<p>An assay method using surface plasmon wherein an immunoreaction field and a detection field are provided independently from each other so that a high sensitivity, a high accuracy and an excellent specificity that is essentially required for immunoassay can be achieved; an assay device; and an assay kit. An assay method which comprises at least the following steps (a) to (g): step (a): a step for bringing a sample into contact with particles carrying a ligand having been immobilized on the surface of the same; step (b): a step for reacting the particles with a ligand-enzyme conjugate; step (c): a step for further reacting the same with a substrate; step (d): a step for isolating the product obtained in step (c); step (e): a step for bringing the product into contact with the surface of a thin metal film of a plasmon excitation sensor; step (f): a step for irradiating the plasmon excitation sensor with laser light and measuring the amount of fluorescence emitted from an excited fluorescent dye; and step (g): a step for calculating the amount of the analyte which is contained in the sample on the basis of the measurement data.</p>
