An investigation of the Sasshogawara geothermal field by means of a borehole x-ray device is reported upon. The theory and experimental procedures of the technique are described. Results are reported in terms of the location; depth and nature of strata; temperature gradient; quantity of gas; and nature of the steam. The results provided fundamental data for the selection of power plant sites. During the drilling severe problems were encountered due to the extreme hardness of the rock. It was concluded that in the Sasshogawara field the geological structure is sufficiently complicated to make detailed geological mapping a virtual impossibility.