COP3 in Kyoto 1997 imposed 6 to 8% reduction of CO2 emission from the base year 1990 on Japan, US, and EU (Kyoto Protocol). One of the possible strategy to achieve this target for Japan is to replace the consumption of petroleum by conventional natural gas and LNG. Exploration and development of marine gas hydrate is a challenging option to accelerate a change of energy supply structure as well as to increase domestic energy resource potential. Marine seismic surveys for the last decades have identified strong BSRs (bottom simulating reflectors) in several areas offshore Japan. Nankai Trough, in particular, is characterized by extensive and wide distribution of high amplitude BSR ( approximately 36000 km (super 2) ), which have been attracting growing interest of geologists from both the industrial and scientific viewpoints. In 1995, MITI-JNOC has launched R&D project to explore and develop methane hydrate (= gas hydrate), and conducted stratigraphic drilling in Nankai Trough off Tokai in late 1999. LWD deployment and preliminary core analysis including core-temperature measurement, determination of chloride concentration of the interstitial waters have revealed that pore saturation of gas hydrate reaches up to 60-90%, surprisingly high concentration!, almost ten times larger than those of the Blake Ridge sediments, ODP Leg 164. The first exploration drilling in Nankai Trough seems to favor a positive and optimistic view for the development and even commercialization of marine gas hydrate deposits.