A very rare and heavy rain‐storm for a few hours in Shobara‐City, Hiroshima Prefecture, induced a lot of slope failures and debris flows in the afternoon on July １６, ２０１０. Damaged area was located within only a square of about ５km. This mountainous area is geologically covered by Takata rhyolitic rocks, Kisa andesitic rocks, and Bihoku sedimentary rocks. Because the surface soil layer was not thick, most of failure‐depths were ０．５‐１．５m. Countless small holes and fissured bed‐rocks, through which water‐flow pushed out surface soil layer, were observed around most of slope failures. Almost displaced materials turned to debris‐flows or a kind of liquefied state and roared down into main stream, and then flowed down as a large hyper‐concentrated flow. The rainfall as a cause of this disaster was statistically about２４０years of return‐period to two‐hour‐rainfall, and about ５，７００years to three‐hour‐rainfall in this district. Areal ratio of slope failures steeper than ３０° to this same watershed area was ７．０％ for Shinodo‐area, and ４．７％ for Saki‐Oto‐area.


Key words：Sediment‐related disasters, heavy rainfall, return‐period, rhyolitic rocks, Shobara‐City