PROBLEM TO BE SOLVED: To calculate a base line with high precision and improve the accuracy of a peak height, etc., even in cases where signal characteristics abruptly change on a raw spectral waveform.SOLUTION: A bottom point detection unit 21 detects all of bottom points in a trough between peaks on a spectral waveform that is an object to be processed, and a specific section extraction unit 22 extracts a section, among sections between two bottom points adjacent on m/z axis, whose section length is less than a prescribed section length. An intra-section interpolation unit 23 replaces a signal waveform within the section with a straight line in which the intensity value of bottom points at both ends of each extracted section is linearly interpolated. A termination determination unit 24 determines whether the gentleness of the waveform due to the processed data smoothed by such an interpolation has reached a prescribed permissible state, and, if the permissible state is not reached, performs processing over
