[en] This paper deals with a new method of estimating the accumulated damage (D-value), a proposal for design d-value spectrum and their applications to nuclear piping system seismic designing. The earthquake waves employed to calculating d-value spectrum are artifical and world famous earthquakes. Then we can present a design d-value spectrum that is not influenced by input earthquake characters since we take an envelope spectrum of these d-value spectra. Applicational example indicate where D-value is evaluated from the design d-value spectrum. The ratio of the D-value to the critical one corresponding to a safety limit for design, which will be obtained by experiments, shows a good index for safety reliability of nuclear piping systems. (author)