A flat, elevated lesion measuring 5 mm in diameter was found in the gastric body of an 80-year-old man. A biopsy showed moderately differentiated adenocarcinoma, and endoscopic ultrasonography revealed a hypoechoic mass located in the submucosa. Endoscopic submucosal dissection was subsequently performed, and a pathological examination revealed a tumor composed of adenocarcinoma and neuroendocrine carcinoma with submucosal infiltration. The pathological diagnosis was gastric mixed adenoneuroendocrine carcinoma (MANEC). An additional gastrectomy procedure was performed, and no recurrence was noted for at least three years. This case is interesting with respect to the carcinogenesis of endocrine cell carcinoma and MANEC.