In this paper, I took up the pamphlet called 'the details of housing improvement' presented by the Union for the Improvement of Living Conditions in the Taisho period of Japan, and studied the theory for floor planning of houses that were shown to the pamphlet. The principle of room composition was classifying main rooms for public and private spaces, and the principle of plane constitution was segregating irregularly territories for public and private spaces. By the principle of plane constitution, the dwelling house was built as that of the central living-room type.