This chapter focuses on the formation of the theory of action in school reform for School as Learning Community at Hamanogo Elementary School. It attends particularly to the theory of action in school reform undertaken by Toshiaki Ose, the founding principal of Hamanogo Elementary School. Ose desired a theory of school reform that could offer strategies to bridge the gaps between practice and theory while being cognizant of the limits and the challenges involved in the practice. This chapter elucidates that both Ose and Manabu Sato shared an awareness of the theory of school reform. Hamanogo Elementary School was the first pilot school of school reform for School as Learning Community and the pilot school for the school reform plan formulated by the education board of the local government. Significantly, Ose first constructed the “espoused theory”, “design theory”, and “theory in use” for School as Learning Community at Hamanogo Elementary School.