Moutain side ditch is constructed at the top of cutting slopes around road and it drains the surface water that flowed from upper part. About 70% of a country is composed of mountains so occurrence of cutting face by road general observation is necessary. Instability of cutting face is increased by permeation of underground water by rainfall the summer, pore water pressure increases. However, moutain side ditch is constructed on the top of cutting slopes has some troubles. For example, difficulty of qualify control and lack of drainage faculty. Therefore, stability variation of cutting slope is analyzed by hydraulic conductivity and construction depth of mountain side ditch and effective depth of mountain side ditch is decided in this paper.