An immunoassay device capable of assaying amount of an antigen by allowing a labeled antibody to specifically bind to a antigen analyte in a sample and assaying a label of a bound product, an interior of a single device has four regions comprises: (1) a first region where the antigen in the sample reacts with a first antibody that is the labeled antibody capable of specifically binding to the antigen, (2) a second region where first antibody that has not bound to the antigen reacts with a second biotin- or avidin-bound antibody, (3) a third region where, depending on whether the second antibody is biotin-bound antibody or avidin-bound antibody, either avidin or biotin is immobilized by immobilization means so as to be unable to move to the fourth region, and the second antibody is captured by the immobilized avidin or biotin, and (4) a fourth region where the label of the first antibody that has bound to the antigen is detected, being constructed in such a way that a solution can move sequentially through each region, the first antibody, which is the labeled antibody such that an antibody component is an F(ab′) fragment or reduced IgG, the F(ab′) fragment or reduced IgG being bound with the label in a predetermined proportion, is included in the first region or an adjacent region, and the second antibody is a biotin- or avidin-bound antibody, being of anti-idiotype antibody against the first antibody and a type that cannot bind to the bound product of the antigen and first antibody, and is included in the second region or an adjacent region.