Terahertz spectroscopy enables to detect relative weak energy such as hydrogen bonding and intermolecular vibration. Therefore it is possible to identify the crystalline state of some material, such as quality of a crystal, and polymorphism. However, quantitative analyses have not been achieved in terahertz spectroscopy since an influence involving the scattering of terahertz waves is unavoidable. Only a few papers reported on quantitative analysis. In this study, we intend to establish a method for quantitative and high sensitive determination of a component from the mixed samples using pretreatment and a principal component analysis of terahertz spectra.