The purpose of present study is to clarify runoff mechanism and to understand hydraulic processes in mountainous basins. Considering multilayer flow in a slope, generation of overland flow for large scale flood is important. The method of multilayer runoff analysis which can calculate overland flow, subsurface flow and vertical infiltration flow based on soil property and rainfall form is newly proposed in this paper. Characteristics of overland flow in mountainous basins can be expressed rationally using our method. A method is applied to the Kusaki dam basin (254km2). The results of runoff analysis match well with the observed data in Kusaki dam basin and overland flow is expressed for peak time in a large scale flood.