The clay layer in the landslide face is about 3mm thick. Accordingly, the constants of shearing strength are as follows;c'≈0.φ'>0The reason is that the shearing conditions of sliding face are not consolidated-undrained but consolidateddrained. Therefore, the safety factor of landslide face can be expressed as;Fs=tanφ'∑ (W cos θ-uw) /∑W sin θThe pore water pressure Uw acting on the slide face, has direct effects upon the occurrence of the landslide which is caused by a heavey rain.When the sliding movement passes a certain value, the c' and φ' are changed by the negative dalatancy etc. So the safety factor Fs is given by the followng formula, Fs=c'L+tanφ'∑ (W cos θ-uw+u) /∑ W sin θIn the formula, U shows excess hydrostatic pressure.In the sliding, when the safety factor becomes fixed after reaching a certain value, it doesn't increase any more even if the pore water pressuer becomes higher.