Kuju-iwoyama, the most intense surficial activities in central Kyushu, is situated in an explosion crater of Kuju volcano. It exists in central part of Kuju volcano. High heat flow values higher than 100mW/m² are observed around Kuju volcano. Simulation of the regional high heat flow was conducted in terms of conductive cooling of magma reservoir. As a result, it is shown that the magma is not molten at present but maintains high temperature of 600°C at a depth of 5km. From the depth, magmatic steam is supplied to the two-phase geothermal reservoir beneath Kuju-iwoyama.