The New Energy and Industrial Technology Development Organization (NEDO) has been conducting a research project named "Deep-Seated Geothermal Resources Survey" since 1992 in order to establish a desirable direction for development of deep geothermal resources that exist beneath the already developed shallow reservoirs. A deep drill hole, WD-1, reached a depth of 3,729 m in July 1995 by applying the latest drilling techniques, such as a top-drive drilling system, enabling the collection of highly valuable information for understanding the characteristics of deep geothermal systems. Side-track drilling of WD-1 was started from a depth of 2,200 m in September 1996, targeting productive fractures expected near the boundary of the granite in a depth range from 2,800 to 3,000 m. We successfully encountered large lost circulation at some depths, and the side-track drilling was terminated at a depth of 2,963 m in January 1997.