Recently, in Japan, heavy rainfall observed during very short period, so-called Guerilla rainfall, which may be considered as one phenomenon of climate change, has highlighted as one of serious natural hazards that may cause slope failure. It should be considered that in the case of heavy rainfall, slope stability against rainfall heavily depends on amount of rainfall, which infiltrates into subsoil. Therefore, it is very important to develop a method that can clarify amount of rainfall infiltration into subsoil from total amount of rainfall, which is one of dominant factor on slope stability, and investigate mechanism of rainfall infiltration into subsoil. From such a viewpoint, this study discussed field data of surface flow, variation of soil moisture content and variation of pore water pressure monitored at investigated slope, Nakhon Nayok, Thailand.