Rainwater retention tank systems are designed to capture rainwater from roof surfaces and store it as an effective water supply for later use. Such systems are known as RSD (Rainfall-Storage-Discharge) systems because they involve three processes: rainfall, storage, and discharge. Here we develop a design method for each process and use the water balance for the tank to estimate the control effect for a given volume of rainwater tank retention. We also develop an algorithm for calculating the nonlinear behavior of the stored water volume in the rainwater tank and the simulated flow variation of the tank. The simulated results are represented in the form of tank capacity-peak outflow curves. In the case that a storage tank of 10 m(3) per 100-m(2) area is installed for the roof surface of all buildings in a small area, the results show that the peak flow and total outflow are reduced by up to 20% and 18%, respectively. Thus, RSD systems are useful for controlling runoff from rooftops and effective in reducing the final discharge rate of an urban drainage system.