We report the case of a ball-valve gastric tumor associated with anomalous junction of the pancreatico-biliary ductal system (AJPBDS) and a right-sided round ligament, misdiagnosed preoperatively as advanced gastric cancer with pancreatic head invasion. A 72-year-old woman presented with chest pain, but laboratory data showed only anemia. Gastroscopy revealed a bleeding polypoid gastric tumor in the anterior wall of the stomach, herniating into the duodenum (ball-valve syndrome), and a Bormann type-2 tumor in the posterior wall. Ultrasonography showed gallbladder stones, dilatation of the intrahepatic bile duct and pancreatic duct, and a left-sided gallbladder (attributed to a right-sided round ligament with anomalous branches of the portal veins). Laparotomy revealed that the gastric tumors were not advanced cancer invading the pancreatic head. Intraoperative cholangiography showed an AJPBDS, causing dilatation of the intrahepatic bile duct and pancreatic duct. We performed distal gastrectomy and cholecystectomy without biliary diversion. Microscopy revealed that the polypoid tumor was a hyperplastic polyp.