A new colorimetric method of determination of boric acid was devised, using ion-exchange resin, mannit and bromthymolblue. With this method determination of boric acid becomes capable by adding one ounce of mannit and ten cc. of resin to an ordinary set for the investigation of mineral waters. This method is suitable for the waters.containing little amount of boric acid and for the
samples of small amount. Thermal waters in Tottori Prefecture contain four mg of fluorine and ten mg of metabolic acid per liter in average.