Research on forecasting of slope failures has been carried out in various fields. A critical line is the boundary that considers rainfall either as safe or dangerous and it is generally used as a criterion for populace evacuation. However, the conventional forecasting is not accurate because most forecasts are based on linear critical line. In this research, the methodology to determine non-linear critical line by using Radial Basis Function Network (RBF Network) is proposed. Forecasting results using linear and non-linear critical lines are compared. Furthermore, criteria to determine warnings for evacuation are discussed. Finally, the proposed forecasting that used unexamined data is carried out to reaffirm the forecasting by non-linear critical line.