Wavelength spectrums of peaks detected on a chromatogram based on observation data to be processed are extracted to create a spectrum set {Sn′} in which the intensity values of the spectrums are normalized (S10, S11). One wavelength spectrum is selected from the set, and a vector of the wavelength spectrum at each point in time of measurement based on the observation data is projected so as to be perpendicular to the vector of the selected spectrum (S12 to S14). The vectors of the wavelength spectrums in the set {Sn′ } are also similarly projected (S15). Consequently, the selected spectrum is erased from the set {Sn′}. The processes from S12 to S16 are repeated until the set {Sn′ } does not include a spectrum, and the obtained signals are added (S17). The signal resulting from the addition is a signal indicating the waveform shape of an unknown baseline. A baseline spectrum is obtained by fitting the signal to a chromatogram at each wavelength obtained from the observation data, and a baseline signal at each wavelength is calculated from the baseline spectrum and the baseline chromatogram. As a result, a baseline can be automatically estimated without setting of a parameter and the like by a user.