The goal of the present study was to clarify the propagation processof a specific idea in classroom and how the process relates to their understanding of lesson content. The target of the analysis was a learning unit consisting　of 7 hours about state of water in science class in which 27 fourth graders attended. In the analysis, keywords related to the specific important idea was chosen. An analytic method was developed to extract the keywords in classroom discourse and track in chronological order. In order to measure the students’ acceptance of the idea, their mental models for state transition of water, which were repeatedly drawn on a sheet of paper, were evaluated. As a result, the following five characteristics were found: (1) the finally accepted idea has speculative nature in the beginning of discussion, (2) teacher’s revoicing acts which emphasize differences among students’ ideas have important role in the idea propagation from a small group to a whole class discussion, (3) the acceptance of new ideas was decided based on the consideration of situational factors, (4) the negative feedbacks offered by peers suppress the acceptance of idea, and (5) Constructive discourse is a necessary condition for student’s personal acceptance of the propagated ideas.