The debris produced at so-called“ Osawa Rock Fall”located along the west slope of Mt. Fuji are transported through the narrow stream called“Iwa Toi”and make big deposits on“ Osawa Fan”area. Topographic and quantitative variation of“ Osawa Fan”are discussed in this paper. Airphotogrammetric plofile survey comparing the airphotographs taken in three different stages, geologic and topographic field survey are carried out for this purpose. Results of this study are as follows;
1) This area is formed of four topographic planes, such as the original plane consisting of Fuji volcanic ejectas, the plane of old stage fan, of middle stage fan and of new stage fan. Exactly, the last plane is classified to high and low-level plane along the streams.
2) Topographically, Osawa fan area is classified to some sections such as the transporting section, the transporting section accompanying some deposition, the depositting section the reeroding section of fan deposits and the re-depositting section.
3) In this area the total volume variation of river bed deposits increased 195, 900m³in volume during 4 years from 1958 to 1962 and decreased 170, 600m³ from 1962 to 1967.
4) Debris derived from Osawa Rock Fall are not transported directly toward the lower area of the fan, but deposit temporarily at the upper part of the fan, then re-erosion is open from the front zone of the deposits to transport them to the lower parts to form small fan gradually. The range of deposition of fan also deviated gradually from north to south.