[en] For the purpose of appraising the applicability of geochemical explorations to the fault investigation, the radon exploration by α-track (track etch) and scintillator methods was carried out with subsidiary measurements of CO₂ and O₂ concentration in the Oku-Tango area of Kyoto Prefecture. Through this exploration and summarization of former reports on radon exploration, the following problems and points requiring special attention in practical investigations were made clear. 1) The radon exploration is not always effective for the fault investigations; as Gomura and Chuzenji active faults in the Oku-Tango area, some active faults show no anomalous radon concentration in the ground air. 2) It is problematic to directly regard the high radon concentration as an indicator of active faults because some of these anomalies are caused by the high emanation rate of radon and thoron from the soil materials and blockaded ground-air circulation system around the measuring holes. 3) In conducting radon exploration, not one but both α-track and scintillator methods should be employed in order to ensure the existence of the radon anomalies. Also other gases such as CO₂ and O₂ should be measured whose behavior would assist in concluding the existence of faults. (author)