The purpose of this study is to refine Japanese elementary science activity structures by using a CSCL approach to transform the classroom into a knowledge-building community. We report design studies on two science lessons in two consecutive years and describe the progressive refinement of the activity structures. Through comparisons of student activities on- and off-line, it was found that the implementation of a CSCL environment facilitated students' idea-centered activity. The task requirement for students to engage in collective and reciprocal activities reflecting on their own ideas was also effective if it required students to use their conceptual understanding for producing something concrete.