Every year, heavy monsoon rains cause numerous slope failures, especially in developing countries in Asia. The authors have previously developed a simple method for predicting groundwater rise and rainfall infiltration processes on natural slopes with relatively shallow depths, based on parametric studies using the finite element method assuming a semi-infinite homogeneous slope. The vertical infiltration and lateral seepage flow were numerically modeled. In the present study, an analytical method that considers the surface flow and the subsurface flow is proposed. A recent disaster in Japan is used to validate the proposed method. The proposed method has four modules: (1) infiltration process, (2) groundwater flow in vertical and lateral directions, (3) surface water flow, and (4) slope stability assessment. The proposed model allows rainfall-induced slope failures to be simply predicted for broad areas based on an infinite homogeneous slope assumption and a simplified hydrological process. The results confirm that the proposed method can simulate actual phenomena with high accuracy.