Izu-Oshima is a volcano island, which has a 3-4 km across summit caldera and a central cone, in Izu Islands about 120km SW of Tokyo. A large eruption has repeatedly occurred every 100-200 years for a long period of time and many craters are scattered over on the flank of the volcano along the NNW-SSE direction. The eruption at the summit and flank in November, 1986, forced the short time evacuation of all the some 10,000 residents from the island.