may be one of the causes of gallbladder cancer, and close attention should, therefore, be paid to any occurrence of heterotopic gastric mucosa in this region.