This paper focuses on in-school teacher training (konaiken) based on social studies which is practiced at an elementary school in Western Japan. The purpose of this paper is to clarify why this elementary school chose social studies as main subject for their in-school teacher training. In order to answer this question, observation, interview, and discourse analysis techniques were employed. The analysis of konaiken reveal several key points. Firstly, the system works for promoting more comprehensive ability with the subject pedagogy. Secondly, the system strengthens the teacher group's collegiality. Finally, the system enhances the relation between the school/teacher and the neighbor communities/residents. Additionally, the system supports the professional development for educating mentor teachers. The authors conclude that the reason why the school focuses on social studies is because the school principal can reconstruct the meanings of the purpose, comprehensiveness and potential effects of the elementary social studies for the school reform, and social studies is considered a weak subject by most teachers, so they can engage in the professional development as beginners equally. These findings imply that the advantages of or preferences for each subject as in-school teacher training need to be considered carefully.