Fossil power plays an important role in the supply of electric power as well as nuclear power. However, the accumulated operating time of most fossil power plants now exceeds 100000 hours, which is the designed life of high-temperature components. In addition to the problem of aging of fossil power plants, costs need to be reduced maintaining supply reliability. This paper describes current maintenance and diagnosis techniques used for aged fossil power plants, and the RBI/RBM method as a new maintenance method that reduces cost while maintaining reliability. Remaining issues to be resolved for applying RBI/RBM are also discussed.