Soil has a structure in which various particles are arranged as a result its geological history. Therefore, by determining the current structure of the soil, it is possible to estimate the stress history of the soil and to predict how it will behave. In particular, the orientation of clay particles is a notable feature in landslide clay that has experienced large deformation. Accordingly, the particle orientation should be useful for estimating movement direction, and stress state. In this study, with the aim of deepening understanding of the microscopic structure of landslide clay, the formation process of landslide clay in the landslide zone of a Tertiary formation was investigated by using the ring shear test to compare the shear plane and the slip surface of an undisturbed specimen. Our investigation found that (1) the microscopic structure of shear planes in a residual strength state due to large deformation exhibits slickensides and striations, (2) the shear layer is formed from a folium of peds arranged parallel to the shear plane, and (3) the shear layer has a fully oriented structure differing from that of the dispersed structures in the local area.