The Kuju volcano group is situated at central Kyushu, Japan. Kuju-iwoyama is an explosive crater of Mt. Hosshoyama which is one of the Kuju volcano group and shows the most intense geothermal (mainly fumarolic) activity in the Kuju area. In the Iwoyama area, we have measured heat and water discharges from fumaroles, steaming grounds, hot springs and thermal conduction through soil. Total heat discharge from the area amounts to 2.38×10⁷ cal/sec (99.2 MW) and total water discharge amounts to 65.2 kg/sec. Also total volcanic gas emissions were estimated, based on chemical analyses of volcanic gases and the total water discharge. As a result, daily volcanic gas emissions were estimated as follows ; CO₂ 166 tons/day, H₂S 53 tons/day, SO₂ 26 tons/day, S 3.2 tons/day, HCl 3.2 tons/day and HF 0.07 tons/day. A quantitative hydrothermal system was proposed, based on the total heat and water discharge. We assume that the hydrothermal system is composed of the surface zone, the shallow thermal water reservoir and the deep heat source (the magma reservoir). From the heat source, the heat and steam are transferred upward by magmatic gas and thermal conduction. Cooling of the magmatic steam on the way from the magma reservoir to the thermal water reservoir is calculated by using the method developed by YUHARA (1968). We also assume that the depth of the thermal water reservoir is 2 km depth. At the depth, the magmatic steam mixes directly with the meteoric water and forms there the thermal water reservoir. Thermal water discharged from the reservoir rises up and it ejects from the ground surface. When the thermal discharge is measured and the depth and the temperature of the heat source are assumed, we can estimate the magmatic steam flow rate and the temperature of the steam at 2 km depth. The steam flow rate and the temperature are estimated to be about 20.3 kg/sec and 990℃, respectively, assuming that the depth of the magmatic reservoir is 5 km and the temperature is 1, 000℃. The temperature of the thermal water reservoir is estimated to be about 370℃, from the mean enthalpy of the reservoir, that is, the thermal state of the reservoir is near the critical point of water. The estimated temperature of the reservoir is nearly equal to the observed maximum temperature of fumaroles. The percentage of the magmatic steam flow to the total water discharge is about 41% and that of the total water discharge to annual precipitation of the area concerned is about 47%. The above mentioned three features are clearly different from those of the ordinary geothermal areas. These may be characteristic features of the hydrothermal system which exists below the active volcano having high temperature fumaroles. Chemical compositions of fluid at the thermal water reservoir and at the magma reservoir are estimated, based on the above mentioned hydrothermal system and the chemical equilibrium of the gas. As a result, it is clarified that the estimated chemical composition at the magma reservoir is very similar to that of the gas in chemical equilibrium with the granitic magma. An important characteristic feature of volcanic gases from Iwoyama is very low content of HCl in comparison with that of sulphide. This fact shows that much HCl remains in the underground or was eliminated on the way from the magma reservoir to the ground surface. Otake and Hatchobaru geothermal area where the geothermal power plants are operated at present, is situated at the western direction of Iwoyama. The distance between them is about 5 km. At Otake and Hatchobaru, much thermal water of sodium chloride type are drawn from production wells. The existence of thermal water of sodium chloride type below the Otake and Hatchobaru may be closely related with the very low content of HCl in volcanic gases from Iwoyama. In this case, we must consider a larger scale of the hydrothermal system.