The immunoassay method of this invention measures the content of a subject substance in a sample. The method includes the steps of: (a) preparing a mixed solution by mixing the sample and an antibody solution including a first monoclonal antibody and a second monoclonal antibody capable of specifically binding to the subject substance; and (b) measuring an optical property of the mixed solution. The first monoclonal antibody is capable of binding to a first epitope of the subject substance, and the second monoclonal antibody is capable of binding to a second epitope of the subject substance different from the first epitope. Each of the first and second epitopes exists singly in the subject substance.
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