We experienced a case of biliary intraductal papillary mucin-producing neoplasm diagnosed before operation by using an ultraslim upper endoscope without assistance of the guide wire. The patient, who had a Billroth II partial gastrectomy and partial resection of left liver lobe history, was admitted in our hospital because of jaundice. Imaging examination indicated dilatation of the intrahepatic and extrahepatic biliary ducts and pancreatic duct was normal. During the process of endoscopic retrograde cholangiography, a large amount of mucus was found extruded from the dilated orifice of papilla. A pediatric upper endoscope was inserted into the common bile duct and then into the left hepatic duct without the assistance of guide wire. The lesion was located and biopsy was taken. Final histology after surgery indicated the patient suffered from biliary intraductal papillary mucin-producing neoplasm. Slim upper endoscope maybe a useful instrument in some patients for peroral direct cholangioscopy, instead of the sophisticated mother-baby endoscope system.