The present invention provides a blood analyzer. The blood analyzer comprises a flow cell in which a first measurement sample flows, a light source for irradiating the first measurement sample flowing in the flow cell, a PIN photodiode for detecting a side scattered light from the first measurement sample irradiated by the light source, and an avalanche photodiode for detecting a side fluorescence light from the first measurement sample irradiated by the light source. The blood analyzer also comprises a signal processing part which has a low pass filter for reducing high frequency noise included in the side fluorescent light signal. A cut-off frequency of the low pass filter is set so that the blood analyzer can classify white blood cells in the first measurement sample into at least four groups comprising neutrophils, lymphocytes, monocytes, and eosinophils, based solely on the side scattered light signal and the side fluorescent light signal.