This study aims to suggest an optimum chemical index for volcanic eruption prediction by using the quality of hot spring water near an active volcano at Yufudake and Kuju Mountain. The research points are Yufuin Spa and Nagayu Spa. Nagayu Spa was confirmed to accept the influence of volcanic gas at Kuju Mountain. Volcanic gas is rich in carbonate. Twenty kinds of chemical materials are evaluated by ion chromatography, titration, and inductively coupled plasma atomic emission spectroscopy. The coefficient of variance is 10％ for the magnesium, calcium, chlorine, and hydrogen carbonate ions. Especially, the hydrogen carbonate ion range is from about 2.3％ to 3.6％, having an extremely low ratio compared to other materials. Therefore, it can be suggested that the hydrogen carbonate ion is a useful material for volcanic eruption prediction at Kuju Mountain.