This study analyzed the behavior system of pre-service elementary school teachers’ photosynthesis experimentation performed in the class studying science textbooks and teaching methods using the CHAT method from a sociocultural perspective. Of 11 pre-service teacher groups that participated in photosynthesis experiment activities, this study selected 2—the first due to its highest score and the other due to its lowest score in the peer evaluation—and analyzed their inquiry diaries. The analysis was performed in accordance with the six CHAT components. Each pre-service teacher from Group A who received the highest score in the peer evaluation began with a different subject element, but all of them shared a single perspective on the remaining components and performed photosynthesis experiment activities without any particular conflict or discord, pursuing the goal of developing their specialization as science teachers. It was also found that a similar behavior system enabled them to deduce their scientific results. In contrast, all pre-service teachers from Group B who received the lowest score carried out the photosynthesis experiment using the existing tools and materials according to the official rules, but they experienced a certain amount of conflict and discord. They showed different goals for photosynthesis experiment activities from each other, further showing problems in drawing scientific conclusions. Therefore, when CHAT analysis identified that some pre-service teachers failed to attain the same goal and drew unscientific conclusions, they should be brought to identify the same goal, most of all, through discussion between group members, and they should be provided with a chance of trial and error so that they could reestablish the subject element of CHAT and successfully draw scientific conclusions.