An unknown spectrum is compared to reference spectra registered in spectral libraries and the similarity is estimated for each reference. In practical analysis, the completely same spectra can't always be obtained even though the same compounds are measured. Many practical conditions: the used instrument, the parameters for measuring, sample condition and statistical fluctuation, bring some ambiguity into a measured spectrum. A library search routinely informs us of the matched compounds with a greater similarity than the criterion determined by a user. The user identifie the unknown spectrum by taking the other conditions into consideration as well.