The author determined the radon content of the thermal waters of Misasa, Tottori Prefecture Japan, at various times after flowing out, and obtained the following results: 1) The velocity of variation is expressed by a simple equation with respect to the radon content. At thermal temperaturee of 41.5-41.8°C and room temperature of 24.3-28.0°C, a following exrerimental equation was obtained, u:;ing a beaker with a inner diameter of about 17 cm and a height of 27 cm, log a = -0,0048t + 2.59 a: radon content in Mache unit. t : time in minute. 2) A close relation was observed between the room temperature and the rate of variation of radon, and also between the thermal temperature and the rate of variation of radon, that is, the rate of variaton of radon was proportional to the room temperature and the thermal temperature respectivly, as it is clearly seen from the distribution coefficient of radon. 3) There exists a c1ose relation between the velocity of variation of radon and the surface area being in contact with air, that is, the velocity was generally proportional to the square root of the surface area, or to the radius of the surface. 4) The velocity of variation of radon content was roughly inversely proportional to relative humidity of air. 5) Betweeen the range of sodium chloride concentrations from 0.1 to 40.1 g. per liter no difference was proved concerning the velocities of variation in radon content.