Origin of hot spring water in the Yakushima island was clarified using δD and δ¹⁸O values and Cl^-. δD and δ¹⁸O values and Cl^-concentrations of river water in the Yakushima island showed zonation. Both δD and δ¹⁸O values and Cl^-concentrations decreased with increase of height because of isotope effect and sea salt migration caused by air.
δD and δ¹⁸O values and Cl^-concentrations of two hot spring waters were in agreement with those in river waters at 600 to 800 m in height and 1200 m in height respectively. Therefore two hot spring waters were thought to derive from river waters 600 to 800 m in height and 1200 m in height. As hot spring temperature calculated from both the normal geothermal gradients in Japan and altitude differences between recharge area and hot springs can reach actual temperature, thermal origin of hot spring does not need volcanic activity.