PURPOSE: To enable the monitoring of changes in the size of a cell during measurement, by dividing a period of measuring time into small terms to plot a particle size distribution curve per small term from a particle size distribution data for each sample as obtained in each small term.

CONSTITUTION: Blood is diluted with a dilution device 1 to such an extent that white corpuscles are measurable and divided into (n) pieces in equal quantities. Then, a red corpuscle dissolving agent is added to samples S1, S2...Sn to measure white corpuscles with an analyzer 2 and a particle size distribution data obtained by measurement in each small term is stored into a memory 3. The data stored in the memory 3 is calculated with a computing unit 4 in each small term and a particle size distribution curve is plotted from the particle size distribution data obtained. From among the particle size distribution curves, those presenting the greatest number of crests are selected and the particle size distribution curve selected is analyzed to calculate the ratio of the number of cells contained in the crests to the total number of white corpuscles.