Attention was focused on whether the incidence of gastric cancer could be decreased by eradication of Helicobacter pylori (H. pylori) infection after H. pylori infection was found to be an important risk factor for gastric cancer,. The Japan Gast Study Group (JGSG) planned a randomized study, with the primary endpoint being the development of gastric cancer after H. pylori eradication. To design a study with a fairly small sample size and short follow-up period, the conclusion was reached that the study should be conducted in patients who had undergone endoscopic mucosal resection (EMR) for early gastric cancer because they have the highest incidence of recurrent gastric cancer. There were no differences of age, gender, tumor site, histology, tumor depth, and tumor size between the two groups. The incidence of metachronous gastric cancer in eradication group was significantly lower than in control group in the analysis ignoring observation period (Odds ratio: 0.353, 95% CI: 0.161-0.775, p=0.009). Kaplan-Meier analysis revealed that the cumulative incidence of gastric cancer was different between the two groups. The Japanese Society for Helicobacter Research has published a guideline recommending that H. pylori infection should be treated by eradication therapy following this study. New evidence obtained from our study in Japan may be helpful for the prevention and eventually elimination of gastric cancer worldwide.