Abstract
　Eruption of Mt. Unzen recurred on November 17, 1990, after 198 ycars of silence. At first, the attention of the local governments was poured to the danger of repetition of Mt. Mayuyama Landslide (1792). However, continuous volcanic activity at Fugendake peak caused a great piling-up of volcanic ash at the headwaters of the Mizunashi River, which stimulated seven debris flows between May 15 and June 3, 1991. Inhabitants were advised to evacuate and no injuries were reported in this period, although a pyroclastic flow occurred on June 3 and killed 43 people. Pyroclastic flows were repeated many times there after, and the local governments forced the inhabitants to evacuate from delineated dangerous area.
　The present paper describes the policy taken by the governments of Nagasaki Prefecture and Shimabara City against the risk of large-scale landslides, pyroclastic flows and debris flows. Establishment of prediction and warning system and other types of risk management is also reported. Finally several proposals of the authors are shown.

【キーワード】：噴火，土石流，火砕流，山体崩壊，予警報システム，防災対策，危機管理
Key words: volcanic eruption, debris flow, pyroclastic flow, mountain collapse, prediction and warning system, disaster prevention, risk management

*長崎大学工学部社会開発工学科
Department of Civil Engineering, Faculty of Engineering, Nagasaki University