A blood analyzer, comprising: a sample preparing part comprising a first chamber for preparing a first measurement sample comprising blood, a hemolyzing reagent, and a staining reagent and a second chamber for preparing a second measurement sample comprising blood and a diluent; a first detection unit comprising a flow cell in which the first measurement sample flows and which is connected to the first chamber, a light source for irradiating the first measurement sample flowing in the flow cell, a PIN photodiode (54, 57) for detecting scattered light from the first measurement sample irradiated by the light source, and an avalanche photo diode for detecting fluorescence light from the first measurement sample irradiated by the light source; a second detection unit comprising a second flow cell in which the second measurement sample flows and which is connected to the second chamber and electrodes that detect changes of direct current resistance therebetween caused by the second measurement sample flowing therebetween; a signal processing part configured for processing a first detection signal from the PIN photodiode (54, 57) and a second detection signal from the avalanche photodiode, and a third detection signal from the electrodes, wherein the signal processing part is configured to amplify the second detection signal with an amplifier and reduce high frequency noise included in said amplified second detection signal; and an analysis part configured for classifying white blood cells in the first measurement sample into at least four groups comprising neutrophils, lymphocytes, monocytes, and eosinophils based on the first and the second detection signals processed by the signal processing part, and for counting red blood cells in the second measurement sample based on the third detection signal processed by the signal processing part, wherein the signal processing part comprises a low pass filter for reducing the high frequency noise of the amplified second detection signal, and wherein the low pass filter of the signal processing part is selected in such a way that the maximum allowable level of the noise contained in the amplified second detection signal after the noise reduction by said second low pass filter is less than 13% with respect to the output voltage range of the amplifier. a sample preparing part comprising a first chamber for preparing a first measurement sample comprising blood, a hemolyzing reagent, and a staining reagent and a second chamber for preparing a second measurement sample comprising blood and a diluent; a first detection unit comprising a flow cell in which the first measurement sample flows and which is connected to the first chamber, a light source for irradiating the first measurement sample flowing in the flow cell, a PIN photodiode (54, 57) for detecting scattered light from the first measurement sample irradiated by the light source, and an avalanche photo diode for detecting fluorescence light from the first measurement sample irradiated by the light source; a second detection unit comprising a second flow cell in which the second measurement sample flows and which is connected to the second chamber and electrodes that detect changes of direct current resistance therebetween caused by the second measurement sample flowing therebetween; a signal processing part configured for processing a first detection signal from the PIN photodiode (54, 57) and a second detection signal from the avalanche photodiode, and a third detection signal from the electrodes, wherein the signal processing part is configured to amplify the second detection signal with an amplifier and reduce high frequency noise included in said amplified second detection signal; and an analysis part configured for classifying white blood cells in the first measurement sample into at least four groups comprising neutrophils, lymphocytes, monocytes, and eosinophils based on the first and the second detection signals processed by the signal processing part, and for counting red blood cells in the second measurement sample based on the third detection signal processed by the signal processing part, wherein the signal processing part comprises a low pass filter for reducing the high frequency noise of the amplified second detection signal, and wherein the low pass filter of the signal processing part is selected in such a way that the maximum allowable level of the noise contained in the amplified second detection signal after the noise reduction by said second low pass filter is less than 13% with respect to the output voltage range of the amplifier.