A 58-year-old woman was found to have a 45-mm abdominal mass adjacent to the pancreas on screening ultrasonography, and subsequent esophagogastroduodenoscopy revealed a small gastric cancer (13 mm in diameter). We initially performed endoscopic submucosal dissection (ESD), and routine histological examination of the specimen sectioned at 2-mm intervals revealed a poorly differentiated adenocarcinoma and signet ring cell carcinoma confined to the mucosa without lymphatic-vascular capillary involvement or ulceration. These findings satisfied the expanded criteria for ESD we reported recently. We next performed laparoscopic excisional biopsy of the abdominal mass, and histological examination with immunohistochemical staining revealed a metastatic lymph node (LN) resulting from the gastric cancer. Distal gastrectomy with extended lymphadenectomy was then performed and histological examination indicated no residual cancer cells or any additional LN metastases. After the above-described clinical course, further analysis was conducted because of the highly unusual nature of this case; 60 additional deep-cut sections from the resected specimen were performed, with one section showing lymphatic involvement in the mucosa on hematoxylin and eosin staining. This case suggests practical limitations in determining lymphatic involvement through routine histological examination, which may not always be able to detect LN metastasis.