Choledochocele is the least common type of choledochal cyst. Choledochocele has a potential for carcinogenesis, but only a handful of cases with concurrent ampullary carcinoma have been reported. We recently experienced a case of choledochocele with tubulovillous adenoma with focal high-grade dysplasia, which was successfully and completely resected en bloc via endoscopic papillectomy. Prior to endoscopic papillectomy, the endoscopic ultrasonography was able to accurately diagnose the ampullary tumor arising in choledochocele. Herein, we would like to report this case and discuss the potential of malignant transformation and the usefulness of endoscopic ultrasonography in choledochocele.