In this study, availability analysis is focused to use the flood control system for pollutant reduction derived from non-point sources. Big tunnel stormwater storage system named "Donryu" is under construction in Nishihadukashi river basin for flood prevention. This paper shows that basic investigations of stormwater and pollutant runoff in this basin continuously carried out from 2004 to clarify the runoff characteristics and effectiveness of pollutant reduction measures. Water level and water quality are automatically monitored by electric sensors. Continuous stormwater samplings were carried out in several rainfall events. Stormwater and pollutant runoff analysis by numerical models had good performance to simulate observed data. Some water quality characteristics are defined by the observation and simulation. Finally, pollutant reduction measures in near future are discussed.