Peak determination is executed with respect to the mass spectrum of a sample to generate a peak list. For each of the plurality of peaks contained in the peak list, a Kendrick mass (KM) of a designated monomer is calculated. An RKM is calculated, the RKM being a fractional part of a value obtained by dividing the KM by the integer mass of the monomer, or a remainder of dividing a nominal Kendrick mass (NKM) by the integer mass of the monomer. A plurality of peaks contained in the peak list and satisfying a grouping condition, including the permissible range of the RKM of the starting point peak, are grouped.