This article describes the history of the development of endoscopic diagnosis of early gastric cancer in Japan. The definition of early gastric cancer and the classification have opened up the research of early gastric cancer. With the continuous progress of endoscopy technology, the screening of early gastric cancer has gradually transitioned from X-ray to endoscopy; the diagnosis of lesion area and depth of invasion has become more refined; the morphological characteristics of early gastric cancer found under endoscopy have become more superficial and small. Understanding these changes can help improve the diagnosis of early gastric cancer under endoscopy.