Japan Atomic Energy Research Institute (JAERI) played a leading role in basic research in the field of atomic energy research and development, while Japan Nuclear Cycle Development Institute (JNC) did a major role in FBR cycle development and high level waste disposal. According to Japanese government's decision in December 2001, JAERI and JNC was merged as of October 1, 2005. The new organization, Japan Atomic Energy Agency (JAEA) is an institute for comprehensive R and D for atomic energy, and which is the largest research and development institute among Japanese Governmental organization. Its missions are basic research on atomic, R and D for nuclear fuel cycle, decommissioning and disposal for own facilities and waste, contribution to safety and non-proliferation, etc. The JAEA owns a number of nuclear facilities: research reactors such as JRR-2 and Joyo, prototype reactors such as ATR 'Fugen' and FBR 'Monju', fuel cycle plants such as Uranium Enrichment Demonstration Plant at Ningyo-toge, MOX fuel plants at Tokai, Reprocessing Plant at Tokai, and Hot Laboratories such as JRTF and FMF. As a part of preparation of the mergence, JNC and JAERI have jointly developed a comprehensive cost estimation method for decommissioning, based on decommissioning and upgrading experiences ofmore » JAERI and JNC. This method has adopted more estimation formulae for typical decommissioning activities than ever, so as to be more reliable. JAERI and JNC had estimated by using the comprehensive estimation method for decommissioning, and concluded the total cost for decommissioning would be 600 billion yen (approx. 5 billion USD). (authors)« less