A 71-year-old man who tested positive in a pepsinogen test also underwent upper gastrointestinal endoscopy. A 0-IIc lesion was identified at the posterior wall of the antrum. Biopsy findings indicated endocrine cell carcinoma. The patient underwent distal gastrectomy, and subsequent histopathological examination of the resected specimens showed a moderately differentiated adenocarcinoma (tub2) in the mucosa with transformation to small cell carcinoma in the submucosal and muscle layers. The final diagnosis was of a mixed adenoneuroendocrine carcinoma. We herein describe a case report of this patient with this rare form of gastric carcinoma.