The author induced one dimensional Analysis of Slope Stability based on the results of Analysis of Slope Stability-I (Shinsabo. No. 85. 1972) which expresses the basic nature of the mechanism of landslide in the simplest style. The Analysis is able to give the analytical explanation to the phenomena in the indoor experiments which the sliding surface of landslide is formed near the underground water surface under the presence of its water surface parallel to the ground surface. The author presented the equation deciding whether the sliding surface of landslide is formed on the underground water surface or the upper surface of the impermeable layer, these two cases are possible to occur, when the slope is made of one permeable and impermeable layer. It is guided as one variation of the Analysis that the landslide with a part of its sliding surface on the underground water surface is able to occur without the removing of sand particles by piping phenomenon, when the underground water surface intersects the ground surface or approaches to it.