The research s motivated by a lack of theoretical tools to analyse the Japanese mathematics lessons with their specificities: the approach by problem solving, the collective dimension of the teaching, and the focus on the mathematical thinking. We introduce the concept of collective milieu in order to take into account the collective dimension of teaching, the flow of lesson as a whole, and the epistemological dimension of the lesson. This concept is developed based on the two theoretical frameworks, Theory of Didactical Situations on the one hand and the Anthropological Theory of the Didactic on the other, which share the same notion of milieu but in different ways. In this paper, after presenting the theoretical tool, we will exemplify in one case of Japanese lesson how and to what extent the specificities of Japanese lessons could be characterized with this tool.