This paper is a historical reflection on the boundary making concept of etranger (foreigners) in French Indochina. It focuses on the case of Japanese migrants present in the Indochinese colonial society. In the first part is introduced the main aspects of the history of Japanese migrants in French Indochina between the years 1882 and 1952. Next, the chapter seeks to define the concept of "foreigners" from the point of view of colonial power by looking at the evolution of the legal definition. This work tries to show the importance and the complexity of fixing the status of foreigner for the colonial power. Then, the concept of etranger is analyzed at the meso level, that of interactions between communities of Indochinese colonial society, especially in the cities of Hanoi and Saigon. Finally, in the last part, the concept of etranger is looked at the micro level by presenting the trajectories of various Japanese migrants who had spent their life in Indochina. The chapter brings, with these three levels of analysis, elements to understand how the concept of etranger is polysemic and how populations categorized as such are nevertheless social actors in the colonial society.