PURPOSE: To exactly and efficiently inspect blood by inputting the data from a blood cell counter to an analyzing apparatus and deciding that an increase in monocytes is suspected by specific decision conditions.

CONSTITUTION: The blood cell counter 1 dilutes and hemolyzes the blood, reduces the red blood cells and platelets, determines a white blood cell grain size distribution from a white blood cell sample, and inputs the same to the analyzing apparatus 2. The apparatus 2 sets the boundaries between the small- size particles and middle-size particles and the boundaries between the middle- size particles and large-size particle in the grain size distribution, designates the sizes of the particles and the number of the particles in the boundaries as XT1 and XT2 respectively and designates the ratio of the number of the middle-size particles with respect to the total number of the white blood cells as MCR. The apparatus sets a, b as constants and decides that the increase in the monocytes is suspected in the case of equation I. The blood inspection is exactly and efficiently executed in this way without depending on inspector's skill.