Primary duodenal adenocarcinoma is a rare malignant neoplasm accounting for 0.3% of all gastrointestinal tract carcinomas. We herein present one case of duodenal adenocarcinoma after duodenal neuroendocrine carcinoma. Poorly differentiated duodenal neuroendocrine carcinoma with liver metastasis (TxNxM1) was confirmed, and eight cycles of palliative chemotherapy (5-fluorouracil/etoposide/cisplatin) were administered. The patient was then in a clinically complete response status. About 1 year later, newly developed adenocarcinoma was detected at the same site. It was completely surgically resected, and the patient was cured.