We report a case of biliary cystadenocarcinoma of the liver with superficial spread to the extrahepatic bile duct. Preoperative endoscopic retrograde cholangiography revealed communication between a 4.5-cm cyst in segment 4 of the liver and the bile duct. From the findings obtained by peroral cholangioscopy and intraoperative cholangioscopy, the granular mucosa in the bile duct was diagnosed as superficially spreading cancer. The right posterior segmental bile duct and the right anterior segmental bile duct were resected at the point where the spread of cancer was no longer traceable and left lobectomy plus caudate lobectomy was carried out. This achieved radical resection, leaving the resected margin of the bile duct free from cancer. Histopathologically, well-differentiated papillary adenocarcinoma was found on the inner surface of the cyst, and the cancer had superficially spread from the cyst to the extrahepatic bile duct via the 2.5-mm diameter communication between the cyst and bile duct. The cancer was limited only to the mucosal layer all over the lesion. When performing radical surgery for biliary cystadenocarcinoma, it is recommended that cholangioscopy be performed to examine whether the cancer has superficial spread to the extrahepatic bile duct or not. Bile duct resection should be carried out, depending on the extent of the superficial spread, so that the resected margin of the bile duct is free from cancer.