A chromatogram data processing system having an impurity detector including a differential chromatogram creator, a purity curve creator and a determiner. The differential chromatogram creator calculates a differential coefficient of an absorbance spectrum with respect to wavelength at a local maximum or local minimum absorption wavelength of the target component, and creates a differential chromatogram which shows a temporal change of the differential coefficient. The purity curve creator creates a purity curve which shows a temporal change of the difference between the degree of similarity of a spectrum on the target peak to a reference spectrum and a threshold of the degree of similarity which is determined taking into account the influence of noise components. The differential chromatogram and the purity curve are displayed on a display unit in a superposed form on a wavelength chromatogram created by a wavelength chromatogram creator.