1. Radon content of 166 thermal waters in IWAI, TOTTORI YOSHIOKA, HAMAMURA, TOGO, SEKIGANE, MISASA, and KAlKE Hot Springs was measured by I. M. Fontactoscope in the years 1950-1951. Of which 66 samples showed a radon content over 30×10(-10) curie units per liter. Namely, 50 springs in Misasa, 6 in Sekigane, 9 in Hamamura, and one in Togo belonged to the radioactive spring in the definition by Ministry of Social Welfare. The highest Radon content (1150×10(10) curie units per liter) was recorded in Hisuino-Yu in Misasa, where five springs had a radon content over 360×10(-10) curie units per liter. 2. No marked difference was proved between the results obtained this time and the data in the former reports concerning the radon content of these thermal springs. 3. Radon content proved higher in the springs which issue from granite than in
the springs of other districts. No definite relation was proved between the radon content and water temperature. The radon content was generally high in simple thermals or in weak sodium chloride springs, low in sulfated springs and in saline springs which had a comparatively
high sulfate content.