My PhD project is about arithmetic teaching in supplementary educational institutions for Japanese children abroad. Children attend regular schools on weekdays (following a non-Japanese curriculum) and then come to supplementary schools on Saturday. At a general level, research has identified various problematic issues due to the increasing diversity of children’s backgrounds. However, little/no research has focused on didactic (content-related) aspects of those difficulties, like how the students cope with following two different curricula of mathematics. Arithmetic was chosen to focus the study on a crucial domain, present throughout primary school and structured differently in different school systems. The theoretical framework for this project is based on the anthropological theory of the didactic (ATD), particularly the notion of praxeology. We conduct a praxeological analysis of arithmetic curricula in Japan, Denmark and Sweden, and based on the resulting reference model, I will analyse classroom observations and interviews with teachers and pupils of the Japanese supplementary schools in Denmark and Sweden.