PURPOSE: To easily measure mean red blood corpsucle by monitoring the count values at two points of the discrete levels of a red blood corpsucle counter.

CONSTITUTION: The output of a particle detector is sent to two threshold circuits 2, 3. The particle signal past the threshold circuit 2 is sent to a frequency-voltage converter 4, from which the voltage corresponding to the number of particles per unit time is generated. On the other hand, a potentiometer 9 is moved by a servo motor 8 in such a manner that the input signal of the control circuit 7 of the servo motor 8 is made zero, and the voltage from a reference voltage generating circuit 10 which the potentiometer 9 detected at that time is read. The voltage of this time is regulated with a converting circuit 11 so as to coincide with the scale of a recorder 12, whereby the continuous mean particle volume is measured and recorded.