Intraductal ultrasonography (IDUS) were performed in patients with extrahepatic bile duct cancer and compared to other diagnostic modalities and to resected specimens. Endoscopic ultrasonography (EUS) is a non-invasive diagnostic method useful for screening patients with bile duct cancers and determining whether they are resectable or not. While, EUS was not useful for the differential diagnosis of advanced and early tumors, and less useful in case of bile duct tumors located at the hilus hepatitis. IDUS proved useful without blind spot even in case of bile duct cancers at the hilus hepatis. IDUS was especially useful for the differential diagnosis of advanced and early tumors. IDUS is the very accurate diagnostic modality which make up for EUS and essential to determine the appropriate operation plan.