A 58-year-old Japanese man was diagnosed with differentiated adenocarcinoma of the stomach. Histological findings of the resected specimen revealed well- to moderately-differentiated tubular adenocarcinoma (tub1, tub2), 13 mm in diameter, which invaded into the submucosa (SM1, 300 μm) and lymphovascular lumen (ly1). Serum antibody against Helicobacter pylori (Hp) and the (13)C-urea breath test were negative, and there were no atrophic changes in the tumor-adjacent mucosa. The immunohistochemical analysis showed that gastric mucin (MUC5AC) was strongly positive and intestinal mucin (MUC2) was weakly and partially positive. According to these results, the final diagnosis of Hp-negative well-differentiated early gastric cancer was made.