It is general to design the sewer network in current urban basin to calculate the minimum construction cost depending on sewer route setting of the shortest distance, and precedent researches on optimal design of sewer network is focused on cost-benefit of sewer network. Also, regional torrential rains currently occurred frequently increase the risk of local heavy rainfall in the urban basin with increasing of impermeable areas due to urbanization while exceeding of discharge capacity in the urban basin sewer network, so limitations on existing sewer design have came to the fore. Thus, this study has control the flow changes in pipes as the function objective that can be controlled depending on configurations of sewer through controlling of superposition effects between runoff hydrograph to determine the minimum overflow volume optimal sewer network focusing on that optimal sewer network to maximize the flooding prevention effect, and through application of excessive rainfall events by design frequency 10 years, reduction effects of inundation have been analyzed. Resulting in that, in case of overflow volume minimum sewer network, among 33 nodes, 20 sewers were connected to other direction than current sewers, and overflow volumes for each point about excessive rainfalls show that reduction rate against current sewer network was between 24% and 46%.