A 65,000kW geothermal power plant is under operation at Yanaizu-Nishiyama, Fukushima, Japan since 1995. Microearthqukes around the Yanaizu-Nishiyama site were observed in the exploration/construction phase of this power plant, and, hence, seismic monitoring network consists of 5 stations was deployed in 1988. Most of the microearthquakes had hypocenters around a fault, which is considered as a production zone, at a depth of 2 km, although the hypocenters and production zone are not fully overlapped. An earthquake with a magnitude of 4.9 occurred in November, 2009, and considerable damage was brought to the power plant and residents around the site. The hypocenter of the earthquake was in the center of the cloud of microearthquakes and the estimated fault plane solution (FPS) by JMA showed that a fault with a NW-SE azimuth and a dip around 45° had a slip in normal fault mode. The seismic activity was low prior to the earthquake, and the hypocenters of the aftershocks showed a re-activation of branching seismic structure extending to southeast side of the cloud of the microearthquake hypocenters. We have investigated relationship between production/injection rate, spatio-temporal distribution of the hypocenters, and magnitude of the microearthquakes. We have found that seismic activity and magnitude is not clearly correlated to the human operation to the reservoir since the start of the operation of the power plant. Microearthquake multiplets, which are clusters of events with close similarity because of common source mechanism and earth transfer function, were identified, and the time series of occurrence, location, and orientation of the multiplet seismic structure were investigated. The origin time of the multiplet events distributed randomly with a little correlation to the production/stimulation of the reservoir. The seismic structure of the multiplet showed that there are three trends in the orientation of the structures, suggesting that they are correlating to the orientation of the fracture system inside the active microearthquake zone. The trigger mechanism and control factor of the large earthquake at this site has not been fully interpreted, however their characteristics as earthquake has been clarified to some degree throughout this study.