Almost all gastric cancers including intestinal type and diffuse type develop from background of H. pylori infection of the gastric mucosa. In treatment of gastric cancer, the development of endoscopic technique expanded the indication of endoscopic treatment for early gastric cancer. Residual cancers due to incomplete resection occur within two years after endoscopic treatment. However, metachronous cancers continue to occur for long-term after endoscopic treatment of primary cancer. Multi-center, randomized controlled trial in Japan showed that H. pylori eradication significantly reduced the incidence of metachronous cancer after endoscopic resection. Severe atrophy is high risk factor on metachronous gastric cancer after successful eradication. Because H. pylori eradication after endoscopic treatment of early gastric cancer have been covered under public health insurance, positive eradication therapy for H. pylori infection is expected to reduce metachronous cancer.