This study aims to specify the essential nature of a child-friendly educational environment through an analysis of the Children's Houses for 3- to 6-year-old children planned by Maria Montessori whose philosophy and method have spread all over the world. Based on her theory of how children grow and develop, Montessori designed an educational method along with a unique physical environment in which to put it into practice. In the present study, an intensive content analysis of 12 Montessori's books, consisting of her own writings and dictations of her own lectures, was conducted and revealed: (1) Montessori's grand theory of child development governed her educational method and its physical environment which was essentially planned to evoke and support children's competence and autonomy, (2) The 8 properties of the physical environment, which were indispensable to the education of young children, were identified, e.g., the environment must (a) be attuned to children's body size and their physical and cognitive abilities; (b) be responsive to children's behavior; and (c) activate children's spontaneous activities, (3) These properties were surely embodied in the physical environment in all of the Montessori Children's Houses. What we can learn from Montessori, now and for the future, is discussed.