To ensure an accurate measurement by describing a line for demarcating a region on a scattergram where a specified particle is present and removing a region where a non-platelet is present from a platelet region before measuring the platelet.

A sheath flow cell 5 forms a flow of fine sample liquid containing platelets which is illuminated with a laser light emitted from a continuous emission laser 21. Intensity signals S1-S3 of scattering light from particles and fluorescence thereof are detected by a photodiode 27 and photomultipliers 30, 31. A signal processor obtains feature parameters based on these signals S1-S3. A microcomputer produces a two-dimensional scattergram by combining the feature parameters and demarcates the erythrocyte region and the platelet region. Subsequently, a region where schistocytes are present is demarcated in the platelet region, the number of dots on the scattergram is counted along with the number of particles in each region, and then the number of high purity platelets from which the schistocytes are removed is determined.