After the disastrous 1995 Kobe earthquake, a new national project has started to drastically improve seismic observation systems in Japan. This project was requested to construct a broadband seismograph network with a spacing of 100 km covering the whole of Japan utilizing existing broadband stations as much as possible. National Research Institute for Earth Science and Disaster Prevention (NIED) has constructed a new broadband seismograph network consisting of 73 stations called NIED F-net. The broadband seismograph (STS-1/2) and strong motion velocity-type seismograph (VSE-355G2/G3) are installed in 30 to 50 m long vaults at each F-net station. All the data obtained from the F-net are archived at NIED and provided to researchers and general public through the Internet. The NIED F-net has been routinely analyzing moment tensor solutions for earthquakes of magnitude greater than 3.5 based on the unified hypocenter catalog maintained by the Japan Meteorological Agency. High quality data provided from the dense F-net are utilized to address a lot of interesting research issues: seismogenic stress field, the source process of large earthquakes, structure of the Earth’s interior, Earth’s background free oscillations, and very low frequency earthquakes.