The author determined the radon content in the ground waters of Misasa and its vicinity by the I. M. fontactoscope from August to November, 1952. The results and conclusions are as follocvs : A close correlation was observed between the radon contents in the spring-waters and that in the ground-waters near the springs, so it seems possible to prospect the existence of radioactive springs by determination of the radon content in groundwaters in the place where no radioactive spring was found as yet. The highest radon content in 53 samples was 741 Mache (2697×10(-10) Curie/l) of the Mr, shigemasa Matsubara's well-water, and this record ecceeds the highest radon content in thermal waters of Misasa known up to the present time. Though the temperature of its well-water was 24.0°C in the afternrnoon of November 25th, 1952, the hot-springs of Yamadaku-kyodoYu, Gunzeshinsenryo, Koyoen, and so on, issue neare by the well. Therefore, the author points out that if a boring is tried in the place where the well is the centre, a most strongly radioactive hot spring in the world may issue.