We report the first known case of an annular pancreas associated with pancreaticobiliary maljunction without cholangiectasis in an infant, aged 2 years and 5 months in Japan. Only two other cases have been reported in Japan both of which were in adults. In our case, the main clinical features were abdominal pain, vomiting and an increasing level of plasma amylase. Endoscopic retrograde cholangiopancreatography (ERCP) was not successful in demonstrating the pancreaticobiliary maljunction due to duodenal stenosis. At operation, a complete type of annular pancreas was found with no enlargement of the common bile duct. We could visualize the pancreaticobiliary maljunction using cholangiopancreatography from the gallbladder during the operation. We then performed duodeno-duodenostomy (side-to-side anastomosis, diamond anastomosis) and portal jejunostomy (Roux en Y anastomosis). The infant was discharged in a good condition at 13 days after the operation. The pattern of the pancreatic ducts was Millbourn's 2a and the type of the duct in the annular portion was Yumura's type I. These results correspond to Lecco's hypothesis that the ring formation originates from the ventral pancreas. It has been further suggested that the cacogenesis and/or dysplasia of the ventral pancreas plays a role in the development at the stage of the pancreaticobiliary maljunction.