Many of fossil power plants, which were constructed during 1960’s and 70’s and exceed more than 10,000 working hours, have presently been operating while they have been receiving progressive damage, mainly from creep, as the time proceeds. [1, 2] Furthermore, by shifting the base load of power from fossil power plants to nuclear power plants, they are faced to even more severe operating condition such as daily or weekly startup and shutdown in order to correspond to rapid change of the demand for power. As the consequence of this trend, the materials degradation was accelerated. Therefore, a nondestructive technique is now highly required for safely operating plants and predicting the remaining life. It is also important for the technique to be simple and quick-operating to cope with the large number of measuring points.