An earthquake of Mw 7.7 occurred in the Indian Ocean off the southern part of Sumatra Island (3.5° S and 100.1° E) around 21:42 local time, and generated a tsunami, which hit in North and South Pagai Islands and Sipola Island of Mentawai Islands lying off the west cost of Sumatra Island. This is a report of a joint field survey on this tsunami disaster conducted by the Port and Airport Research Institute and National Institute for Land and Infrastructure Management together with the Ministry of Marine Affaires and Fisheries of Indonesia and other. In addition, results of numerical simulation on the tsunami are also indicated.
　In the survey, tsunami inundation and runup heights were measured in North and South Pagai Islands. Interview investigation was also conducted to understand people’s evacuation action in these islands. The major results of the survey are as follows:
1) Relatively large tsunami hit Pagai Islands comparing to the magnitude of earthquake. The tsunami height was about 6 m in these islands, and especially the 7 – 8 m tsunami attacked Malakopa, Sabeugunkgung and Magoiru in North Pagai, resulting in losses of human lives and houses.
2) A person who heard the sound of tsunami approaching in night hours run to hill with transmitting the information of the tsunami coming to other residents, and therefore all residents in a costal side of the village could evacuate to save their lives. Prompt tsunami detection and dissemination of its information led to reduction of human losses especially for local tsunamis.
3) There was a case that many people were left dead or missing in the area where a bridge crossing a small river was destroyed by the tsunami coming up in the river. This disaster indicates importance of plan and arrangement of save evacuation routes and sites.
4) Although the Indonesian were well known on tsunamis, almost of them failed to escape from the tsunami immediately after the ground motion due the earthquake, because they judged no tsunami based on weaker ground motion than the 2007 Bengkulu earthquake which generated no tsunami affecting in the Pagai Islands. From this fact, we should consider more careful education on tsunami disasters to avoid people’s wrong decision.