A 9-year-old male patient was admitted due to projectile vomiting and epigastric abdominal pain for 15 days. He presents no jaundice, no abdominal mass, and no hepatosplenomegaly, but presented severe epigastric tenderness on palpation. Abdominal ultrasonography showed diffuse swelling of the pancreas with diffuse dilatation of the intra- and extrahepatic bile ducts. MR cholangiopancreatography (MRCP) revealed diffuse dilatation of the common and left main bile ducts without anomalous union of pancreatobiliary duct (AUPBD), and we diagnosed type IVa choledochal cyst which is not associated with AUPBD. We report a case of type IVa choledochal cyst which was presented with acute pancreatitis.