City water, amounting 2883m³, was pumped into a well of 1800m deep at Matsushiro, central Japan. The pumping pressure was 50 to 14 bars, Several days after the start of the fluid injection, a series of microearthquakes commenced two to four kilometers away from the well. Detailed investigations of those earthquakes confirmed that the unusual seismic activity was triggered by the water injection. Most of the supplied water seems to have permeated through the Matsushiro earthquake fault. Intrinsic permeability of the fault zone was estimated as 0.01-0.1 darcy, which are reasonable values for fractured rocks.