The Heisei-eruption of Mt. Unzen had started on November 17 in 1990 and continued in the beginning of 1995. Dangerous volcanic phenomena such as pyroclastic flows and debris flows had occurred intermittently during the eruption, resulting in 44 of casualties and 2,511 destroyed houses. Though no one had experienced and estimated the occurrences of such events, the central government of Japan and the Nagasaki prefecture government soon started to ensure the appropriate measures for reducing the expansion of disasters. The first approaches were preparation of hazard-map for evacuation and execution of temporary works such as sediment removal and sensor installation. Furthermore, they had developed many new methods such as unmanned construction and executed the measures to mitigate debris flows and pyroclastic flows disasters. The measures were based on three viewpoints; (1) We have to cope with various phenomena by them. (2) They should be executed in short term. (3) The safety of workers has to be ensured. Inhabitants, academic persons and administrators had coped with the Mt. Unzen volcanic disaster in cooperation. As a result of the execution of the measures, the safety zone of life in the Shimabara city was restored and the communities were reconstructed. Thus, the preventive measures in Mt. Unzen were successful. Recently there is destruction of social infrastructures by volcanic disasters in the world. I can introduce the volcanic sabo work executed by the central and local governments in Mt. Unzen as a model case of the mitigation or prevention measures against volcanic disasters.
Keywords: sediment movement, volcanic disaster, volcanic sabo work, hazard map, volcanic sabo master plan