With a newly devised method of Ra. B determination using Dithizone extraction the author measured the Ra. B contents of Misasa and Sekigane Hot Springs. This method is applicable to the weakly radioactive waters. Radium B content of Misasa Hot Spring rangeS from 3.0 to 7.2×10(-9) Curie units per liter and the distance of its radon soure from the issuing point was estimated to be 3-25土5 minutes.