The aim of the current research is twofold: 1. Present a theoretical approach to study mathematics teacher knowledge and the conditions for developing it in direct relation to teaching practice. 2. Present and analyse a case of open lesson using this theoretical approach. In order to conduct their analysis, the authors have developed a new technology about the open lesson practice and some elements of theory as well. For this study, the authors collected data from three sources: the lesson plan of the teacher; the real-time observation of the lesson; the discussion after the lesson. In conclusion, the current study indicates why open lessons represent, to Japanese teachers, an attractive element of a professional learning community of teachers.