A water injection experiment was carried out by the scientific drilling program named the 'Nojima Fault Zone Probe' during the two periods 9-13 February and 16-25 March 1997. The pumping pressure at the surface was approximately 4 MPa. The total amount of injected water was 258 m(3). The injection was made between depths of 1480 m and 1670 m in the Disaster Prevention Research Institute, Kyoto University (DPRI) 1800 m borehole drilled into the Nojima Fault zone. A seismic observation network was deployed to monitor seismic activity related to the water injections. Seismicity suddenly increased in the region not far from the injection hole 4 or 5 days after the beginning of each water injection. These earthquakes were likely to be induced by the water injections. Most of the earthquakes had magnitudes ranging from -2 to +1. Numerous earthquakes occurred during the first injection, but only one could be reliably located and it was approximately 2 km north of the injection site. Between the two injection periods, earthquakes concentrated in the region approximately 1 km northwest of the injection site. During and after the second injection experiment, earthquakes were located approximately 1.5 km west of the injection site. Those earthquakes were located approximately 3 km or 4 km from the injection point and between 2 km and 4 km in depth. Values of intrinsic permeability of 10(-14)-10(-15) m(2) were estimated from the time lapse of the induced seismic activity. The coefficient of friction in the area where the induced earthquakes occurred was estimated to be less than 0.3.