An object of the present invention is to provide an
immunoassay that enables the rapid and convenient quantitative measurement with high detection sensitivity of a target substance in a
liquid phase without the need of immobilization and washing steps and enables the
visualization of an
antigen. The present invention achieves the object by performing sequentially the steps of:(a) bringing, in a
liquid phase, a complex comprising a polypeptide containing an
antibody light-chain variable region and a polypeptide containing an
antibody heavy-chain variable region into contact with an
antigen in a specimen for measurement, wherein either or both the polypeptides are labeled with a fluorescent dye(s);(b) detecting the
fluorescence of the fluorescent dye(s) or measuring the
fluorescence intensity; and(c) calculating the amount of the
antigen contained in the sample or visualizing the antigen using the
positive correlation between the antigen concentration and the
fluorescence intensity of the fluorescent dye(s) as an indicator, thereby measuring the concentration of the
target antigen existing in the test substance.