An immunoassay comprises the steps of: (a) mixing a whole blood sample with sensitized insoluble carrier particles smaller than erythrocytes to cause an immune agglutination reaction; (b) introducing the resulting immune agglutination reaction mixture including agglutinated particles and unagglutinated particles to a flow cell, irradiating the particles passing through the flow cell with laser light, and detecting scattered lights generated thereby; (c) setting a threshold value for distinguishing unagglutinated particles from agglutinated particles and a threshold value for distinguishing the agglutinated particles from blood cells with regard to intensity of the scattered light; and (d) distinguishing and counting the unagglutinated particles, the agglutinated particles and the blood cells from the scattered lights detected in the step (b), in reference to the threshold values set in the step (c).