1. A method for classifying and counting leukocytes, which comprises: (1) a step of staining cells in a sample obtained from a hematological sample by treatment with a hemolytic agent, with a fluorescent dye; (2) a step of introducing the sample containing the stained cells into a flow cytometer to measure first scattered light, second scattered light different from the first scattered light and fluorescence of the respective cells; (3) a step of obtaining scattered light peak intensities and scattered light widths of the respective cells based on the measured first scattered light, obtaining scattered light intensities of the respective cells based on the measured second scattered light, and obtaining fluorescece intensities of the respective cells based on the measured fluorescence light; (4) a step of classifying the cells into a first group and a second group based on the scattered light peak intensities and the scattered light widths, the first group including leukocytes and second group including platelet clumps; (5) a step of classifying the leukocytes included in the first group into at least lymphocytes, monocytes and granulocytes based on the scattered light intensities and the fluorescence intensities; and (6) a step of counting the classified lymphocytes, the classified monocytes and the classified granulocytes.