A technique for measuring the degree of an antigen-antibody reaction by preparing a suspension of insoluble microscopic carrier particles of at least one type carrying an antigen, an antibody or a hapten, forming an agglutination promoting or inhibiting reaction system among the insoluble carrier particles based on an antigen-antibody reaction using the suspension and one or more antigen, antibody or hapten, irradiating the solution of the reaction system with laser light and detecting the light scattered from the reaction system at one or more specific angles, detecting a signal indicative of one or more specific frequency bands from the resulting scatter spectrum, and thenceforth calculating the quantity of antigen, antibody or hapten in a specimen on the basis of the detected signal. The intensity spectrum output of filter 11 for frequency band selection is in the form of a square root and is converted into the original intensity spectrum by means of a squaring circuit 12. The output of the squaring circuit is recorded by a recorder 10.