Alocal heavy rain hit Minamata City of Kumamoto Prefecture on 20th July in 2003. A large scale of disaster occurred in Hogawachi Atumari district and Fukagawa Shinyashiki district of Minamata City at about 4 in the morning(87mm/h of rain at 4)and it claimed the lives of 19 people. Both areas are composed of extremely weathering andesite in geology. The inclination of the collapsed slope of Hogawachi Atumari district is about 30° and that of Shinyashiki district is about 40°. The volume of the debris flow of the Atumari River was estimated in 1-1.5×10⁵m³ and that of the slope failure of Shinyashiki district was estimated in 100-150m³ from the results of brief surveying or measurement of a laser profiler. The debris flow of the Atumari River flew straight down from the collapsed slope without damming up there. It is supposed that the debris flow was caused by the slope failure due to a rise of pore pressure of much seepage water into the weathering andesite layer with many cracks. The slope failure of Shinyashiki district which is called a surface failure was caused by the action of piping flow among the shallow talus accumulation layer. Sediment of the debris flow in the Atumari River consisted of 2 kinds of materials (muddy and gravelly one). It is said that the debris flow of the Atumari River ran for several ten minutes.