Abstract
Precursory phenomena before the large earthquakes have been investigated in many countries. In China and some other countries, they made a success of predictions of the large earthquakes by catching precursory phenomena. Variation of radon content of the underground gas and water is also one of these phenomena.
The gas monitor for continuous measurement of radon concentration was designed. Radon concentration, temperature, conductivity and rate of volume of self-spouting of hot spring has been observed at Odoi, Izu Peninsula, Central Japan.
These data effected by the air-temperature but not so effected with the production rate of spring. According to the results of these investigation during these five years, the increasing radon contents are observed at three times of the earthquake swarms in the east part of sea off Ito.
