An increasing amount of damage results from debris flow in urban areas and mountainous regions covering most of the lands in Korea. It is believed that the analysis of the rainfall that gave rise to debris flow needs to be preceded in order to reduce damage from mudslide. The Spatial Runoff Assessment Tool (S-RAT) model, a distribution-type rainfall–runoff model, was adopted to analyze the rainfall causing debris flow. In order to estimate debris flow expansion, the couple modeling was performed using the Rapid Mass Move Simulation (RAMMS) model, a two-dimensional numerical analysis model for debris flow. In addition, for the correction of the aerial images on actual areas damaged by earth and sand, the optimal parameters were estimated by the NSI method, including sediment concentration (Cv), flow velocity (V), viscosity–friction coefficient (Mu), and turbulent friction factor (Xi), The result showed that the frequency of the actual rainfall causing the Umyeonsan landslide was more than 200 years, and the damaged area around the Raemian apartment was simulated as 65,430 m2,with the frequency of 200 years.