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This research aims at the study of the occurrence of runoff phenomena and its characteristicsthrough the dynamic (in terms of time and space) behaviors of soil water flow in the mountainouswatershed.In this paper, the study of the runoff problems was carried out, mainly using the various soilmoisture suction data of sub-surface soil water observed at 3 points in a study watershed.After some careful examinations and study of these data, the results are summarized as follows.1) On the dynamic behaviors of soil water in the surface layer, there is a great difference betweenthe surface soil on the foot slope close by the mountain stream and the other points.2) It seems that when the surface soil of the slope is dry or in the initial stage of runoff, the wet soilof the slope in the proximity of a stream or the stream itself contribute considerably to the areaproducing runoff.It is infered that the area producing runoff expands in the direction of the slope as rainingprolonged.3) The results of this observation made in the study watershed proved the propriety of the model of"Partial Area Concept".4) In this watershed the flow in the surface layer can be divided into sub-surface flow and laminarsurface flow, by border of about 7 mm of storage depth.5) The parameters P and K in the storage function equation (1) are closely related to the behaviorsof the flow on the surface layer and the average scale of the area producing runoff for each occurrence.