Since the introduction of the effective rainfall method by Suzuki and Kobashi (1981) in the field of sediment hydrology, there have been many studies on the occurrence of debris flow and slope failure using the effective rainfall method. These studies can be divided into two types: those related to the prediction of sediment‐related disasters and analytical studies on the mechanism of sediment disasters. The advantage of using the effective rainfall method for analytical studies of slope failures is that the method is closely linked to practical tank modeling and is now an important model for groundwater analysis and for estimating water storage in hillslopes. However, the history, significance, and role of the effective rainfall method are still unclear. In this paper, the origin of effective rainfall, its relationship with the tank model, and the role of effective rainfall on slope failure are discussed. First, it is shown that the distinction between the effective rainfall and the antecedent‐precipitation index (API) is based on whether or not the tank model is taken into account. Next, it is clarified that the effective rainfall can be derived by coupling the tank model to the unit hydrograph method. The tank model is also shown to be a model that represents the water storage at a representative point in a saturated subsurface flow in the soil layer of hillslopes. It is shown that the current effective rainfall corresponds to the water storage capacity by saturated subsurface flow. Finally, the significance of the effective rainfall and the one‐stage tank model for slope failure is discussed by applying the model to sediment‐related disaster cases. As an application, it is shown that slope failure can be classified into four types based on the occurrence of water storage and the drainage conditions of the soil‐slide surface.