Kogajajima, composed of hornblende andesite and dacite lavas, is a small volcanic island 0.5 by 1 km in diameter and is at the summit of a seamount 5 by 10 km in diameter and more than 800 m high. The eastern half of the island is hydrothermally altered. Several active fumalores and hot springs were recognized in our survey in 2005 and 2006. In addition to the young K-Ar age suggesting late Pleistocene in age, the fumarolic activities implies that the volcano is still active in recent years.