A 71-year-old man was treated for NSAIDs-induced gastric ulcer and esophagogastroduodenoscopy performed. Endoscopic findings identified a small depressed lesion─approximately 5mm in size─at the lesser curvature of the antrum. Biopsy specimens showed Group 2, but using magnifying endoscopy with NBI we diagnosed the lesion as cancer. ESD was performed and histopathological findings showed a minute well-differentiated adenocarcinoma in the surface of the mucosal layer with neuroendocrine carcinoma invasion to the submucosal layer. The cancer detected on pathological analysis was 2.4 mm in diameter, and therefore a gastric microcancer. Due to the presence of approximately 1,000μm invasion in the submucosal layer with concurrent intravenous invasion, laparoscopy-assisted distal gastrectomy with lymph node dissection was performed. The final lesion was diagnosed as pT1b2N0cM0 in Stage ⅠA. Ten months after surgery, the patient had no recurrence of the lesion. A portion of the tubular adenocarcinoma in this case was immunohistochemically positive for CD56. It was considered that sampling in this case detected an initial transitional change from adenocarcinoma to neuroendocrine carcinoma. The fact that this cancer invaded the submucosal layer─despite being only 2.4 mm in diameter─indicated high grade malignancy of gastric neuroendocrine carcinoma.