Typical mountainous basins in Japan have a small drainage area and steep topography. On such small catchments, the average flow-state during flood events can be estimated by the average resistance law. In this paper, the average flow-state and the average resistance law in a study basin were discussed using the relationship between storage and discharge. Ochozu study located in Fukuoka Forest of Kyushu University Forests is a small moutainous basin with a basin area of 0.095㎞[2] and a stream length of 256 m. Inthe basin, rivsr discharge and rainfall have been observed from 1982 to 1987 and from 2001 through 2002. Fifty-three flood events were selected for this study. The storage-discharge relationship was expressed by a strage function model and the oarameters of the model were optimally searched by simple genetic algorithm. The parameter corresponding to the gradient of the storage-discharge relationship varied from event to event and the average resistance law was estimated by this gradient. The results indicated that the average flow-state in the basin was classified into the predominance of overlamd flow described by Manning's formula and that of subsurface runoff described by Darcy's law. The threshould between overland flow and subsurface runoff was estimated to be a basin storage of 7.6mm that is approximately equivqlent to the thickness of zone of major hydrologic activity.