A waveform data processing device 30 capable of accessing a storage device 40 for storing data on an observed waveform such as a chromatogram, information on a starting point and an ending point of a peak cluster consisting of a plurality of peaks close one another present on the observed waveform, and information on a position of each peak included in the peak cluster and a positive/negative direction of the each peak, includes baseline determination means for determining, based on the data and the information stored in the storage device 40, a shortest straight line or shortest line segments from the starting point of the peak cluster as a beginning point to the ending point of the peak cluster as a finishing point satisfying all following conditions, and determining the straight line or the line segments to be a baseline of the peak cluster: (1) in a section where positive peaks are contiguous, a baseline passes below the observed waveform, becoming a straight line or line segments convex downward; (2) in a section where negative peaks are contiguous, a baseline passes above the observed waveform, becoming a straight line or line segments convex upward; and (3) in a section where positive and negative peaks are contiguous, the shape of a baseline is not influenced by the observed waveform.