The experiment in that the landslide was artificially made to occur by the excavation at the lower part of slope and to pour water into the ground has been done on the natural slope in the mudstone area in the Tertiary. And the process of shearing deformation of the ground was grasped as the early stage of the occurrence of landslide. Consequently, in this experiment it was presumed that the rise of porewater pressure and ground-water level did not influence directly on the occurrence of landslide and the lowering with time of the strength of the ground affected it. Furthermore, assuming that the ground showed the behavior of the Voigt body during the early two days and after that the Bingham flow and the yield stress lowered with time, the phenomena were explained for the present.