The system of preservation district for groups of historic buildings is important for preserving the cultural values of historic cities. In order to promote the conservation of these values, this paper analyses the Floor Area Ratio (FAR) and Building Core Ratio (BCR) in each block and lot of Imai cho preservation district. As conclusion, it is important to know the relation of the FAR and BCR with the historic buildings in 3 terms: historical architectural function, plot plan structure, and historic development of the urban structure. Both indicators help to understand the historic development of Imai-cho, and may be used as an important tool for the conservation of urban historic area.