In the case of reactivated landslides, the clay particles become oriented along the slip surface due to repeated sliding and, in the case of gravelly clay, it is assumed that the gravel also tries to change its orientation with respect to the slip surface. As a slip surface composed of gravelly clay, due to the formation of clay brought about by weathering and repeated sliding, causes fewer landslides than a slip surface composed only of clay. It is thought that gravel influences the strength properties of the clay. Therefore, the effects of gravel on the slip surface and the shear properties were clarified in this study, and the extent to which gravel can improve the insufficient shear strength of the clay on the slip surface was examined. The shear properties of kaolin and kaolin-mixed gravel specimens were compared in ring shear tests under different normal stresses, and the transition of gravel during shearing was observed using X-ray CT scans. Both peak strength and residual strength were seen to increase with an increasing gravel content. It was also found that the difference between peak strength and residual strength decreased with an increasing gravel content. The X-ray CT scans showed that the gravel was aligned parallel to the shear surface after shearing. Therefore, it is expected that mixing gravel into the clay of the slip surface, where landslides are likely to occur, will prevent landslide events because gravel resists sliding and helps maintain strength, due to interlocking and friction.