A blood analyzer, comprising: a blood sample supply section for supplying a first blood sample and a second blood sample divided from a blood sample; a sample preparation section for preparing a first assay sample by mixing the first blood sample, a first hemolytic agent and a first nucleic acid-staining fluorescent dye, and a second assay sample by mixing the second blood sample, a second hemolytic agent different from the first hemolytic agent and a second nucleic acid-staining fluorescent dye, wherein the first hemolytic agent and the first nucleic acid-staining fluorescent dye are contained in a first reagent, said first reagent is a reagent for detecting a blood cell group composed of lymphoblasts and nucleated red blood cells, and the second hemolytic agent is a hemolytic agent for the measurement of nucleated red blood cells; a flow cell for flowing the first assay sample and the second assay sample; a light source for irradiating a flow of the first assay sample or the second assay sample in the flow cell; an optical detecting section (243, 246, 248) for receiving first fluorescence and first scattered light selected from forward-scattered light and side-scattered light emitted from the irradiated first assay sample, and receiving second fluorescence and second forward-scattered light from the irradiated second assay sample; and a controller configured to perform data processing operations comprising: discriminating a cell group composed of lymphoblasts and nucleated red blood cells from other blood cells, contained in the first assay sample, on the basis of the first fluorescence and the first scattered light, counting the discriminated cells contained in the cell group, discriminating nucleated red blood cells from other blood cells, contained in the second assay sample, on the basis of the second fluorescence and the second forward-scattered light, counting the discriminated nucleated red blood cells, obtaining an information on the presence of the lymphoblasts in the blood sample, on the basis of the number of the cells in the cell group and the number of the nucleated red blood cells, and outputting the information on the presence of the lymphoblasts in the blood sample. a blood sample supply section for supplying a first blood sample and a second blood sample divided from a blood sample; a sample preparation section for preparing a first assay sample by mixing the first blood sample, a first hemolytic agent and a first nucleic acid-staining fluorescent dye, and a second assay sample by mixing the second blood sample, a second hemolytic agent different from the first hemolytic agent and a second nucleic acid-staining fluorescent dye, wherein the first hemolytic agent and the first nucleic acid-staining fluorescent dye are contained in a first reagent, said first reagent is a reagent for detecting a blood cell group composed of lymphoblasts and nucleated red blood cells, and the second hemolytic agent is a hemolytic agent for the measurement of nucleated red blood cells; a flow cell for flowing the first assay sample and the second assay sample; a light source for irradiating a flow of the first assay sample or the second assay sample in the flow cell; an optical detecting section (243, 246, 248) for receiving first fluorescence and first scattered light selected from forward-scattered light and side-scattered light emitted from the irradiated first assay sample, and receiving second fluorescence and second forward-scattered light from the irradiated second assay sample; and a controller configured to perform data processing operations comprising: discriminating a cell group composed of lymphoblasts and nucleated red blood cells from other blood cells, contained in the first assay sample, on the basis of the first fluorescence and the first scattered light, counting the discriminated cells contained in the cell group, discriminating nucleated red blood cells from other blood cells, contained in the second assay sample, on the basis of the second fluorescence and the second forward-scattered light, counting the discriminated nucleated red blood cells, obtaining an information on the presence of the lymphoblasts in the blood sample, on the basis of the number of the cells in the cell group and the number of the nucleated red blood cells, and outputting the information on the presence of the lymphoblasts in the blood sample. discriminating a cell group composed of lymphoblasts and nucleated red blood cells from other blood cells, contained in the first assay sample, on the basis of the first fluorescence and the first scattered light, counting the discriminated cells contained in the cell group, discriminating nucleated red blood cells from other blood cells, contained in the second assay sample, on the basis of the second fluorescence and the second forward-scattered light, counting the discriminated nucleated red blood cells, obtaining an information on the presence of the lymphoblasts in the blood sample, on the basis of the number of the cells in the cell group and the number of the nucleated red blood cells, and outputting the information on the presence of the lymphoblasts in the blood sample.