1. A method for classifying and counting leukocytes, which comprises: (1) a step of staining cells in a hematological sample by treatment with a hemolytic agent, with a fluorescent dye capable of differentiating in fluorescence intensity between mature leukocytes, leukocytes with abnormal DNA amount and immature leukocytes; (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 cells; (3) a step of obtaining scattered light peak intensities and scattered light widths of the 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 fluorescence intensities of the 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 the second group including coincidence cells and platelet clumps; (5) a step of classifying the leukocytes included in the first group into mature leukocytes, leukocytes with abnormal DNA amount and immature leukocytes based on the scattered light intensities and the fluorescence intensities of the leukocytes included in the first group; and (6) a step of counting the classified mature leukocytes, the classified leukocytes with abnormal DNA amount and the classified immature leukocytes.