An abundance of specific species of phytoplankton in a phytoplankton group in which a plurality of kinds coexists is calculated in a simple manner. Based on a reference sample intensity ratio r0, a reference sample total fluorescence intensity I0, and an existing quantity K0 of specific species of phytoplankton, an intensity ratio rd of other species of plankton is calculated. An analysis sample that is expected to have similarity with the reference sample is irradiated with the excitation light, an intensity of fluorescence emitted from the analysis sample is measured in each of wavelength bands A and B, and an intensity ratio r is calculated. A total fluorescence intensity I is measured, and an existing quantity K of the specific species of the phytoplankton is calculated based on the intensity ratio rd of other species of plankton, the intensity ratio r, and the total fluorescence intensity I.