Congenital biliary dilatation (CBD) is a congenital malformation of focal dilatation of the extrahepatic bile ducts, including the common bile duct, and is often associated with pancreaticobiliary maljunction (PBM). In this article, we report a CBD case that presented with focal dilation of the common bile duct without PBM (Todani's classification type Ib). The patient was a 32-year-old man who visited a doctor with a chief complaint of abdominal distension. Computed tomography revealed cystic dilatation of the common bile duct, and the patient was referred to our institution. Magnetic resonance cholangiopancreatography showed cystic dilatation of the common bile duct with a maximum diameter of 7 cm; however, evaluating the presence of PBM was challenging. Endoscopic ultrasonography showed small gallstones and debris in the dilated common bile duct and no thickening of the gallbladder wall. Endoscopic retrograde cholangiopancreatography revealed no PBM or markedly elevated bile amylase levels. Based on these findings, the patient was diagnosed with Todani Type Ib CBD. Since this patient did not have pancreatobiliary reflux, it was unclear whether the risk of developing biliary tract cancer was high, and since the treatment was highly invasive, the decision was to follow up without surgical treatment.