Abstract: An assay method capable of efficiently trapping an analyte in a certain amount of a reaction liquid in a short reaction time, the method being characterized in that only a part of a solid phase coated with a substance that specifically binds to the substance to be assayed is immersed in a pool of the reaction liquid containing the analyte, and this part of the solid phase is exchanged during the reaction, so that any surface of the solid phase can be immersed in the reaction liquid once in a predetermined time, and an assay kit for the assay based on this principle. In comparison with the conventional assay methods wherein the entire area of a limited solid phase surface is brought into contact with a certain amount of a reaction liquid, or wherein a test solution is diluted to bring the entirety of a certain surface area of the solid phase into contact therewith, the inventive method can increase reaction speed, which in turn results in trapping of the analyte on the solid phase efficiently in a short time.