Around the mountain stream of Suishou-Valley, Oku-Chichibu, there is an exceptional distribution of a mixed forest of Pterocarya rhoifolia and Chamaecyparis pisifera. This report aims to clarify the reason for this phenomenon in the area, especially with a focus on topographical stability, and to give a consideration to the process of future succession. In the past, P. rhoifolia dominated stands probably existed after a vast disturbance in Suishou-Valley. However, the mixed forest with a few conifers has been currently established. It suggests that the P. rhoifolia dominated forest has been shifted to a C. pisijera dominated forest because of the long term, sustaining stability of site condition.