An object is to provide an immunoassay method requiring neither a solid-phase immobilization step nor a washing step, enabling quick and simple quantitative measurement of a target substance in a liquid phase and capable of visualizing an antigen. Such an object is attained by measuring the concentration of a target antigen present in a test substance by sequentially performing a step (a) of bringing an antibody light-chain variable region polypeptide and an antibody heavy-chain variable region polypeptide labeled with a fluorescent dye into contact with an antigen in a test substance in a liquid phase; or bringing an antibody heavy-chain variable region polypeptide and an antibody light-chain variable region polypeptide labeled with a fluorescent dye into contact with an antigen in a test substance in a liquid phase; a step (b) of measuring the fluorescence intensity of the fluorescent dye; and a step (c) of computationally obtaining the level of the antigen contained in the test substance with reference to a positive correlation between the concentration of the antigen in a liquid phase and the fluorescence intensity of the fluorescent dye.
