The existence of hot-spring waters of high salinity was well known in nonvolcanic regions, but their origin has been unknown for a long time. We began our geochemical investigation of hot-spring waters discharged in the non-volcanic regions of the southwestern part of Japan in 2003. Through our studies we expect to find hot-spring waters derived from deep fluid characterized by CO2-bearing saltwater, such as Arima-type thermal water, which originates from dehydrated metamorphic fluid released from the Philippine-Sea plate subducting into the interior of the earth.