<p>It is intended to provide an immunoassay method by which a single-step assay can be conveniently carried out within a short time and which is applicable to various antigens and enables the achievement of an accurate and quick quantitative analysis. Namely, an immunoassay apparatus having the following four parts: (1) the first part wherein an antigen in a sample is reacted with a first antibody that is a labeled antibody capable of specifically binding to the above antigen; (2) the second part wherein the first antibody not binding to the antigen is reacted with a second antibody that is a biotin- or avidin-conjugated antibody; (3) the third part wherein avidin (in the case where the second antibody is a biotin-conjugated antibody) or biotin (in the case where the second antibody is an avidin-conjugated antibody) is immobilized with an immobilization means to thereby prevent the migration thereof into the fourth part and then the second antibody is supplemented with the avidin or biotin thus immobilized; and (4) the fourth part wherein the label of the first antibody having been bound to the antigen is detected; wherein the first antibody is a labeled antibody comprising an F(ab') fragment or reduced IgG as the antibody moiety and a label having been bound to the F(ab') fragment or reduced IgG at a definite binding ratio, while the second antibody is biotin- or avidin-conjugated antibody that is an antiidiotype antibody against the first antibody being incapable of binding to the complex of the antigen with the first antibody.</p>
