As collaborative problem-solving skills are increasing in importance, Project-Based Learning (PBL) is being implemented in various ways, but the results differ depending on the participants. This study shows that the results of PBL depend on student interaction type and learning motivation. The difference in the levels of interaction and learning motivation between two groups of students utilizing different discussion processes was qualitatively explored. The participants in this study consisted of 8 students in the 6th grade of elementary school, and the intervention was conducted over the course of 10 lessons. The group for which the leader arbitrarily resolved conflicts had a type of inactive interaction and the level of motivation for learning was outside of the acceptable range compared to the group that took part in a democratic discussion. As the project progressed, it changed to a type of inactive interaction. Thus, the results suggest that the roles of individuals in a group should be adjusted to account for to the characteristics of PBL and the structure of the task; furthermore, an inquiry into how learning motivation changes according to social interactions with others should be conducted.