This study attempts to interpret the urban form of the Japanese cities through the natural spatial reference of front and back which is deeply rooted in Japanese culture. The aim is to provide workable guidance for interpreting the urban form, and for creating diverse and active commercial spaces in Japan and beyond. Based on the physical built environment, this study proposes a parameter of interface which is defined by components taken from both surface of the building and the objects around. The interface is categorized into four types by the combination of main-component of front (F)/back (B) and sub-component of front (f)/back (b), namely Ff, Fb, Bf, and Bb. The approach is demonstrated in the typical commercial district of Shibuya, Tokyo. By analyzing the distributional features of the interface quantitatively, the scopes and attributes of front and back-side space are figured out. According to the distributional similarity of the interface, we sort the urban space into front-side space defined by Ff and back-side space defined by Fb, Bf, and Bb. Thereinto, the ambiguous back-side space defined by Fb and Bf supports diverse possibilities and highly involves in the everyday life of Japanese people.