TOPMODEL is a strong-parameter model with both advantages existing in each of the distributed-parameter and lumped-parameter models. It can consider the physical characteristics in the rainfall-runoff process in mountainous basins to some extent and has lately attracted considerable attention in the research field of rainfall-runoff analysis. In the TOPMODEL, water balance in groundwater storage is calculated by a simple lumped-parameter model A basin area is horizonally divided into small square grids and phenomena in surface soil layers, such as surface runoff and groundwater recharge, are expressed by distributed-parameter models on each square grid. Time-verying runoffcontributing area during flood events can also be calculated in the distributed-parameter models by judging the generation of surface runoff from the soil moisture defict in each square grid. In this paper, the TOPMODEL was applied to the long- and short-term runoff analysis in the Ochozu study basinwith a basin are of 0.095km2, and the applicability of TOPMODEL to the small moutainous basin was discussed through the prediction accuracy of river discharge. Seven unknown parameters in the model were optimally searched by a simple genetic algorithum(SGA) with the fitness function defined by a relative error between observed and calculated river discharges. The results indicated that the SGA-optimized TOPMODEL could successfully predict the river discharge in a rainy season.