Abstract
This study reports how a teacher succeeded in improving his epistemological knowledge on student learning through his participation in the design study. The teacher has been involved in our collaborative design studies for several years. We videotaped our research meetings, collected his reports on practices, and interviewed with him. Our analyses manifested that the teacher gradually developed his epistemological expertise through his collaboration with learning scientists in research meetings. Furthermore, our analysis on the interview with the teacher suggested that several variables such as the participatory structure, previous experiences by the teacher, and his academic interests in the design study itself, facilitated his development.