The occurrence of an adenoendocrine cell carcinoma on the ampulla of Vater is rare, especially when the component of adenocarcinoma is not located on the mucosa of the ampulla. A 76-year-old man was referred to our hospital for further investigation of a mass lesion on the ampulla. EGD revealed SMT like mass lesion on the ampulla. Endoscopic ultrasonography showed an ampullary hypoechoic mass. We performed pylorus-preserving pancreatoduodenectomy on the basis of the diagnosis of poorly differentiated adenocarcinoma of the ampulla of Vater. Postoperative pathological examinations revealed two different components of the tumor;malignant endocrine cells, and adenocarcinoma. The component of adenocarcinoma was located on the Ap lesion. We deducted that the adenocarcinoma appeared on the epithelium of Ap, then grew and spread into the direction of duodenum lumen, degenerating to endocrine cells.