1. A blood analyzer comprising: a blood specimen supplying portion for supplying a blood specimen; a sample preparation portion for preparing a measurement sample without using a fluorescence-labeled antibody by mixing the blood specimen supplied from the blood specimen supplying portion, a hemolyzing agent that contains a cationic surfactant and a nonionic surfactant, and a fluorescent dye that stains nucleic acid; a light source for irradiating light onto the measurement sample prepared by the sample preparation portion; a first detector for detecting fluorescence emitted from each of cells contained in the measurement sample irradiated with light by the light source to output fluorescence intensity of each of the cells; a second detector for detecting scattered light emitted from each of the cells contained in the measurement sample irradiated with light by the light source to output scattered light intensity of each of the cells; an information processing portion configured to: a) distinguish lymphocytes from leucocytes contained in the cells based on fluorescence intensities outputted from the first detector and scattered light intensities outputted from the second detector; b) obtain an indicator selected from the group consisting of a measure of central tendency of fluorescence intensities of the distinguished lymphocytes, a dispersion of fluorescence intensities of the distinguished lymphocytes, a kurtosis of a particle distribution of fluorescence intensities of the distinguished lymphocytes and a skewness of the particle distribution of fluorescence intensities of the distinguished lymphocytes; and c) obtain a ratio between a B-lymphocyte count and a T-lymphocyte count based on the obtained indicator in b) and a predetermined relationship value between B-lymphocyte and T-lymphocyte, wherein the relationship value directs one-to-one relationship between the indicator and the ratio; and an output portion for outputting the ratio obtained by the information processing portion.