Joints and zeolite-veins in both metamorphic rocks and quartz diorite exposed along the Nakagawa River were studied. Fractures with zeolite-veins are most developed in three areas, the Nakagawa hot spring area, Hokizawa, and Higashizawa. They follow two prevailing directions: N--S with minor right-lateral displacement and N60/sup 0/E with minor left-lateral displacement. The two fractures should represent a conjugate set that was produced by stress with maximum principal axis of N30/sup 0/E-S30/sup 0/W. Distribution and prevailing directions of fractures are illustrated. Geothermal gradients are measured in two newly opened boreholes, at the Nakagawa hot spring area and Higashizawa. The geothermal gradients are 12.60 +- 0.48/sup 0/C/100m at the Nakagawa hot spring and 5.55 +- 0.24/sup 0/C/100m at Higashizawa. Temperature-depth relationships in the two boreholes are given. Seismic observation was made at the Higashizawa. In five days 43 shocks were recorded, of which 20 are thought to have occurred 2 to 20km from the observation station, that is, in and very near the Tanzawa mountains. None have shallower hypocenters than 2 km in depth.