We report a patient (an 80-year-old woman) with anemia and fecal occult blood, who had an emergency operation for carcinoma of the cecum (well-differentiated adenocarcinoma without local lymph node metastasis). Postoperative magnetic resonance cholangiopancreatography, cholangiography, and upper gastroduodenal endoscopy showed a tumor of the ampulla of Vater, and pylorus-preserving pancreatoduodenectomy was performed. Histology of the resected tumor was that of small-cell carcinoma, and immunohistochemistry showed positive staining for neuron-specific enolase, chromogranin A, and synaptophysin, confirming the neuroendocrine nature of the tumor. As the histology of the tumor was distinct from cecal carcinoma, and no tumors were found in other organs, the tumor was diagnosed as primary small-cell neuroendocrine carcinoma of the ampulla of Vater. The patient died due to multiple liver metastases of the carcinoma of the ampulla of Vater 7 months after the pancreatoduodenectomy. The clinical and morphological features of this disease have been reported in nine individuals previously.