In the present invention, in an immunoassay method utilizing colloidal gold whereon an antibody or antigen has been immobilized, a simple procedure enables the sensitivity to be adjusted and the measurement range to be expanded to match the target of detection. It was discovered that performing a blocking treatment using both a non-biologically derived component and a biologically derived component on the surface of the colloidal gold particles whereon an antibody or antigen has been immobilized enables the adjustment of measurement sensitivity through the types, concentrations, etc., of both components, and enables the use of a non-biologically derived component that heretofore could not be used because colloidal gold agglomerates with the use of a non-biologically derived component alone.