Natural radioactivity (γ-ray intensity) often increases over faults and fractured zones which rise to shallow subsurface zones near the surface. This is caused by the γ-ray emitted by bismuth-214(214Bi) which is a daughter nuclide of radon-222(222Rn) rising through the fractures from the deeper subsurface zones. It is possible, therefore, to estimate the position of faults and fractures, and the conduits for hot spring waters, by measuring the intensity of γ-ray by gamma-ray spectrometer. ◎　Radioactivity measurement is done using helicopters (heli-borne) and motor cars (car-borne) for reconnaissance of wide areas, but for detailed survey of relatively small areas, back-packing light-weight spectrometers (man-borne) are believed to be the most cost-effective method. ◎　Profile lines are set at right angles to the inferred faults. Stations are set at 5m intervals and measurement is done during good weather. Increase of water content of the ground by rain will change the absorption of radon gas and affect the measured values.   Examples of recent natural radioactivity measurements. ■Hot spring survey at Ecchu Shoukawa Kyou, Toyama Pref. (1997) ■Hot spring survey at Shimoda area, Shizuoka Pref. (1997) ■Hot spring survey at Kmogawa, Chiba Pref. (1996) ■Hot spring survey at Lake Yamanaka, Yamanashi Pref. (1995) ■Hot spring survey at Mizuho, Shimane Pref. (1993) ■Hot spring survey at Kii Tanabe, Wakayama Pref. (1993)