To predict the runout distance and inundated area of rainfall-induced debris flows, we develop a depth-averaged two-dimensional numerical model, in which the debris and water mixture is assumed to be a continuous, incompressible, unsteady fluid. The model is based on the continuity equations and Navier-Stokes equations. As raster grid networks of digital elevation model in Geographic Information Systems (GIS) can be used as the finite difference mesh, the continuity and momentum equations are solved numerically using the finite difference method. All the input and output data are processed in GIS. The model is applied to simulate the rainfall-induced debris flow of July 20, 2003, in Minamata City, southern Kyushu, Japan. The simulation reproduces the propagation and deposition of the debris flow and the results are in good agreement with field investigations. The two-dimensional numerical simulation of rainfall-induced debris flows using GIS can be used to estimate the flow range, and to define potentially hazardous areas for homes and roads.