In order to study hydrogeologically the earthquake prediction at Tokai district in Central Japan, concentration of groundwater radon was measured as the counting rate by an α -ray detecting method at Daito, Fukuroi, and Fujieda observation stations.
Although radon counting-rate seems to change under the influence of tectonic stress related to the earthquake occurrence, configuration of the changes of radon counting-rate at each the station has variety because of different hydrogeological condition such as confined or unconfined aquifer and Tertiary or Quaternary aquifer.
The changes of radon counting-rate at Daito observation station have much particular connection with the earthquakes deeper than 140 km which occur from the subduction of Ocean plate.