PURPOSE: To improve a method for measuring the absorbance of a reaction solution or the intensity of diffused light therein, by a method wherein nonsoluble carrier particles whose average particle sizes are less than 0.1 micron are made to carry antibodies or antigens and a light is applied thereon.

CONSTITUTION: By applying a light of 0.2W0.52 microns on the reaction solution of antibodies or antigens supported by nonsoluble carrier particles of minute particle sizes, and of antigens or antibodies in a substance to be inspected, it is made possible to measure the progress of the reaction in a quantifying manner in high density of carrier particles. This is because water solution wherein said carrier particles are suspended transmits excellently the light within the abovementioned range, and because the degree of the progress of the reaction of the antigens with the antibodies in the abovementioned reaction mixture solution corresponds very well to the absorbance of the abovementioned irradiation light or the intensity of the diffused light thereof. Used as the nonsoluble carrier particles are particles of latex of organic macromolecules or those of inorganic oxide such as silica and alumina.