This paper is based on the records of Jeju’s migration in the former half of the 20th Century which can be said to be a tumultuous time in Korea’s history both internally and externally. It aims to go over the background and significance of the migration. With the effort, it intends to find some evidences that may lead into reevaluating our awareness and attitude towards the Jeju Japanese as to why the Jeju people migrated to Japan during those times. What made the JeJu people go to Japan and what made it possible for them to cross over? Three main reasons were deducted from this investigation. First of all, the Jeju people’s migration to Japan was a direct result of the ‘land investigation project’ which was one of the most representative Japanese colonization policy. Secondly, we could take the direct transportation factor which made it possible for the population leak of Jeju into account. The regular passenger liner between Osaka and Japan called the Kimikayomaru(君代丸) was set up. Such launching of passenger liners is also intimately connected with the change in the labor market followed by industrialization within Japan. Finally, we could take the unique characteristics of the Jeju people’s cultural community into consideration. While other Korean Japanese from other regions form relationships by forming organizations from a province, city or county, the people from Jeju formed relationships according to villages and identified themselves with their own region.