PURPOSE: To measure particles real time, by counting the photoelectron pulses of laser scattering beams due to particles in a fluid and providing (M+1) pulse count circuits corresponding to the addition number M of times in the moving average processing thereof to successively operate the same.

CONSTITUTION: Laser beam 1 is condensed to the fluid in a measuring area 3 and scattering beams due to particles are allowed to reach a photomultiplier tube 7 and the output of the photomultiplier tube 7 is inputted to a pulse wave form shaping circuit 10. The circuit 10 is connected to pulse count circuits 21W2(M+1) to count pulses. Then, count operation is performed by the signal from a shift register 51 and a count value is latched by latch circuit 31W3(M+1) and subsequently compared with a set value by a comparator 55. At this time, the pulse count circuits 21W2(M+1) are successively operated. That is, (M+1) pulse count circuits are provided corresponding to the addition number M of times in the moving average processing of photoelectron pulse count to be successively operated and, therefore, a particle characteristic can be measured real time in a fluctuation reduced state.