PURPOSE: To obtain a preservation method without reducing the concentration of a reagent for measuring fluorescent immunity during preservation by adding a low- molecule protein which does not contribute to antigen antibody reaction.

CONSTITUTION: When preserving a reagent for measuring fluorescent immunity, a low-molecule protein which does not contribute to a corresponding antigen antibody reaction is added by housing the reagent for measuring fluorescent immunity in a preservation container. A protein with a lower molecule than a fluorescence-labeled substance in the reagent for measuring fluorescent immunity is easily adhered to the inner wall surface of a preservation container, thus almost totally preventing the adhesion of the fluorescence-labeled substance and hence preventing reduction in the concentration of the fluorescence-labeled substance floating in the reagent for measuring fluorescent immunity during preservation and attaining an accurate fluorescent immunity by eliminating a problem where the amount of fluorescencelabeled substance which is constrained near the surface of a slab-type optical waveguide due to antigen antibody reaction is reduced. Further, addition of a low-molecule protein to the reagent for measuring fluorescent immunity prevents the fluorescence-labeled substance from being adhered to the inner surface of the container, thus eliminating the need for silicon coating to the inner surface of a preservation container.