The Sengan area including Onuma, Matsukawa and Kakkonda power plants (total capacity of 82MWe) is one of the largest geothermal areas in Japan. The andesitic magma of Quarternary in age is assumed to be ultimate geothermal heat source in this area, because high temperature thermal manifestations with acidic alteration zones are distributed around the andesite volcanoes such as Yakeyama and Hachimantai. Exploitation data in the Matsukawa, Kakkonda and Onuma indicate the geothermal reservoirs are composed of various kinds of fractures developed in the NeogeneTertiary formations (sedimentary and pyroclastic rocks). High temperature fluids may rise from deep horizon (probably basement rocks) along the faults and are stored or circulated in the fractured-type reservoir. Recent results of geothermal exploration show there is no positive evidence indicating the presence of deep and widely distributed high temperature hydrothermal system which have been considered to be origin for shallower hydrothermal systems found in the exploited areas. Geothermal resources of above 150 °C stored in the Neogen Tertiary formations can be roughly calculated to be about 4, 000MWe, 30 years by the volumetric method.