From the engineering geological, hydrogeological and geochemical points of view, the characteristics of water quality in groundwater (partly stream water) from the Rokko mountain area is summarized as follows; 1) water quality in groundwater 'through Nunobiki granodiorite shows Ca- Na-HCO₃ type, and Na-HCO₃ type is greater than Ca-HCO₃ type in groundwater from Rokko granite, 2) the groundwater is characte-rized by higher alkalinity, higher pH and being rich in fluoride content than the stream water, 3) mineral and common water exist irregularly and alternatively within fault-sheared zone, 4) chondrite-normalized REE pattern in groundwater is regarded as an in-dex of water-rock interaction in deep underground, 5) from measurements of tritium, the turnover time of groundwater in a fractured system is about 40 years and the mean velocity of downward displacement flow about 10 to 17m/y, 6) some amount of stream water is supplied into groundwater contained in fractures or faults in base rock which. are crossing a stream, usually in a certain small amount, but during heavy rains dis- charge of supplied water increases suddenly to a great extent, and 7) a crustal deforma-tion occurs at or around the Rokko mountain area, increasing the underground supply of carbon dioxide gas and leading to very active water-rock interaction.