A patient in whom pancreatic pseudocyst after the surgery for traumatic pancreatic injury was successfully treated with endoscopic cystoduodenostomy (ECD). A 27-year-old man beaten on the abdomen with a hammer underwent emergency surgery due to intraperitoneal hemorrhage and free gas development. Pancreatojejunostomy (Letton-Wilson technique) was performed because the pancreas completely ruptured just above the superior mesenteric vein was diagnosed as type IIIb pancreatic injury. Right hemicolectomy was performed because of the extensive perforations and crush of the right side of the colon. A pancreatic juice fistula observed postoperatively was resolved with conservative therapy, and he was discharged on the 40th disease day. Epigastric pain developed about three months after he was injured. Examinations resulted in the diagnosis as pancreatic pseudocyst occurring from the caudal side of the ruptured pancreas. The fluid pooled in the pseudocyst was drained by endoscopically puncturing the elevated site on the posterior wall of the duodenal bulb with a needle knife. Then, all associated symptoms were resolved and the pseudocyst disappeared. Since then, he has been followed for about one year without recurrence, with his pancreatic function being normal. Endoscopic cystoduodenostomy (ECD) can reliably puncture even a small cyst when it is expanded on the wall. It has other advantages that it can easily change a fistula to an internal fistula even in cases with the communication to the pancreatic duct, such as the present case, and that it is unlikely to produce a pancreatic juice fistula. Therefore, ECD is considered easy, safe, and effective for treating pseudocyst after pancreatic injury.