This study investigates the characteristics of Japanese primary school mathematics lessons that adopt a problem-solving approach. We argue that these characteristics are reflected in three key aspects: collective teaching and learning, the lesson as a "drama" (i.e., its structured flow), and the focus on mathematical knowledge. In this paper, we refer to these aspects as the collective dimension, chronological dimension, and epistemological dimension of Japanese mathematics lessons, respectively. Using the Anthropological Theory of the Didactic (ATD) as a theoretical framework, we analyze the mathematics lessons of a Japanese primary school, focusing on the evolution of "milieus" and the three geneses of mathematical knowledge ("topogenesis," "chronogenesis," and "mesogenesis"). Our aim is to highlight these dimensions and further identify the distinct features of Japanese lessons, as well as to demonstrate how, and to what extent, the evolution of "milieus" effectively captures the structure and dynamics of the Japanese lessons.