This chapter examines the development of school reform at Hiromi Elementary School from 1998 to 2000 and elucidates the theory of action in school reform for School as Learning Community. It focuses mainly on Principal Masaaki Sato. His theory of action in school reform for SLC was formed and comprised the children’s “active, collaborative and expressive learning”, the articulation of “espoused theory” and “design theory” of school reform, and the difference between “theory through practice” and “endless ‘excellence practice’” based on Manabu Sato’s theory. Further, in contrast to Masaaki Sato’s pursuit of learning, Naohiro Miyazaki, Director of Professional Development Department, stressed the uniqueness of the pursuit of learning as captured by the department and individual teachers because “organizational learning” at Hiromi Elementary School developed with respect for and emphasis on the diversity of the “theory in use” of learning.