In Kagoshima Prefecture, which is located in the southern part of Kyushu Island, Japan, there are a lot of volcanoes such as Mt. Sakurajima, Mt. Kirishima, Mt. Kaimon etc.. Consequently most of the surface ground is covered with various volcanic products. The non-welded part of pyroclastic flow deposits is locally called Shirasu in Japanese that is classified into sandy soil and forms steep slopes. In the rainy season (June similar to September) the slope failures often occur due to heavy rainfall on such steep slopes. In this paper a synthetic system to predict such slope failures and to prevent natural disasters due to rainfall system is proposed based on the combination of field measured data in real time with those obtained by some in-situ tests, the laboratory soil tests and the numerical models.