International comparative studies on mathematics teaching and learning often provide unitary and harmonious images of classroom practices. This paper aims to complement those studies by describing the complex aspects of those practices. Adopting activity theory as a framework, this paper considers classroom teaching and learning of mathematics as an activity system embedded in socio-historical contexts, analyzes its contradictions, and makes a comparative analysis of different practices of mathematics teaching and learning in the same country. As a case in point, mathematics lessons in Japanese secondary schools are considered. The initial activity system of mathematics lessons in Japanese modern schools came from the whole-class instruction system and then gradually evolved into the problem-solving style lesson. The latter has been facing challenges, and then a new activity system has emerged from a new paradigm of school education. It is argued that activity systems analysis enables us to understand the contradictions prevailing in mathematics education and that comparative analysis of more than one system in the same culture can provide valuable insights.