This study reports how a teacher in the design research succeeded in developing his epistemological perspective which finally led to the development of pedagogical content knowledge on knowledge building. In the design study, teachers collaboratively plan, do and reflect on their lessons with learning scientists and subject matter experts. We see this practice as a new lesson study where teachers are involved in reciprocal cognitive apprenticeship with collaborative experts. The analysis of discourse by a teacher in the design meeting, his reports on lessons he was involved in, and regular interviews with him clarified several issues for the success.