In 1974, an exploration bore-hole, No. SA-1, had been drilled up to 500 meters in depth in the Satsunan geothermal area, southern Kagoshima. Temperature at the bottom is over 240°C and the chemical composition of thermal water gives a type of Na-Ca-Cl called as “coastal thermal water” which is derived from sea water. Rocks in the bore-hole suffer extremely hydrothermal alteration characterized by the presence of chloritic clay minerals. By interaction between the coastal thermal water (sea water) and rocks, chlorite has been formed from volcanic glass and mafic minerals through the stage of Al-montmorillonite, Mg, Fe-saponite and interstratified mineral of chlorite and montmorillonite (saponite). An interstratified mineral of sericite and montmorillonite occurs locally above the part of the lost-circulation of drilling-mud. Gypsum is found through the all core. Mordenite and clinoptilolite occur in the upper part and wairkite occurs in the lower part. Zeolite such as laumontite, however, cannot be detected in the middle part. It is considered that all alteration minerals in the No. SA-1 have been formed by interaction between coastal thermal water and rocks under the recent physicochemical condition.