PURPOSE: To automatically detect the quality of measurement during a measuring job, by detecting a liquid drop grown coarse at a particle expanding section with a particle detecting section to count a partial area of a peak distribution of a pulse outputted from a particle detecting section with a peak discrimination.

CONSTITUTION: A particle as liquid drop grown coarse at a particle expanding section 2 is inputted into a particle detecting section 3 to measure and as each particle is detected, a pulse signal outputted from the detecting section 3 is counted with a counter 4. A peak discriminator 6 discriminates a peak at least at a part in a pulse height distribution of the pulse signal and the number of pulses is counted with a counter 7 for detecting counting deficiency, for example, in a range of 8 in the drawing. Counts thus obtained have 0 or a very small value when operation is normal and when the operation is poor, many more pulses are to be counted to allow detection of abnormality. Other discriminator 6 and counter 7 are required to count pulses, for example, in a range of 9 in the drawing.