Ampullary adenoma is a benign neoplasm with malignant potential that arises from the glandular epithelium of the ampulla of Vater. When the tumor is confined to the ampulla, abdominal sonogram and CT scan can show dilatation of the common bile duct or pancreatic duct, but the mass itself may not be seen. And even biopsies are done, the confirmation of malignant change is frequently missed, and it may be impossible to assess the presence of carcinoma in situ or invasive carcinoma without complete exicision of the lesion. So complete surgical resection is recommended because of extensive growth of the ampullary adenoma and its malignant potential. Recently, we experienced a case of about 0.8 x 1.0 cm sized tumor of the ampulla of Vater that was diagnosed as villous adenoma on endoscopic biopsy, and malignant change was found on resected surgical specimen.