This study aims at improving the accuracy of a currently used flood runoff model by taking into account the soil water storage (SWS) before flooding and continuously changing the effective rainfall input into the model. In this study, SWS was estimated by using a "long-term hydrological assessment model considering snow process" (LoHAS) first. Then, the runoff rate, which is changing hour to hour, was determined by using the estimated SWS as the initial value of the runoff. Next, the estimated effective rainfall was input into the storage function model. Compared to the accuracy of the current flood runoff model, the accuracy of the new model in terms of the reproduced hydrograph was improved. The accuracy in reproducing the second peak of a two-peak flood and in reproducing flood runoff caused by snowmelt and rainfall was particularly improved.