This study presents a decision support system for flood-control facility planning. The system is composed of two models: a flood inundation prediction model (Model 1) and a flood damage estimation model (Model 2). Model 1 calculates the inundation depths of a river basin with and without a flood control facility for any given storms. Model 2, using the depths obtained by Model 1, computes the damage amounts of the public and private properties as a function of inundation depth. The analysis using the two models yields the reduction of damage as an amount of money owing to the flood-control facility. The models were applied to flood damage simulations for the Kanda river basin with highly urbanized catchment located in the Tokyo Metropolis. The simulations yielded the damage reductions in two cases: the maximum experienced storm by Kanogawa typhoon in 1958 and a set of several storms with different return periods. In the simulation, the GIS data on the assets in the Kanda river basin was fully utilized to calculate the damage of the assets because of the inundation. The estimated reductions of flood damage signified the effectiveness of the flood-control channel project.